Green Synthesis of Carbon Quantum Dots from Stale Soy Milk Composited Zinc Oxide (ZnO/CQD) for Photodegradation of Malachite Green

Green Synthesis of Carbon Quantum Dots from Stale Soy Milk Composited Zinc Oxide (ZnO/CQD) for Photodegradation of Malachite Green | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Green Synthesis of Carbon Quantum Dots from Stale Soy Milk Composited Zinc Oxide (ZnO/CQD) for Photodegradation of Malachite Green Hendri Widiyandari, Putri Lestari, Azza Arba Nurul Ummah, Alief Almasyah Akbar Mastura, and 2 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5372134/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 21 Mar, 2025 Read the published version in Nanotechnology for Environmental Engineering → Version 1 posted 7 You are reading this latest preprint version Abstract Visible light-driven photocatalysts are widely investigated to produce high removal efficiency in removing organic pollutants. Carbon quantum dots (CQD) are a plausible candidate for enhancing photocatalytic activity and play an essential role in malachite green (MG) degradation. Biomass waste, stale soy milk, contains lactic acid, which is utilized as a carbon precursor to prepare CQD. ZnO photocatalysts were composited with CQD derived from stale soy milk by green synthesis for the first time. The presence of CQD and their effect on morphology, surface area, decrease in band gap energy, and reduced electron-hole recombination. Indicating that the photocatalytic activity of ZnO/CQD in MG degradation was confirmed after 90 minutes, reaching 84% with a reaction rate constant of 0.01137 k/min -1 . Furthermore, the reusability study after four reaction cycles revealed that ZnO/CQD were stable, and scavenger tests were performed to identify the active sites. As a result, we believe that CQD from stale soy milk composited with ZnO is an excellent photocatalyst candidate for removing organic pollutants. CQD ZnO/CQD Photodegradation Malachite Green Ciprofloxacin Stale soy milk Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 1. Introduction The textile sector has grown significantly to fulfill the population's desires. However, the textile industry's waste must be handled appropriately. According to global data, the world produces 92 million tons of textile waste annually. Malachite green (MG), a green triphenylmethane dye waste, is one of the textile industry's wastes. MG is a dye used in silk, wool, flax, leather, cotton, and other textiles [ 1 , 2 , 3 ]. According to the Occupational Safety and Health Administration (OSHA, USA), MG is a class 2 dangerous chemical to human health since it could cause cancer, mutagenesis, teratogenicity, and pulmonary toxicity [ 4 , 5 ]. A concrete method is needed to eliminate MG from aqueous solutions. Many methods have been used to treat these wastes, including precipitation, coagulation, flocculation, adsorption, activated carbon, and ozonolysis. However, these methods transport contaminants from phase to phase since MG wastes stable compounds that are exceedingly difficult to degrade [ 4 , 6 ]. Furthermore, the operational costs of these approaches are substantial, requiring alternative methods for treating MG. Advanced oxidation processes (AOPs) are chemical methods that oxidize organic pollutants into non-hazardous molecules by generating reactive free radicals. AOPs have recently acquired favor as an efficient method for pollutant degradation because, as reported, reactive free radicals generated during AOP processing efficiently degrade or eliminate pollutants [ 6 , 7 ]. Photocatalysts are included in the AOPs process suitable for degrading MG [ 8 ]. Photocatalytic AOPs mostly use semiconductor metal oxides as photocatalysts, such as TiO2, ZnO, CuO, and NiO [ 9 , 10 ], because of their high photostability, non-toxicity, facile synthesis process, and environmental sustainability [ 11 , 12 ]. Researchers widely use ZnO for the photodegradation of organic pollutants because they have high photosensitivity, low cost, oxidation resistance, and thermal stability [5;14]. ZnO has favorable characteristics for water processing, so ZnO is a metal oxide semiconductor suitable for use as a photocatalyst. ZnO nanoparticles, ZnO nanowires, nanorods, and ZnO nanoflowers are some of the most commonly used ZnO structures. This study employs ZnO nanoflower due to its large surface area compared to other ZnO structures. However, ZnO only performs under the excitation range of UV light because of its large band gap that only absorbs 3% − 5% of sunlight [ 14 ]. Furthermore, the high rate of electron and hole recombination reduces photocatalytic activity performance, requiring a material that would enhance ZnO photocatalytic activity [ 15 ]. Carbon Quantum Dots (CQD) are one of the best carbon types for improving ZnO's photocatalytic activity [ 16 ], mainly because they could reduce the recombination rate of electrons and holes as electron acceptors [ 17 ]. Furthermore, CQD has excellent photoluminescence capabilities, biocompatibility, fluorescence conversion ability, electron transport, and photosensitivity [ 18 ]. Combining ZnO and CQD produces more photogenerated electron-hole pairs on ZnO to convert long wavelengths of light into short wavelengths. In addition, CQD has conjugated π bonds to increase the adsorption ability of organic pollutants on the ZnO surface. Based on research conducted by [ 19 ] mentioned that the carbon precursor commonly used in the synthesis of CQD to increase quantum yield is derived from biomass waste sources. This renewable step is an effective way to treat waste sustainably. Furthermore, biomass-derived CQD obtained from natural resources provide beneficial economic properties and functional groups with excellent fluorescence and sensing capabilities to tackle surface passivation, which could enhance target molecule sensitivity [ 20 , 21 , 22 ]. Biomass waste is derived from agricultural and food waste [ 23 ]. For example, Wang and his colleague Zhou have conducted a green synthesis of CQD derived from soy milk; Saxena et al. 2012 synthesized peanut shells into carbon nanosphere sources; Prasannan and Imae, 2013 synthesized CQD from orange peel waste; Dubey et al 2015 have synthesized CQD from soy nuggets as a carbon source, and Hojaghan et al. 2021 have synthesized CQD from soybean pieces as a carbon source [ 24 , 25 , 26 , 27 , 16 ]. The source of CQD used in this research is stale soy milk because it contains lactic acid as a carbon source. In addition, the used of stale soy milk as a research novelty, increasing economic value and reducing the amount of stale soy milk waste in the environment. Conventional methods typically produce CQD. However, this process involves the usage of numerous hazardous and toxic compounds that are harmful to the environment. Therefore, green synthesis is a method that attracts attention today to produce CQD [ 28 ]. Green synthesis utilizes non-toxic natural materials that are environmentally friendly and renewable [ 29 ]. In addition, green synthesis has high stability and produces high quantum yield. Sariga et al. 2023 mentioned using green synthesis methods to obtain high biocompatibility and non-toxic, economical, and environmentally friendly CQD [ 19 , 30 ]. As a result, this research aims to green synthesize CQD from stale soy milk composites with ZnO to develop ZnO/CQD photocatalytic material. ZnO/CQD is produced and investigated for photocatalytic activity on malachite green. 2. Methods 2.1 Materials The materials used were stale soy milk (obtained from soy milk producers in Surakarta, Central Java, Indonesia), C 2 H 6 O (≥ 99.8%, Sigma Aldrich), ZnO commercial, NaOH (97%, Sigma Aldrich), Zn(NO 3 ) 2 .6H 2 O (≤ 100%, Smart Lab), C 6 H 5 Na 3 O 7 .2H 2 O (99%, Merck), malachite green (99%, Loba Chemie). 2.2 Fabrication of ZnO nanoflower The synthesis of ZnO nanoflower was conducted using the hydrothermal method. A Certain amount of Zn(NO 3 ) 2 .6H 2 O (7.44 g) and C 6 H 5 Na 3 O 7 .2H 2 O (2.94 g) were dissolved in 20 mL of distilled water. Then, the solution was stirred, and NaOH 1 M was gradually added until pH = 13. Stirring for 100 minutes at room temperature, the solution was put into a 100 mL autoclave and heated at 130°C for 12 hours. The solution was filtered and washed using distilled water and C 2 H 6 O until a white precipitate was obtained and then dried at 100 ° C for 4 hours and then calcined at 500 ° C for 3 hours with a heating rate of 5 ° C min − 1 . 2.3 Fabrication of CQD Green synthesis of CQD solution from stale soy milk by hydrothermal method. Stale soy milk (56 mL) was added to 96% ethanol (24 mL), then placed in a 100 mL autoclave and heated for 12 hours at a temperature variation of 120°C, 150°C, and 180°C. The solution was filtered, extracted, and placed in a centrifugator at 2000 rpm for 40 minutes. 2.4 Fabrication of ZnO/CQD The synthesis of ZnO/CQD started by dissolving 0.2 grams of ZnO nanoflower into 45 mL of distilled water and 1.5 mL of CQD solution and stirring for 2 hours. Next, the solution was heated at 100°C for 8 hours. The solution was filtered and washed using distilled water and C 2 H 6 O until a white-brown precipitate was obtained, and finally, it was dried at 100°C for 4 hours. 2.5 Material characterization X-ray diffraction (XRD, D8 Advance Bruker, Germany) to identify crystallographic properties with an x-ray source of Cu radiation (λ = 1.54184 Å; voltage = 40 KV; current = 35 mA) where the 2θ angle is 20° − 80°. Fourier transform infrared (FTIR) to examine the functional groups of the final product. A UV-Vis spectrophotometer (Thermo Scientific Genesys 150, USA) was used to identify the absorbance peaks in the CQD solution and the degradation results before and after irradiation. UV-Vis diffuse reflectance (Analytic Jena Specord 200 Plus, Germany) to calculate band gap energy, while to calculate surface area, N2 adsorption-desorption, and porosity parameters using Brunauer-Emmett-Teller (BET, Micromeritics Tristar II Plus 3020). Photoluminescence (PerkinElmer LS 55) to evaluate electron-hole recombination and Field emission scanning electron microscope/Energy dispersive x-ray (FE-SEM/EDX, JEOL JIB 4610F) to identify morphology. 2.6 Photocatalytic experimental procedure The photocatalytic activity of ZnO/CQD was investigated using a solar simulator (Peccel/PEC-L01) as a visible light source with a wavelength range of 400 nm − 800 nm. A 250 mL beaker was used for this reaction. The beaker was placed on a stirrer at 11 cm from the visible light source while maintaining the temperature at 25°C. Photocatalyst material of as much as 0.1 gram was dissolved in a ten ppm MG dye solution of 50 mL and then stirred in the dark (without irradiation) for 30 minutes to establish adsorption-desorption balance. For MG dye, they stirred again under visible light exposure for 150 minutes with an interval of 30 minutes. After the photodegradation process, the samples were centrifuged at 5000 rpm for 20 minutes to separate the photocatalyst from the solution. The pure supernatant liquids of MG and ciprofloxacin were evaluated using a UV-Vis spectrophotometer, where the degradation efficiency of both pollutants can be calculated using the following equation: $$\:Degradasi\:\left(\%\right)=\:\frac{{C}_{0}-\:{C}_{t}}{{C}_{0}}\:x\:100=\frac{{A}_{0}-\:{A}_{t}}{{A}_{0}}\:\:x\:100$$ C 0 , A 0 , C t , and A t are MG's initial concentration and absorption, respectively; concentration and absorption t minutes after irradiation. 2.7 Scavenger test Reactive free radicals produced throughout the photocatalytic process are essential in photodegradation, and scavengers examine them. This experiment aims to trap reactive free radicals using ZnO/CQD to determine the role of reactive free radical species in the degradation of MG pollutants. 0.001 grams of ammonium oxalate (AO) to detect h + , 0.001 grams of benzoquinone (BQ) to detect ⋅O 2− , and 10 mL of isopropanol (IPA) were used to trap reactive ⋅OH. 3. Result and discussion 3.1. Physicochemical properties of materials The FESEM characterization results of samples (a) ZnO commercial, (b) ZnO nanoflower, and (c) ZnO/CQD can be seen in Fig. 1 . ZnO commercial shows an arbitrary structure; ZnO nanoflower shows a slightly budding nanoflower structure consisting of a pistil in the center and surrounded by petals. ZnO/CQD shows a slightly budding nanoflower structure with a thorn shape in the center and surrounded by petals. The morphology of ZnO Nanoflower and ZnO/CQD has a smooth surface and uniform size. The addition of CQD is known to improve the morphology of ZnO/CQD. This is possible because CQD serves as a scaffold for Zn ions to connect and crystallize, hence determining the direction and pattern of ZnO crystal development, resulting in a more organized morphology. Due to their small size, CQD has numerous nucleation sites, including -OH, -COOH, and -NH 2 . When CQD is added to the ZnO precursor, it can operate as a nucleation center, directing ZnO development to an instructed morphology, such as nanoflowers. TEM analysis examined the microstructure and crystallographic features of ZnO/CQD. In addition to SEM, the composite of CQD on ZnO can be confirmed from the TEM image of ZnO/CQD, as shown in Fig. 2 (a). Based on Figure (a), it is known that the CQD attached to the ZnO surface form an intense contact, thus illustrating the successful formation of the ZnO/CQD composite. Heterogeneous ZnO primary particles form the ZnO/CQD structure with a size of 50 nm. In addition, it is provided with CQD on the ZnO surface with a size of 10 nm and a lattice edge of 0.2 nm, as shown in Fig. 2 (b). The presence of CQD on the ZnO surface has been confirmed. The sharp spots in the selected area electron diffraction (SAED) pattern in Fig. 2 (c) indicate the polycrystalline nature of ZnO/CQD. The EDS spectrum image of the ZnO commercial is shown in Fig. 1 (d) with elemental mapping of Zn, O, and C at 40.9, 37.6 and 21.5 At%, respectively. The EDS spectrum of the ZnO nanoflower is shown in Fig. 3 (e) with elemental mapping of Zn, O, and C of 36.8, 33.9, and 29.3 At%, respectively. In addition, the Figure shows that the mapping of each element is evenly and uniformly distributed on the ZnO nanoflower. In the spectrum of ZnO/CQD, the mapping of Zn (46%), O (44.2%), and C (9.9%) shows that the distribution of CQD is uniform and evenly distributed across the surface of ZnO nanoflower, as shown in Fig. 2 . Figure 2 depicts the homogenous distribution of components in ZnO composited using CQD. CQD is equally distributed in the ZnO matrix, which may lead to the morphological evolution to nanoflower. According to Fig. 3 (a), no other compound diffraction peaks were found in this study's XRD pattern produced by the photocatalyst material. The diffraction peaks produced consist of (100), (002), (101), (102), (110), (103), (200), (112), (201), (004), and (202). The diffraction peak results follow the JCPDS 36 1451 data. Based on the database, it can be seen that the crystal structure produced is hexagonal wurtzite. The calculated crystal sizes of ZnO commercial, ZnO nanoflowers, and ZnO/CQD are 20.64 nm, 20.05 nm, and 23.15 nm, respectively. The addition of CQD is known to increase the crystal size of the photocatalyst material. The increase in crystal size is known to increase charge mobility and reduce electron-hole recombination. In addition, the increase in crystal size could also expand the light absorption area to improve the performance of photocatalytic activity [ 32 ]. The FTIR spectra of ZnO commercial, ZnO nanoflower, and ZnO/CQD are shown in Fig. 3 (b) and the corresponding functional groups in Table 1 . The peaks below 800 cm − 1 show the characteristics of Zn and O bonds; this can be seen in the appearance of a strong absorption band at the peak of 433 − 432 cm − 1 , which is indicated by Zn-O stretching vibration; it shows that the frequency generated from the Zn-O bond is quite strong [ 8 , 10 ]. Likewise, at 542–547, cm − 1 which indicates the presence of O-Zn-O stretching vibration [ 5 ]. Epoxy groups, highly reactive functional groups consisting of two carbons and one oxygen, have been identified at 904–910 cm − 1 . This occurs because of the C-O stretching vibration in the epoxy group. The epoxy group is carbon quantum dots (CQD), which act as a ZnO composite [ 10 ]. In addition, the peak at 1369–1371 cm − 1 indicates O-ZnO stretching vibrations resulting from a shift in C = O stretching at 1353 cm − 1 . Meanwhile, 1556 cm − 1 shows C = O stretching vibration, which indicates that the resulting molecule contains carbonyl groups [ 33 ]. In addition, at 1654 cm − 1 , carbonylic groups (O-C-O) asymmetric vibrations are present. This carbonylic group comprises a carbon atom bonded to two oxygen atoms [ 10 ]. The 1981–1987 cm − 1 peak shows the presence of C-H bending, included in the aromatic compound with weak bonds. Aromatic compounds have remarkable stability due to resonance in the ring structure. In addition, aromatic compounds are organic, which means they contain carbon atoms bound to other elements, namely oxygen. In addition, another peak appears at 2970 cm − 1 , which indicates the presence of C-H stretching [ 20 ]. The resulting functional groups are almost the same as ZnO/CQD made from other natural materials, so in general, the existence of these functional groups shows promising results for photocatalyst materials Table 1 Functional groups of FTIR spectra Band Positions (Cm − 1 ) Chemical Functional Group Source ZnO Commercial ZnO nanoflower ZnO/CQD - 434 433 Zn-O stretching vibration 8 543 542 547 O-ZnO-O streching 5 904 909 910 Epoxy groups 10 1369 1371 1371 O-ZnO stretching vibration 8 1556 - - C = O stretching vibration 33 1654 - - Asymmetric vibration of O-C-O 10 1981 1987 1985 C-H bending 5 - - 2970 C-H stretching vibration 20 Adsorption desorption isotherm measurements of N 2 at 77 K were used to determine the adsorption ability, pore size distribution, and specific surface area of the ZnO/CQD composite. The adsorption-desorption isotherm of N2 showed a type IV isotherm with a type H4 hysteresis in the range of ca. 0.4-1.0 P/P0, as shown in Fig. 3 (c). The BET surface area was 6.418 m 2 /g. This shows an increase from previous research [ 35 ], which only amounted to 5.1672 m 2 /g for ZnO/CQD composites. Surface area dramatically affects the occurrence of direct contact with contaminants on the photocatalyst surface. Therefore, increasing the surface area of the photocatalyst can lead to more effective direct contact, which in turn can increase the photocatalytic activity [ 36 ]. In addition, increasing the surface area of the photocatalyst can also increase the adsorption that occurs, so the ZnO/CQD composite has good adsorption ability [ 34 ]. Based on the Barret-Joyner-Halenda (BJH) pore size distribution curve, as shown in Fig. 3 (d), the pore size distribution of the ZnO/CQD composite is 363 Å or 36.3 nm, which indicates the presence of a mesoporous structure [ 37 ]. The pore size distribution plays an essential role in the degradation of malachite green by ZnO/CQD. This is because the pore structure affects mass transfer, which can improve the sensing performance of more gas-sensitive materials. In addition, the pore size also affects the diffusion of reaction components on the photocatalyst and the adsorption of dissolved oxygen. In this case, the mesoporous structure is more favorable for the diffusion of reaction components than the microporous structure. Therefore, this research's mesoporous structure of ZnO/CQD can improve the photocatalytic activity. CQD that were produced via the green synthesis by hydrothermal method for 12 hours with variations in heating temperature, 120°C, 150°C, and 180°C were analyzed by irradiation under sunlight and UV light (λ = 365 nm). The analysis was used to investigate the formation of a CQD solution. CQD solution is yellowish-golden to brownish-yellow in sunlight, as shown in Fig. 4 (a). The CQD solution is luminescent blue in appearance when irradiated under UV light, as shown in Fig. 4 (b). The analysis results under UV light show a change in the color of the CQD solution from a golden brownish-yellow to a luminous blue. The color difference indicates that the resulting solution is a solution of carbon quantum dots (CQD). When irradiated with UV light, the smaller particle size of CQD can lead to electrons on the surface being excited to a higher energy level. Thus, when the electrons are trapped back in their ground state, the photons emitted are luminous blue [ 28 ]. In this study, variations in heating temperature were performed to obtain the optimal CQD solution. A higher heating temperature could lead to an increase in the mechanical destruction of the material's structure. Therefore, a higher heating temperature could produce the lowest particle size [ 31 ]. In addition, the increase in photoluminescence properties is followed by the lower particle size. Therefore, the CQD solution with a heating temperature of 180°C can produce the brightest blue light compared with the other variations. UV-Vis further characterized the CQD solution to identify the absorbance peak. Figure 5 (a) shows that the 180°C CQD solution has the highest absorbance peak compared with the other variations. This follows the analysis under UV light irradiation, indicating that the 180°C CQD solution emits the best bright blue light. The results of UV-Vis characterization of the 180°C CQD solution revealed two absorption peaks in the UV absorption region, which are at a wavelength of 257 nm and 300 nm − 400 nm, as shown in Fig. 5 (a). The absorption peak at a wavelength of 257 nm is because of the π-π* electronic transition of the conjugated C = C bond. The most substantial absorption peak at a wavelength of 300 nm − 400 nm is due to the n-π* electronic transition of the C = O bond. UV-Vis DRS characterization is used to calculate the band gap energy. The results of UV-Vis DRS characterization show that the photocatalyst material has an absorption peak in the UV light region at a wavelength of 320 nm − 390 nm and a strong absorption peak in the visible light region at a wavelength of 400 nm − 650 nm as shown in Fig. 5 (b). The shift of the absorption peak indicates that adding CQD on ZnO could expand the absorption area from UV to visible light. The shift of n-π* electrons on the C = O bond from the CQD core followed by the π-π* displacement on the C = C bond from the CQD surface results in a shift in the wavelength of ZnO, where the event is called the quantum confinement effect. The quantum confinement effect is a phenomenon that occurs when the physical size of a material decreases, causing changes in the optical properties of the material. As a result of this event, the band gap energy will decrease, as shown in Fig. 5 (c). Consecutively from ZnO commercial, ZnO nanoflower, and ZnO/CQD are 3.2 eV, 3.16 eV, and 3 eV [ 34 ]. The addition of CQD that can expand the light absorption area results in the low energy required for electron excitation from the valence band to the conduction band, so the addition of CQD to ZnO can increase the efficiency of electron-hole pair charge separation [ 35 ]. Photoluminescence characterization is used to determine the separation of charge carriers, recombination of electrons and holes, and electron transfer, as shown in Fig. 5 (d). ZnO commercial and ZnO nanoflower have high-intensity peaks that indicate a high level of recombination of electrons and holes and low charge isolation of photogenerated charge carriers (electrons and holes) [ 36 ]. The high recombination rate of electrons and holes can cause electrons to move back to the ground state quickly, which can reduce the performance of photocatalytic activity. ZnO/CQD have the lowest intensity peak at a wavelength of 395 nm, which indicates violet emission. The low-intensity peak of photoluminescence indicates the high charge isolation of electron and hole photogeneration so that the level of electron and hole recombination decreases. The high charge isolation is very beneficial in creating the best photodegradation and reduction efficiency [ 8 ]. Furthermore, adding CQD can increase crystal defects that prevent electrons from returning to the ground state to improve the performance of the photocatalytic activity. Therefore, this research proves that adding CQD can increase photocatalytic activity, so ZnO/CQD are the most optimal sample for degrading dyes and pharmaceutical waste [ 34 ]. 3.2. Photocatalytic Degradation Performance Evaluation The photocatalyst's degradation efficiency for MG degradation has been analyzed. 100 mg of photocatalyst was dissolved in 50 mL of MG solution under visible light exposure. The degradation efficiency of MG is shown in Fig. 6 (a). In the first stage, the reaction solution was kept in the dark for 30 minutes to determine the photocatalyst's adsorption ability. Then, the solution was reacted under visible light in the next minute. The composite of CQD on ZnO is supposed to enhance the adsorption capabilities of ZnO/CQD. After 30 minutes of irradiation, ZnO commercial and ZnO nanoflower showed 6% and 8% adsorption. However, ZnO/CQD showed an adsorption of 73%. The adsorption parameter is essential in photocatalysis reactions because it can affect the degradation efficiency. In other words, when compared with ZnO commercial and ZnO nanoflower, ZnO/CQD showed the highest efficiency of 84% for 90 minutes, while ZnO commercial was only 23% and ZnO nanoflower was 32%. ZnO/CQD have the highest efficiency due to high charge separation, good light absorption, increased adsorption, and high charge transfer [ 38 ]. Table 2 Kinetics rate of MG reaction Sample Reaction rate constant (k/min -1 ) Linear dependence (R 2 ) Negative control 0,00474 0,74932 ZnO commercial 0,00178 0,93301 ZnO nanoflower 0,00477 0,92558 ZnO/CQD 0,01137 0,48736 The kinetics rate of the malachite green degradation reaction were determined using the first-order kinetics model technique of the Langmuir-Hinshel equation, as illustrated in Fig. 6 (b). Table 2 shows the reaction rate constants and linear regressions for each sample. The reaction rate constant (k) describes the rate of a chemical process. In this situation, k refers to the rate at which a substance degrades under specific conditions. The greater the k value, the faster the degradation will proceed, making the reaction conditions more efficient in speeding up the degradation process. ZnO/CQD exhibit the highest response rate constant value, 0.01137 k/min-¹. It demonstrates that ZnO/CQD are highly effective at degrading MG. However, the R 2 value of ZnO/CQD is only 0.48736, indicating a poor match with the first-order kinetics model. This suggests that the degradation of ZnO/CQD may not completely obey first-order kinetics. This could be due to other reasons, such as variations in the surrounding temperature or the presence of different compounds throughout the reaction. We suppose that adding composite CQD on the surface of ZnO leads to a high k value, which is not followed by the first-order kinetics fit to the model. ZnO commercial and ZnO nanoflower had lower response rate constant values compared to ZnO/CQD, with 0.00178 k/min-¹ and 0.00477 k/min-¹, respectively, despite higher linear regression values of 0.93301 and 0.92558. As a result, it is clear that the reaction rate constant (k) is the most crucial parameter for determining degradation efficiency; the higher the value of k, the faster the degradation process, indicating that the reaction is highly effective. However, the lower R 2 value indicates that this degrading reaction does not entirely adhere to the first-order kinetics paradigm. This shows that the first-order model might not adequately represent the complex degradation mechanism despite the rapid degradation rate. 3.3. Mechanism of Photocatalytic Degradation The mechanism of photocatalytic degradation is shown in Fig. 7 . ZnO/CQD were photodegraded under visible light to examine MG degradation performance. The photocatalyst material is activated when it is directly exposed to visible light with an energy of hv, which must be more significant than or equal to the band gap energy. In addition, electrons are generated on the ZnO nanoflower particles by being stimulated from the valence band to the conduction band, thus abandoning the hole in the valence band. In such cases, electrons are extremely easy to retrieve in their ground state, known as the valence band. The high recombination rate of electrons and holes may inhibit photocatalytic activity. As a result, the importance of composite CQD on ZnO nanoflower particle surfaces is highlighted. Because of the energy difference between the conduction band of ZnO nanoflower and the energy level of CQD, electrons stimulated on its surface can be rapidly transported to the surface of CQD. CQD play as electron acceptors from the conduction band on the surface of ZnO nanoflower particles. Furthermore, CQD act as electron donors, donating electrons to fill holes and extending the lifespan of separated electrons and holes. This can promote an increase in photocatalytic activity. CQD may reduce electron-hole recombination by providing separate routes for electrons and holes, boosting charge separation efficiency. Charge separation in photodegradation is one of the factors that influence photocatalytic activity. The following step is the water ionization process, which involves oxidizing water molecules with holes to form hydroxyl radicals (⋅OH). This method engages MG molecules directly with the ZnO nanoflower's outermost surface layer. The oxygen ionosorption process happens on the surface of the CQD, where oxygen molecules are reduced to create superoxide radical anions (⋅O 2 -). A subsequent stage is the protonation process, which develops after the superoxide radical anion produces hydroperoxyl radicals. These hydroperoxyl radicals can produce ⋅O 2 - and ⋅OH. The final stage involves the breakdown of contaminants by ⋅O 2 - and ⋅OH, which are converted into innocuous molecules such as carbon dioxide and water [ 7 ]. 3.4. Reusability Study and Scavenger Test The reuse study is significant for evaluating the stability and photocatalytic performance of ZnO/CQD and determining the effectiveness of ZnO/CQD. The reuse study was analyzed for up to 4 cycles, showing that the photocatalytic activity is stable and durable. In addition, Fig. 8 (a). shows that the removal efficiency is not significantly reduced, which indicates that the ZnO/CQD have stability and reliable photocatalytic performance. Figure 8 (b). shows the absorbance reduction of each cycle. The first cycle has the highest absorbance because the ZnO/CQD used are still new and active, while in the next cycle, the absorbance has decreased. After four cycles of reusability, Fig. 8 (c). shows that the degradation efficiency rate slightly reduced at the beginning of each stage of the photocatalyst reaction. This indicates that ZnO/CQD have excellent chemical stability and effective industrial application prospects in water treatment. Separate charge carriers (electron and hole) form highly reactive oxidative species, such as ⋅O 2 - and ⋅OH and others that have an essential role in MG degradation. A scavenger experiment was used to analyze the role of reactive oxidative species on the surface of ZnO/CQD during the MG degradation process. Different scavengers such as 10 mL isopropyl alcohol (IPA), 0.001 g ammonium oxalate (AO), and 0.001 g benzoquinone (BQ) were used to trap (⋅OH), (h + ), and (⋅O 2 -) respectively. Figure 8 (d). shows the role of each scavenger agent. 84%, 39%, 33%, and 58% for none, IPA, AO, and BQ, respectively, so that the order of decrease in photocatalytic activity caused by scavenger is AO > IPA > BQ. Based on these results, it is appropriate to say that reactive oxidative species such as (⋅OH), (h + ), and (⋅O 2 -) generated from photogeneration on the photocatalyst surface are responsible for the performance of photocatalytic activity. Therefore, it can be seen that h + and ⋅OH have an essential role in the degradation of MG, but ⋅O 2 - only has a minor impact on the process of MG degradation. The addition of AO in the photocatalysis reaction traps h + at valence band, thus inhibiting charge recombination and providing more e- at conduction band for MG photodegradation. 4. Conclusion Our work demonstrates that CQD from stale soy milk prepared via green synthesis were performed for the first time and were further composited with ZnO photocatalyst for MG degradation, providing the advantages of highly efficient photocatalytic degradation, eco-friendliness, cost-effectiveness, and ease of recyclability. The results reveal that the CQD composite generated from stale soy milk is effective in reducing electron and hole recombination. This CQD composite has the potential to be a cost-effective and environmentally friendly choice for measuring the electron-hole recombination rate of ZnO photocatalysts. The ZnO/CQD composite is capable of producing effective electron-hole pair separation. It provides more photogenerated electrons to reduce O 2 and more photogenerated holes to oxidize H 2 O, which improves the ability to generate free radicals and thus increases photocatalytic activity. The morphology, composition, phase structure, surface area, and reduced band gap energy were determined using TEM, FESEM, EDX, XRD, BET, and UV-Vis DRS. The results revealed that the ZnO/CQD composite was able to degrade 84% MG for 90 minutes with a reaction rate constant of 0.01137 when exposed to visible light. The results were excellent, more than those of ZnO commercial and ZnO nanoflower photocatalysts. Furthermore, ZnO/CQD can be recyclable and reused for four photocatalytic cycles without significantly reducing their degrading performance. Declarations Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Author Contribution H.W: Writing – original draft, Writing – review & editing, Validation, Supervision, Project administration, Methodology, Conceptualization. P.L: Writing – original draft, experimental and data curation. A.A.N.U: experimental and data curation. A.A.A.M: experimental and data curation. H.P: Writing – original draft, Writing – review & editing. O.A: TEM and FE-SEM characterization. Acknowledgement The authors (PL, AANU, and AAAM) thank to The Directorate General of Higher Education, Research, and Technology (DGHERT) of the Ministry of Education, Culture, Research, and Technology for funding this research through Program Kreativitas Mahasiswa (PKM) 2023. Also, the author (HW) thanks to Universitas Sebelas Maret through Penelitian Unggulan Terapan (PUT-UNS) contract number: 194.2/UN27.22/PT.01.03/2024 References Djebbari C, Zouaoui E, Ammouchi N, Nakib C, Zouied D, Dob K (2021) Degradation of Malachite green using heterogeneous nanophotocatalysts (NiO/TiO 2 , CuO/TiO 2 ) under solar and microwave irradiation. SN Appl Sci 255:1–11 Bazazi S, Jodeyri S, Hosseini SP, Arsalani N, Rashidzadeh B, Fathalipour S, Seidi F, Hashemi E (2023) Ball mill-hydrothermal method for one-step synthesis of zinc oxide/carbon quantum dot (ZnO-CQD) nanocomposites as photocatalyst for degradation of organic pollutants. J Photochem Photobiology A: Chem 445:1–11 Sekar A, Yadav R (2021) Green fabrication of zinc oxide supported carbon dots for visible light-responsive photocatalytic decolourization of Malachite Green dye: Optimization and kinetic studies. Optik 242:1–20 Montazer M, Pebdeni AB, Sheikholeslami MN, Abkenar SD, Firoozbakhtian A, Hosseini M, Dragoi EN (2023) Synthesis of cuttlebone/carbon quantum dots/nickel oxide nanocomposite for visible light photodegradation of malachite green used for environmental remediation. Chemosphere 333:1–10 Saad AM, Abukhadra MR, Ahmed SA, Elzanaty AM, Mady AH, Betiha MA, Shim J, Rabie AM (2020) Photocatalytic degradation of malachite green dye using chitosan supported ZnO and Ce-ZnO nano-flowers under visible light. J Environ Manage 258:1–11 Teymourinia H, Alshamsi HA, Al-nayili A, Sohouli E, Gholami M (2023) Synthesis of new photocatalyst based on g-C 3 N 4 /N, P CQD/ZIF-67 nanocomposite for ciprofloxacin degradation under visible light irradiation. J Ind Eng Chem 125:259–268 Iyyappan J, Gaddala B, Gnanasekaran R, Gopinath M, Yuvaraj D, Kumar V (2024) Critical review on wastewater treatment using photo catalytic advanced oxidation process: Role of photocatalytic materials, reactor design and kinetics. Case Stud Chem Environ Eng 9:1–16 Soni V, Sonu, Sudhaik A, Singh P, Thakur S, Ahamad T, Nguyen VH, Thi LP, Quang HHP, Chaudhary V, Raizada P (2024) Visible-light-driven photodegradation of methylene blue and doxycycline hydrochloride by waste-based S-scheme heterojunction photocatalyst Bi 5 O 7 I/PCN/tea waste biochar. Chemosphere 347:1–19 Pivert ML, Poupart R, Gnambodoe MC, Martin N, Wang YL (2019) Direct growth of ZnO nanowires on civil engineering materials: smart materials for supported photodegradation. Microsystems Nanoengineering 57:1–7 Velumani A, Sengodan P, Arumugam P, Rajendran R, Santhanam S, Palasinamy M (2020) Carbon Quantum dots supported ZnO sphere based photocatalyst for dye degradation application. Curr Appl Phys 20:1176–1184 Zouhier M, Tanji K, Navio JA, Hidalgo MC, Paez J, Kherbeche A (2020) Preparation of ZnFe 2 O 4 /ZnO composite: Effect of perational parameters for photocatalytic degradation of dyes under UV and visible illumination. J Photochem Photobiology A: Chem 390:1–12 Nguyen HP, Cao TM, Nguyen TT, Pham VV (2023) Improving photocatalytic oxidation of semiconductor (TiO 2 , SnO 2 , ZnO)/CNTs for NO x removal. J Ind Eng Chem 127:321–330 Chanu LA, Singh KJ, Devi KN (2022) UV light illuminated photodegradation of Malachite green dye using ZnO/CuO nanocomposites. Materials Today: Proceedings. ;65:2865–2870 Ja’farawy MSA, Kusumandari, Purwanto A, Widiyandari H (2022) Carbon quantum dots supported zinc oxide (ZnO/CQD) efficient photocatalyst for organic poluutant degradation – A systemic review. Environ Nanatechnol Monit Manage 18:1–23 Behnood R, Sodeifian G (2020) Synthesis of N doped-CQD/Ni doped-ZnO nanocomposites for visible light photodegradation of organic pollutants. J Environ Chem Eng 8:1–11 Hojaghan HS, Niasari MS, Amiri O, Rashki S, Ashrafi M (2021) Green synthesis, characterization and antimicrobial activity of carbon quantum dots-decorated ZnO nanoparticles. Ceram Int 47:5187–5197 Song S, Wu K, Wu H, Guo J, Zhang L (2019) Multi-shelled ZnO decorated with nitrogen and phosphorus co-doped carbon quantum dots: synthesis and enhanced photodegradation activity of methylene blue in aqueous solutions. RSC Adv 9:7362–7374 Hu X, Li Y, Xu Y, Gan Z, Zou X, Shi J, Huang X, Li Z, Li Y (2021) Green one-step synthesis of carbon quantum dots from orange peel for fluorescent detection of Escherichia coli in milk. Food Chem 339:1–8 Sariga, Kolaprath MKA, Benny L, Varghese A (2023) A facile, green synthesis of carbon quantum dots from Polyalthia longifolia and its application for the selective detection of cadmium. Dyes Pigm 210:1–11 Patra S, Singh M, Subudhi S, Mandal M, Nayak AK, Sahu BB, Mahanandia P (2023) One-step green synthesis of in-situ functionalized carbon quantum dots from Tagetes patula flowers: Applications as a fluorescent probe for detecting Fe 3+ ions and as an antifungal agent. J Photochem Photobiology A: Chem 442:1–3 Tyagi A, Tripathi KM, Singh N, Choudhary S, Gupta RK (2016) Green synthesis of carbon quantum dots from lemon peel waste: applications in sensing and photocatalysis. RSC Adv 6:72423–72432 Gedda G, Sankaranarayanan SA, Putta CL, Gudimella KK, Rengan AK, Girma WK Green synthesis of multi-functional carbon dots from medicinal plant leaves for antimicrobial, antioxidant, and bioimaging applications. Sci Rep ;6371:1–9 Kurian M, Paul A Recent trends in the use of green sources for carbon dot synthesis-A short review. Carbon Trends ;3:1–11 Wang L, Zhou HS (2014) Green Synthesis of Luminescent Nitrogen-Doped Carbon Dots from Milk and Its Imaging Application. ACS Publications 86:8902–8905 Saxena M, Sarkar S (2012) Synthesis of carbogenic nanosphere from peanut skin. Diam Relat Mater 24:11–14 Prasannan A, Imae T (2013) One-Pot Synthesis of Fluorescent Carbon Dots from Orange Waste Peels. ACS Publications 52:15673–15678 Dubey P, Tripathi KM, Mishra R, Bhati A, Singh A, Sonkar SK (2015) A simple one-step hydrothermal route towards water solubilization of carbon quantum dots from soya-nuggets for imaging applications. RSC Adv 5:87528–87534 Malavika JP, Shobana C, Ragupathi M, Kumar P, Lee YS, Govarthanan M, Selvan RK (2021) A sustainable green synthesis of functionalized biocompatible carbon quantum dots from Aloe barbandensis Miller and its multifunctional applications. Environ Res 200:1–10 Bas¸lak C, Oztürk G, Demirel S, Kocyigit A, Dogu S, Yildirim M (2023) Green synthesis of carbon quantum dots from Sideritis vuralii and its application in supercapasitors. Inorg Chem Commun 153:1–7 Najjar M, Nasseri MA, Allahresani A, Darroudi M (2022) Green and efficient synthesis of carbon quantum dots from cordia myxa L. and their application in photocatalytic degradation of organic dyes. J Mol Struct 1266:1–10 Valencia OGR, Carrasco MR, Fuentes JH, German CMRS, Flores ME, Chavez EV (2021) Synthesis of blue emissive carbon quantum dots from Hibiscus Sabdariffa flower: Surface functionalization analysis by FT-IR spectroscopy. Materialia 19:1–8 Li H, Cheng B, Xu J, Yu J, Cao S (2024) Crystalline carbon nitrides for photocatalysis. Royal Soc Chem 2:411–447 Barati A, Shamsipur M, Arkan E, Hosseinzadeh L, Abdollahi H (2015) Synthesis of biocompatible and highly photoluminescent nitrogen doped carbon dots from lime: Analytical applications and optimization using response surface methodology. Mater Sci Eng C 47:325–332 Widiyandari H, Prilita O, Ja’farawy MSA, Nurosyid F, Arutanti O, Astuti Y, Mufti N (2023) Nitrogen-doped carbon quantum dots supported zinc oxide (ZnO/N-CQD) nanoflower photocatalyst for methylene blue photodegradation. Results Eng 17:1–7 Hidayat RN, Widiyandari H, Parasdila H, Prilita O, Astuti Y, Mufti N, Ogi T (2024) Green synthesis of ZnO photocatalyst composited carbon quantum dots (CQD) from lime (Citrus aurantifolia). Catal Commun 187:1–8 Sonu DV, Sudhaik A, Khan AAP, Ahamad T, Raizada P, Thakur S, Asiri AM, Singh P (2023) GCN/CuFe 2 O 4 /SiO 2 photocatalyst for photo-Fenton assisted degradation of organic dyes. Mater Res Bull 164:1–13 Song S, Wu K, Wu H, Guo J, Zhang L (2019) Multi-shelled ZnO decorated with nitrogen and phosphorus co-doped carbon quantum dots: synthesis and enhanced photodegradation activity of methylene blue in aqueous solutions. RSC Adv 9:7362–7374 Kumar R, Sudhaik A, Sonu, Nguyen VH, Le QV, Ahamad T, Thakur S, Kumar N, Hussain CM, Singh P, Raizada P (2023) Graphene oxide modified K, P co-doped g-C 3 N 4 and CoFe 2 O 4 composite for photocatalytic degradation of antibiotics. J Taiwan Inst Chem Eng 150:1–13 Additional Declarations No competing interests reported. Cite Share Download PDF Status: Published Journal Publication published 21 Mar, 2025 Read the published version in Nanotechnology for Environmental Engineering → Version 1 posted Editorial decision: Accepted 04 Mar, 2025 Reviewers agreed at journal 08 Dec, 2024 Reviewers agreed at journal 07 Dec, 2024 Reviewers invited by journal 08 Nov, 2024 Editor assigned by journal 01 Nov, 2024 Submission checks completed at journal 01 Nov, 2024 First submitted to journal 01 Nov, 2024 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-5372134","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":376370939,"identity":"56bb26ae-2731-4c8c-8a71-b8123c7507f4","order_by":0,"name":"Hendri Widiyandari","email":"data:image/png;base64,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","orcid":"","institution":"Sebelas Maret University","correspondingAuthor":true,"prefix":"","firstName":"Hendri","middleName":"","lastName":"Widiyandari","suffix":""},{"id":376370940,"identity":"4842f9f1-7520-4fc9-bd35-9ff67658977f","order_by":1,"name":"Putri Lestari","email":"","orcid":"","institution":"Sebelas Maret University","correspondingAuthor":false,"prefix":"","firstName":"Putri","middleName":"","lastName":"Lestari","suffix":""},{"id":376370944,"identity":"c63f3b7f-add5-470c-b07d-0afb93618df2","order_by":2,"name":"Azza Arba Nurul Ummah","email":"","orcid":"","institution":"Sebelas Maret University","correspondingAuthor":false,"prefix":"","firstName":"Azza","middleName":"Arba Nurul","lastName":"Ummah","suffix":""},{"id":376370945,"identity":"7702bff8-548f-4283-9e32-60cacd810a1b","order_by":3,"name":"Alief Almasyah Akbar Mastura","email":"","orcid":"","institution":"Sebelas Maret University","correspondingAuthor":false,"prefix":"","firstName":"Alief","middleName":"Almasyah Akbar","lastName":"Mastura","suffix":""},{"id":376370947,"identity":"b646ba1a-0bdc-44aa-ba8a-e66f375a298b","order_by":4,"name":"Hanaiyah Parasdila","email":"","orcid":"","institution":"Sebelas Maret University","correspondingAuthor":false,"prefix":"","firstName":"Hanaiyah","middleName":"","lastName":"Parasdila","suffix":""},{"id":376370949,"identity":"a069954b-4680-426c-92e1-b7c7e4f95f72","order_by":5,"name":"Osi Arutanti","email":"","orcid":"","institution":"National Research and Innovation Agency","correspondingAuthor":false,"prefix":"","firstName":"Osi","middleName":"","lastName":"Arutanti","suffix":""}],"badges":[],"createdAt":"2024-11-01 09:08:34","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5372134/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5372134/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s41204-025-00425-9","type":"published","date":"2025-03-21T15:57:54+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":68889159,"identity":"2cbdb807-4ce9-4c6d-aa33-e03d1f8ac560","added_by":"auto","created_at":"2024-11-13 07:11:12","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":1284152,"visible":true,"origin":"","legend":"\u003cp\u003eFESEM of (a) ZnO commercial (b) ZnO nanoflower (C) ZnO/CQD\u003c/p\u003e\n\u003cp\u003eEDS of (d) ZnO commercial (e) ZnO nanoflower (f) ZnO/CQD\u003c/p\u003e","description":"","filename":"Figure1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-5372134/v1/be8fa7fa2ebb723f8d889ad3.jpg"},{"id":68888274,"identity":"1fb71d90-1747-4572-a530-73087f30b422","added_by":"auto","created_at":"2024-11-13 07:03:12","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":1886813,"visible":true,"origin":"","legend":"\u003cp\u003e(a-c) HR-TEM image and elemental mapping of ZnO/CQD, respectively.\u003c/p\u003e","description":"","filename":"Figure2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-5372134/v1/a95fdb1fc3b1905e7e3edd0c.jpg"},{"id":68888277,"identity":"96a73960-a72e-4102-afe1-44cdd45db3aa","added_by":"auto","created_at":"2024-11-13 07:03:12","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":739256,"visible":true,"origin":"","legend":"\u003cp\u003e(a) \u0026nbsp;XRD patterns, (b) FTIR spectra, (c) N\u003csub\u003e2\u003c/sub\u003e Adsorption Desorption Isotherm, and (d) BJH Pore Size Distribution of ZnO/CQDs\u003c/p\u003e","description":"","filename":"Figure3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-5372134/v1/56f57117a94c61bc4db55cee.jpg"},{"id":68888281,"identity":"64c89e19-7586-4556-ad4f-ac7227157709","added_by":"auto","created_at":"2024-11-13 07:03:12","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":799845,"visible":true,"origin":"","legend":"\u003cp\u003eCQDs solution without (a) and with (b) UV light irradiation\u003c/p\u003e","description":"","filename":"Figure4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-5372134/v1/96dd7fe66584cc6de28a0cc3.jpg"},{"id":68889284,"identity":"f477beac-51f7-4e65-9e2a-ba0f79489cad","added_by":"auto","created_at":"2024-11-13 07:19:13","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":641432,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Absorbance of CQDs, (b) band gap energy, (c) UV-Vis diffuse reflectance absorbance, and (d) PL Spectra\u003c/p\u003e","description":"","filename":"Figure5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-5372134/v1/86a246cf75ea2207bb51a113.jpg"},{"id":68889160,"identity":"03efbf8e-74ae-4972-9ef5-54cb845ec89e","added_by":"auto","created_at":"2024-11-13 07:11:12","extension":"jpg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":559990,"visible":true,"origin":"","legend":"\u003cp\u003e(a). Degradation of MG (b) Kinetics rate of MG reaction\u003c/p\u003e","description":"","filename":"Figure6.jpg","url":"https://assets-eu.researchsquare.com/files/rs-5372134/v1/a19a8b45a514c4dcf8766d44.jpg"},{"id":68888276,"identity":"76bd65d2-6565-426c-bcde-77a72c2d512b","added_by":"auto","created_at":"2024-11-13 07:03:12","extension":"jpg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":599239,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic Degradation of ZnO/CQD\u003c/p\u003e","description":"","filename":"Figure7.jpg","url":"https://assets-eu.researchsquare.com/files/rs-5372134/v1/5c8ae18b9cfe279e50be86be.jpg"},{"id":68888280,"identity":"ac90aff5-6425-4acc-9cf8-02e35e36da5c","added_by":"auto","created_at":"2024-11-13 07:03:12","extension":"jpg","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":668613,"visible":true,"origin":"","legend":"\u003cp\u003e(a). Removal efficiency (b). Absorbance (c). C/C\u003csub\u003e0\u003c/sub\u003e of reusability study (d). Scavenger experiment\u003c/p\u003e","description":"","filename":"Figure8.jpg","url":"https://assets-eu.researchsquare.com/files/rs-5372134/v1/95abbdacef3a3d30aa45754c.jpg"},{"id":79120672,"identity":"1357695d-ad30-4c71-ae76-99f778086705","added_by":"auto","created_at":"2025-03-24 16:10:53","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":7960354,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5372134/v1/303d56d4-07ea-4133-9700-1d4d39957567.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Green Synthesis of Carbon Quantum Dots from Stale Soy Milk Composited Zinc Oxide (ZnO/CQD) for Photodegradation of Malachite Green","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eThe textile sector has grown significantly to fulfill the population's desires. However, the textile industry's waste must be handled appropriately. According to global data, the world produces 92\u0026nbsp;million tons of textile waste annually. Malachite green (MG), a green triphenylmethane dye waste, is one of the textile industry's wastes. MG is a dye used in silk, wool, flax, leather, cotton, and other textiles [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. According to the Occupational Safety and Health Administration (OSHA, USA), MG is a class 2 dangerous chemical to human health since it could cause cancer, mutagenesis, teratogenicity, and pulmonary toxicity [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. A concrete method is needed to eliminate MG from aqueous solutions. Many methods have been used to treat these wastes, including precipitation, coagulation, flocculation, adsorption, activated carbon, and ozonolysis. However, these methods transport contaminants from phase to phase since MG wastes stable compounds that are exceedingly difficult to degrade [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. Furthermore, the operational costs of these approaches are substantial, requiring alternative methods for treating MG.\u003c/p\u003e \u003cp\u003eAdvanced oxidation processes (AOPs) are chemical methods that oxidize organic pollutants into non-hazardous molecules by generating reactive free radicals. AOPs have recently acquired favor as an efficient method for pollutant degradation because, as reported, reactive free radicals generated during AOP processing efficiently degrade or eliminate pollutants [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. Photocatalysts are included in the AOPs process suitable for degrading MG [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. Photocatalytic AOPs mostly use semiconductor metal oxides as photocatalysts, such as TiO2, ZnO, CuO, and NiO [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e], because of their high photostability, non-toxicity, facile synthesis process, and environmental sustainability [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. Researchers widely use ZnO for the photodegradation of organic pollutants because they have high photosensitivity, low cost, oxidation resistance, and thermal stability [5;14]. ZnO has favorable characteristics for water processing, so ZnO is a metal oxide semiconductor suitable for use as a photocatalyst. ZnO nanoparticles, ZnO nanowires, nanorods, and ZnO nanoflowers are some of the most commonly used ZnO structures. This study employs ZnO nanoflower due to its large surface area compared to other ZnO structures. However, ZnO only performs under the excitation range of UV light because of its large band gap that only absorbs 3% \u0026minus;\u0026thinsp;5% of sunlight [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. Furthermore, the high rate of electron and hole recombination reduces photocatalytic activity performance, requiring a material that would enhance ZnO photocatalytic activity [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eCarbon Quantum Dots (CQD) are one of the best carbon types for improving ZnO's photocatalytic activity [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e], mainly because they could reduce the recombination rate of electrons and holes as electron acceptors [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. Furthermore, CQD has excellent photoluminescence capabilities, biocompatibility, fluorescence conversion ability, electron transport, and photosensitivity [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. Combining ZnO and CQD produces more photogenerated electron-hole pairs on ZnO to convert long wavelengths of light into short wavelengths. In addition, CQD has conjugated π bonds to increase the adsorption ability of organic pollutants on the ZnO surface. Based on research conducted by [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e] mentioned that the carbon precursor commonly used in the synthesis of CQD to increase quantum yield is derived from biomass waste sources. This renewable step is an effective way to treat waste sustainably.\u003c/p\u003e \u003cp\u003eFurthermore, biomass-derived CQD obtained from natural resources provide beneficial economic properties and functional groups with excellent fluorescence and sensing capabilities to tackle surface passivation, which could enhance target molecule sensitivity [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. Biomass waste is derived from agricultural and food waste [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. For example, Wang and his colleague Zhou have conducted a green synthesis of CQD derived from soy milk; Saxena et al. 2012 synthesized peanut shells into carbon nanosphere sources; Prasannan and Imae, 2013 synthesized CQD from orange peel waste; Dubey et al 2015 have synthesized CQD from soy nuggets as a carbon source, and Hojaghan et al. 2021 have synthesized CQD from soybean pieces as a carbon source [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e, \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. The source of CQD used in this research is stale soy milk because it contains lactic acid as a carbon source. In addition, the used of stale soy milk as a research novelty, increasing economic value and reducing the amount of stale soy milk waste in the environment.\u003c/p\u003e \u003cp\u003eConventional methods typically produce CQD. However, this process involves the usage of numerous hazardous and toxic compounds that are harmful to the environment. Therefore, green synthesis is a method that attracts attention today to produce CQD [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]. Green synthesis utilizes non-toxic natural materials that are environmentally friendly and renewable [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. In addition, green synthesis has high stability and produces high quantum yield. Sariga et al. 2023 mentioned using green synthesis methods to obtain high biocompatibility and non-toxic, economical, and environmentally friendly CQD [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. As a result, this research aims to green synthesize CQD from stale soy milk composites with ZnO to develop ZnO/CQD photocatalytic material. ZnO/CQD is produced and investigated for photocatalytic activity on malachite green.\u003c/p\u003e"},{"header":"2. Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Materials\u003c/h2\u003e \u003cp\u003eThe materials used were stale soy milk (obtained from soy milk producers in Surakarta, Central Java, Indonesia), C\u003csub\u003e2\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003eO (\u0026ge;\u0026thinsp;99.8%, Sigma Aldrich), ZnO commercial, NaOH (97%, Sigma Aldrich), Zn(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e.6H\u003csub\u003e2\u003c/sub\u003eO (\u0026le;\u0026thinsp;100%, Smart Lab), C\u003csub\u003e6\u003c/sub\u003eH\u003csub\u003e5\u003c/sub\u003eNa\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e.2H\u003csub\u003e2\u003c/sub\u003eO (99%, Merck), malachite green (99%, Loba Chemie).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Fabrication of ZnO nanoflower\u003c/h2\u003e \u003cp\u003eThe synthesis of ZnO nanoflower was conducted using the hydrothermal method. A Certain amount of Zn(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e.6H\u003csub\u003e2\u003c/sub\u003eO (7.44 g) and C\u003csub\u003e6\u003c/sub\u003eH\u003csub\u003e5\u003c/sub\u003eNa\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e.2H\u003csub\u003e2\u003c/sub\u003eO (2.94 g) were dissolved in 20 mL of distilled water. Then, the solution was stirred, and NaOH 1 M was gradually added until pH\u0026thinsp;=\u0026thinsp;13. Stirring for 100 minutes at room temperature, the solution was put into a 100 mL autoclave and heated at 130\u0026deg;C for 12 hours. The solution was filtered and washed using distilled water and C\u003csub\u003e2\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003eO until a white precipitate was obtained and then dried at 100\u003csup\u003e\u0026deg;\u003c/sup\u003eC for 4 hours and then calcined at 500\u003csup\u003e\u0026deg;\u003c/sup\u003eC for 3 hours with a heating rate of 5\u003csup\u003e\u0026deg;\u003c/sup\u003eC min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Fabrication of CQD\u003c/h2\u003e \u003cp\u003eGreen synthesis of CQD solution from stale soy milk by hydrothermal method. Stale soy milk (56 mL) was added to 96% ethanol (24 mL), then placed in a 100 mL autoclave and heated for 12 hours at a temperature variation of 120\u0026deg;C, 150\u0026deg;C, and 180\u0026deg;C. The solution was filtered, extracted, and placed in a centrifugator at 2000 rpm for 40 minutes.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.4 Fabrication of ZnO/CQD\u003c/h2\u003e \u003cp\u003eThe synthesis of ZnO/CQD started by dissolving 0.2 grams of ZnO nanoflower into 45 mL of distilled water and 1.5 mL of CQD solution and stirring for 2 hours. Next, the solution was heated at 100\u0026deg;C for 8 hours. The solution was filtered and washed using distilled water and C\u003csub\u003e2\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003eO until a white-brown precipitate was obtained, and finally, it was dried at 100\u0026deg;C for 4 hours.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e2.5 Material characterization\u003c/h2\u003e \u003cp\u003eX-ray diffraction (XRD, D8 Advance Bruker, Germany) to identify crystallographic properties with an x-ray source of Cu radiation (λ\u0026thinsp;=\u0026thinsp;1.54184 \u0026Aring;; voltage\u0026thinsp;=\u0026thinsp;40 KV; current\u0026thinsp;=\u0026thinsp;35 mA) where the 2θ angle is 20\u0026deg; \u0026minus;\u0026thinsp;80\u0026deg;. Fourier transform infrared (FTIR) to examine the functional groups of the final product. A UV-Vis spectrophotometer (Thermo Scientific Genesys 150, USA) was used to identify the absorbance peaks in the CQD solution and the degradation results before and after irradiation. UV-Vis diffuse reflectance (Analytic Jena Specord 200 Plus, Germany) to calculate band gap energy, while to calculate surface area, N2 adsorption-desorption, and porosity parameters using Brunauer-Emmett-Teller (BET, Micromeritics Tristar II Plus 3020). Photoluminescence (PerkinElmer LS 55) to evaluate electron-hole recombination and Field emission scanning electron microscope/Energy dispersive x-ray (FE-SEM/EDX, JEOL JIB 4610F) to identify morphology.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e2.6 Photocatalytic experimental procedure\u003c/h2\u003e \u003cp\u003eThe photocatalytic activity of ZnO/CQD was investigated using a solar simulator (Peccel/PEC-L01) as a visible light source with a wavelength range of 400 nm \u0026minus;\u0026thinsp;800 nm. A 250 mL beaker was used for this reaction. The beaker was placed on a stirrer at 11 cm from the visible light source while maintaining the temperature at 25\u0026deg;C. Photocatalyst material of as much as 0.1 gram was dissolved in a ten ppm MG dye solution of 50 mL and then stirred in the dark (without irradiation) for 30 minutes to establish adsorption-desorption balance. For MG dye, they stirred again under visible light exposure for 150 minutes with an interval of 30 minutes. After the photodegradation process, the samples were centrifuged at 5000 rpm for 20 minutes to separate the photocatalyst from the solution. The pure supernatant liquids of MG and ciprofloxacin were evaluated using a UV-Vis spectrophotometer, where the degradation efficiency of both pollutants can be calculated using the following equation:\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\n$$\\:Degradasi\\:\\left(\\%\\right)=\\:\\frac{{C}_{0}-\\:{C}_{t}}{{C}_{0}}\\:x\\:100=\\frac{{A}_{0}-\\:{A}_{t}}{{A}_{0}}\\:\\:x\\:100$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eC\u003csub\u003e0\u003c/sub\u003e, A\u003csub\u003e0\u003c/sub\u003e, C\u003csub\u003et\u003c/sub\u003e, and A\u003csub\u003et\u003c/sub\u003e are MG's initial concentration and absorption, respectively; concentration and absorption t minutes after irradiation.\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e2.7 Scavenger test\u003c/h2\u003e \u003cp\u003eReactive free radicals produced throughout the photocatalytic process are essential in photodegradation, and scavengers examine them. This experiment aims to trap reactive free radicals using ZnO/CQD to determine the role of reactive free radical species in the degradation of MG pollutants. 0.001 grams of ammonium oxalate (AO) to detect h\u003csup\u003e+\u003c/sup\u003e, 0.001 grams of benzoquinone (BQ) to detect \u0026sdot;O\u003csub\u003e2\u0026minus;\u003c/sub\u003e, and 10 mL of isopropanol (IPA) were used to trap reactive \u0026sdot;OH.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Result and discussion","content":"\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e3.1. Physicochemical properties of materials\u003c/h2\u003e \u003cp\u003eThe FESEM characterization results of samples (a) ZnO commercial, (b) ZnO nanoflower, and (c) ZnO/CQD can be seen in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. ZnO commercial shows an arbitrary structure; ZnO nanoflower shows a slightly budding nanoflower structure consisting of a pistil in the center and surrounded by petals. ZnO/CQD shows a slightly budding nanoflower structure with a thorn shape in the center and surrounded by petals. The morphology of ZnO Nanoflower and ZnO/CQD has a smooth surface and uniform size. The addition of CQD is known to improve the morphology of ZnO/CQD. This is possible because CQD serves as a scaffold for Zn ions to connect and crystallize, hence determining the direction and pattern of ZnO crystal development, resulting in a more organized morphology. Due to their small size, CQD has numerous nucleation sites, including -OH, -COOH, and -NH\u003csub\u003e2\u003c/sub\u003e. When CQD is added to the ZnO precursor, it can operate as a nucleation center, directing ZnO development to an instructed morphology, such as nanoflowers.\u003c/p\u003e \u003cp\u003eTEM analysis examined the microstructure and crystallographic features of ZnO/CQD. In addition to SEM, the composite of CQD on ZnO can be confirmed from the TEM image of ZnO/CQD, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e (a). Based on Figure (a), it is known that the CQD attached to the ZnO surface form an intense contact, thus illustrating the successful formation of the ZnO/CQD composite. Heterogeneous ZnO primary particles form the ZnO/CQD structure with a size of 50 nm. In addition, it is provided with CQD on the ZnO surface with a size of 10 nm and a lattice edge of 0.2 nm, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e (b). The presence of CQD on the ZnO surface has been confirmed. The sharp spots in the selected area electron diffraction (SAED) pattern in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e (c) indicate the polycrystalline nature of ZnO/CQD.\u003c/p\u003e \u003cp\u003eThe EDS spectrum image of the ZnO commercial is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e (d) with elemental mapping of Zn, O, and C at 40.9, 37.6 and 21.5 At%, respectively. The EDS spectrum of the ZnO nanoflower is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e (e) with elemental mapping of Zn, O, and C of 36.8, 33.9, and 29.3 At%, respectively. In addition, the Figure shows that the mapping of each element is evenly and uniformly distributed on the ZnO nanoflower. In the spectrum of ZnO/CQD, the mapping of Zn (46%), O (44.2%), and C (9.9%) shows that the distribution of CQD is uniform and evenly distributed across the surface of ZnO nanoflower, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e depicts the homogenous distribution of components in ZnO composited using CQD. CQD is equally distributed in the ZnO matrix, which may lead to the morphological evolution to nanoflower.\u003c/p\u003e \u003cp\u003eAccording to Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e (a), no other compound diffraction peaks were found in this study's XRD pattern produced by the photocatalyst material. The diffraction peaks produced consist of (100), (002), (101), (102), (110), (103), (200), (112), (201), (004), and (202). The diffraction peak results follow the JCPDS 36 1451 data. Based on the database, it can be seen that the crystal structure produced is hexagonal wurtzite. The calculated crystal sizes of ZnO commercial, ZnO nanoflowers, and ZnO/CQD are 20.64 nm, 20.05 nm, and 23.15 nm, respectively. The addition of CQD is known to increase the crystal size of the photocatalyst material. The increase in crystal size is known to increase charge mobility and reduce electron-hole recombination. In addition, the increase in crystal size could also expand the light absorption area to improve the performance of photocatalytic activity [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe FTIR spectra of ZnO commercial, ZnO nanoflower, and ZnO/CQD are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e (b) and the corresponding functional groups in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The peaks below 800 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e show the characteristics of Zn and O bonds; this can be seen in the appearance of a strong absorption band at the peak of 433\u0026thinsp;\u0026minus;\u0026thinsp;432 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, which is indicated by Zn-O stretching vibration; it shows that the frequency generated from the Zn-O bond is quite strong [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. Likewise, at 542\u0026ndash;547, cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e which indicates the presence of O-Zn-O stretching vibration [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. Epoxy groups, highly reactive functional groups consisting of two carbons and one oxygen, have been identified at 904\u0026ndash;910 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. This occurs because of the C-O stretching vibration in the epoxy group. The epoxy group is carbon quantum dots (CQD), which act as a ZnO composite [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. In addition, the peak at 1369\u0026ndash;1371 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e indicates O-ZnO stretching vibrations resulting from a shift in C\u0026thinsp;=\u0026thinsp;O stretching at 1353 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eMeanwhile, 1556 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e shows C\u0026thinsp;=\u0026thinsp;O stretching vibration, which indicates that the resulting molecule contains carbonyl groups [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. In addition, at 1654 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, carbonylic groups (O-C-O) asymmetric vibrations are present. This carbonylic group comprises a carbon atom bonded to two oxygen atoms [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. The 1981\u0026ndash;1987 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e peak shows the presence of C-H bending, included in the aromatic compound with weak bonds. Aromatic compounds have remarkable stability due to resonance in the ring structure. In addition, aromatic compounds are organic, which means they contain carbon atoms bound to other elements, namely oxygen. In addition, another peak appears at 2970 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, which indicates the presence of C-H stretching [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. The resulting functional groups are almost the same as ZnO/CQD made from other natural materials, so in general, the existence of these functional groups shows promising results for photocatalyst materials\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eFunctional groups of FTIR spectra\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colspan=\"3\" nameend=\"c3\" namest=\"c1\"\u003e \u003cp\u003eBand Positions (Cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eChemical Functional Group\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eSource\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eZnO Commercial\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eZnO nanoflower\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eZnO/CQD\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e434\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e433\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eZn-O stretching vibration\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e8\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e543\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e542\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e547\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eO-ZnO-O streching\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e904\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e909\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e910\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eEpoxy groups\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e1369\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1371\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1371\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eO-ZnO stretching vibration\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e8\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e1556\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eC\u0026thinsp;=\u0026thinsp;O stretching vibration\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e33\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e1654\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eAsymmetric vibration of O-C-O\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e1981\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1987\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1985\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eC-H bending\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e2970\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eC-H stretching vibration\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e20\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eAdsorption desorption isotherm measurements of N\u003csub\u003e2\u003c/sub\u003e at 77 K were used to determine the adsorption ability, pore size distribution, and specific surface area of the ZnO/CQD composite. The adsorption-desorption isotherm of N2 showed a type IV isotherm with a type H4 hysteresis in the range of ca. 0.4-1.0 P/P0, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e (c). The BET surface area was 6.418 m\u003csup\u003e2\u003c/sup\u003e/g. This shows an increase from previous research [\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e], which only amounted to 5.1672 m\u003csup\u003e2\u003c/sup\u003e/g for ZnO/CQD composites. Surface area dramatically affects the occurrence of direct contact with contaminants on the photocatalyst surface. Therefore, increasing the surface area of the photocatalyst can lead to more effective direct contact, which in turn can increase the photocatalytic activity [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]. In addition, increasing the surface area of the photocatalyst can also increase the adsorption that occurs, so the ZnO/CQD composite has good adsorption ability [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e]. Based on the Barret-Joyner-Halenda (BJH) pore size distribution curve, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e (d), the pore size distribution of the ZnO/CQD composite is 363 \u0026Aring; or 36.3 nm, which indicates the presence of a mesoporous structure [\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]. The pore size distribution plays an essential role in the degradation of malachite green by ZnO/CQD. This is because the pore structure affects mass transfer, which can improve the sensing performance of more gas-sensitive materials. In addition, the pore size also affects the diffusion of reaction components on the photocatalyst and the adsorption of dissolved oxygen. In this case, the mesoporous structure is more favorable for the diffusion of reaction components than the microporous structure. Therefore, this research's mesoporous structure of ZnO/CQD can improve the photocatalytic activity.\u003c/p\u003e \u003cp\u003eCQD that were produced via the green synthesis by hydrothermal method for 12 hours with variations in heating temperature, 120\u0026deg;C, 150\u0026deg;C, and 180\u0026deg;C were analyzed by irradiation under sunlight and UV light (λ\u0026thinsp;=\u0026thinsp;365 nm). The analysis was used to investigate the formation of a CQD solution. CQD solution is yellowish-golden to brownish-yellow in sunlight, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e (a). The CQD solution is luminescent blue in appearance when irradiated under UV light, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e (b). The analysis results under UV light show a change in the color of the CQD solution from a golden brownish-yellow to a luminous blue.\u003c/p\u003e \u003cp\u003eThe color difference indicates that the resulting solution is a solution of carbon quantum dots (CQD). When irradiated with UV light, the smaller particle size of CQD can lead to electrons on the surface being excited to a higher energy level. Thus, when the electrons are trapped back in their ground state, the photons emitted are luminous blue [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]. In this study, variations in heating temperature were performed to obtain the optimal CQD solution. A higher heating temperature could lead to an increase in the mechanical destruction of the material's structure. Therefore, a higher heating temperature could produce the lowest particle size [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]. In addition, the increase in photoluminescence properties is followed by the lower particle size. Therefore, the CQD solution with a heating temperature of 180\u0026deg;C can produce the brightest blue light compared with the other variations.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eUV-Vis further characterized the CQD solution to identify the absorbance peak. Figure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(a) shows that the 180\u0026deg;C CQD solution has the highest absorbance peak compared with the other variations. This follows the analysis under UV light irradiation, indicating that the 180\u0026deg;C CQD solution emits the best bright blue light. The results of UV-Vis characterization of the 180\u0026deg;C CQD solution revealed two absorption peaks in the UV absorption region, which are at a wavelength of 257 nm and 300 nm \u0026minus;\u0026thinsp;400 nm, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(a). The absorption peak at a wavelength of 257 nm is because of the π-π* electronic transition of the conjugated C\u0026thinsp;=\u0026thinsp;C bond. The most substantial absorption peak at a wavelength of 300 nm \u0026minus;\u0026thinsp;400 nm is due to the n-π* electronic transition of the C\u0026thinsp;=\u0026thinsp;O bond.\u003c/p\u003e \u003cp\u003eUV-Vis DRS characterization is used to calculate the band gap energy. The results of UV-Vis DRS characterization show that the photocatalyst material has an absorption peak in the UV light region at a wavelength of 320 nm \u0026minus;\u0026thinsp;390 nm and a strong absorption peak in the visible light region at a wavelength of 400 nm \u0026minus;\u0026thinsp;650 nm as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(b). The shift of the absorption peak indicates that adding CQD on ZnO could expand the absorption area from UV to visible light. The shift of n-π* electrons on the C\u0026thinsp;=\u0026thinsp;O bond from the CQD core followed by the π-π* displacement on the C\u0026thinsp;=\u0026thinsp;C bond from the CQD surface results in a shift in the wavelength of ZnO, where the event is called the quantum confinement effect.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe quantum confinement effect is a phenomenon that occurs when the physical size of a material decreases, causing changes in the optical properties of the material. As a result of this event, the band gap energy will decrease, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(c). Consecutively from ZnO commercial, ZnO nanoflower, and ZnO/CQD are 3.2 eV, 3.16 eV, and 3 eV [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e]. The addition of CQD that can expand the light absorption area results in the low energy required for electron excitation from the valence band to the conduction band, so the addition of CQD to ZnO can increase the efficiency of electron-hole pair charge separation [\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e].\u003c/p\u003e \u003cp\u003ePhotoluminescence characterization is used to determine the separation of charge carriers, recombination of electrons and holes, and electron transfer, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(d). ZnO commercial and ZnO nanoflower have high-intensity peaks that indicate a high level of recombination of electrons and holes and low charge isolation of photogenerated charge carriers (electrons and holes) [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]. The high recombination rate of electrons and holes can cause electrons to move back to the ground state quickly, which can reduce the performance of photocatalytic activity. ZnO/CQD have the lowest intensity peak at a wavelength of 395 nm, which indicates violet emission. The low-intensity peak of photoluminescence indicates the high charge isolation of electron and hole photogeneration so that the level of electron and hole recombination decreases. The high charge isolation is very beneficial in creating the best photodegradation and reduction efficiency [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. Furthermore, adding CQD can increase crystal defects that prevent electrons from returning to the ground state to improve the performance of the photocatalytic activity. Therefore, this research proves that adding CQD can increase photocatalytic activity, so ZnO/CQD are the most optimal sample for degrading dyes and pharmaceutical waste [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e].\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003e3.2. Photocatalytic Degradation Performance Evaluation\u003c/h2\u003e \u003cp\u003eThe photocatalyst's degradation efficiency for MG degradation has been analyzed. 100 mg of photocatalyst was dissolved in 50 mL of MG solution under visible light exposure. The degradation efficiency of MG is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(a). In the first stage, the reaction solution was kept in the dark for 30 minutes to determine the photocatalyst's adsorption ability. Then, the solution was reacted under visible light in the next minute.\u003c/p\u003e \u003cp\u003eThe composite of CQD on ZnO is supposed to enhance the adsorption capabilities of ZnO/CQD. After 30 minutes of irradiation, ZnO commercial and ZnO nanoflower showed 6% and 8% adsorption. However, ZnO/CQD showed an adsorption of 73%. The adsorption parameter is essential in photocatalysis reactions because it can affect the degradation efficiency. In other words, when compared with ZnO commercial and ZnO nanoflower, ZnO/CQD showed the highest efficiency of 84% for 90 minutes, while ZnO commercial was only 23% and ZnO nanoflower was 32%. ZnO/CQD have the highest efficiency due to high charge separation, good light absorption, increased adsorption, and high charge transfer [\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eKinetics rate of MG reaction\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"3\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSample\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eReaction rate constant (k/min\u003csup\u003e-1\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eLinear dependence (R\u003csup\u003e2\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNegative control\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0,00474\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0,74932\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eZnO commercial\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0,00178\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0,93301\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eZnO nanoflower\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0,00477\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0,92558\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eZnO/CQD\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0,01137\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0,48736\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eThe kinetics rate of the malachite green degradation reaction were determined using the first-order kinetics model technique of the Langmuir-Hinshel equation, as illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(b). Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e2\u003c/span\u003e shows the reaction rate constants and linear regressions for each sample. The reaction rate constant (k) describes the rate of a chemical process. In this situation, k refers to the rate at which a substance degrades under specific conditions. The greater the k value, the faster the degradation will proceed, making the reaction conditions more efficient in speeding up the degradation process. ZnO/CQD exhibit the highest response rate constant value, 0.01137 k/min-\u0026sup1;. It demonstrates that ZnO/CQD are highly effective at degrading MG.\u003c/p\u003e \u003cp\u003eHowever, the R\u003csup\u003e2\u003c/sup\u003e value of ZnO/CQD is only 0.48736, indicating a poor match with the first-order kinetics model. This suggests that the degradation of ZnO/CQD may not completely obey first-order kinetics. This could be due to other reasons, such as variations in the surrounding temperature or the presence of different compounds throughout the reaction. We suppose that adding composite CQD on the surface of ZnO leads to a high k value, which is not followed by the first-order kinetics fit to the model. ZnO commercial and ZnO nanoflower had lower response rate constant values compared to ZnO/CQD, with 0.00178 k/min-\u0026sup1; and 0.00477 k/min-\u0026sup1;, respectively, despite higher linear regression values of 0.93301 and 0.92558. As a result, it is clear that the reaction rate constant (k) is the most crucial parameter for determining degradation efficiency; the higher the value of k, the faster the degradation process, indicating that the reaction is highly effective. However, the lower R\u003csup\u003e2\u003c/sup\u003e value indicates that this degrading reaction does not entirely adhere to the first-order kinetics paradigm. This shows that the first-order model might not adequately represent the complex degradation mechanism despite the rapid degradation rate.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003e3.3. Mechanism of Photocatalytic Degradation\u003c/h2\u003e \u003cp\u003eThe mechanism of photocatalytic degradation is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e. ZnO/CQD were photodegraded under visible light to examine MG degradation performance. The photocatalyst material is activated when it is directly exposed to visible light with an energy of hv, which must be more significant than or equal to the band gap energy. In addition, electrons are generated on the ZnO nanoflower particles by being stimulated from the valence band to the conduction band, thus abandoning the hole in the valence band. In such cases, electrons are extremely easy to retrieve in their ground state, known as the valence band. The high recombination rate of electrons and holes may inhibit photocatalytic activity.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAs a result, the importance of composite CQD on ZnO nanoflower particle surfaces is highlighted. Because of the energy difference between the conduction band of ZnO nanoflower and the energy level of CQD, electrons stimulated on its surface can be rapidly transported to the surface of CQD. CQD play as electron acceptors from the conduction band on the surface of ZnO nanoflower particles. Furthermore, CQD act as electron donors, donating electrons to fill holes and extending the lifespan of separated electrons and holes. This can promote an increase in photocatalytic activity. CQD may reduce electron-hole recombination by providing separate routes for electrons and holes, boosting charge separation efficiency. Charge separation in photodegradation is one of the factors that influence photocatalytic activity. The following step is the water ionization process, which involves oxidizing water molecules with holes to form hydroxyl radicals (\u0026sdot;OH). This method engages MG molecules directly with the ZnO nanoflower's outermost surface layer. The oxygen ionosorption process happens on the surface of the CQD, where oxygen molecules are reduced to create superoxide radical anions (\u0026sdot;O\u003csub\u003e2\u003c/sub\u003e-). A \u003cem\u003esubsequent stage\u003c/em\u003e is the protonation process, which develops after the superoxide radical anion produces hydroperoxyl radicals. These hydroperoxyl radicals can produce \u0026sdot;O\u003csub\u003e2\u003c/sub\u003e- and \u0026sdot;OH. The final stage involves the breakdown of contaminants by \u0026sdot;O\u003csub\u003e2\u003c/sub\u003e- and \u0026sdot;OH, which are converted into innocuous molecules such as carbon dioxide and water [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e].\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003e3.4. Reusability Study and Scavenger Test\u003c/h2\u003e \u003cp\u003eThe reuse study is significant for evaluating the stability and photocatalytic performance of ZnO/CQD and determining the effectiveness of ZnO/CQD. The reuse study was analyzed for up to 4 cycles, showing that the photocatalytic activity is stable and durable. In addition, Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e(a). shows that the removal efficiency is not significantly reduced, which indicates that the ZnO/CQD have stability and reliable photocatalytic performance. Figure\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e(b). shows the absorbance reduction of each cycle. The first cycle has the highest absorbance because the ZnO/CQD used are still new and active, while in the next cycle, the absorbance has decreased. After four cycles of reusability, Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e(c). shows that the degradation efficiency rate slightly reduced at the beginning of each stage of the photocatalyst reaction. This indicates that ZnO/CQD have excellent chemical stability and effective industrial application prospects in water treatment.\u003c/p\u003e \u003cp\u003eSeparate charge carriers (electron and hole) form highly reactive oxidative species, such as \u0026sdot;O\u003csub\u003e2\u003c/sub\u003e- and \u0026sdot;OH and others that have an essential role in MG degradation. A scavenger experiment was used to analyze the role of reactive oxidative species on the surface of ZnO/CQD during the MG degradation process. Different scavengers such as 10 mL isopropyl alcohol (IPA), 0.001 g ammonium oxalate (AO), and 0.001 g benzoquinone (BQ) were used to trap (\u0026sdot;OH), (h\u003csup\u003e+\u003c/sup\u003e), and (\u0026sdot;O\u003csub\u003e2\u003c/sub\u003e-) respectively. Figure\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e(d). shows the role of each scavenger agent. 84%, 39%, 33%, and 58% for none, IPA, AO, and BQ, respectively, so that the order of decrease in photocatalytic activity caused by scavenger is AO\u0026thinsp;\u0026gt;\u0026thinsp;IPA\u0026thinsp;\u0026gt;\u0026thinsp;BQ.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eBased on these results, it is appropriate to say that reactive oxidative species such as (\u0026sdot;OH), (h\u003csup\u003e+\u003c/sup\u003e), and (\u0026sdot;O\u003csub\u003e2\u003c/sub\u003e-) generated from photogeneration on the photocatalyst surface are responsible for the performance of photocatalytic activity. Therefore, it can be seen that h\u003csup\u003e+\u003c/sup\u003e and \u0026sdot;OH have an essential role in the degradation of MG, but \u0026sdot;O\u003csub\u003e2\u003c/sub\u003e- only has a minor impact on the process of MG degradation. The addition of AO in the photocatalysis reaction traps h\u003csup\u003e+\u003c/sup\u003e at valence band, thus inhibiting charge recombination and providing more e- at conduction band for MG photodegradation.\u003c/p\u003e \u003c/div\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eOur work demonstrates that CQD from stale soy milk prepared via green synthesis were performed for the first time and were further composited with ZnO photocatalyst for MG degradation, providing the advantages of highly efficient photocatalytic degradation, eco-friendliness, cost-effectiveness, and ease of recyclability. The results reveal that the CQD composite generated from stale soy milk is effective in reducing electron and hole recombination. This CQD composite has the potential to be a cost-effective and environmentally friendly choice for measuring the electron-hole recombination rate of ZnO photocatalysts. The ZnO/CQD composite is capable of producing effective electron-hole pair separation. It provides more photogenerated electrons to reduce O\u003csub\u003e2\u003c/sub\u003e and more photogenerated holes to oxidize H\u003csub\u003e2\u003c/sub\u003eO, which improves the ability to generate free radicals and thus increases photocatalytic activity. The morphology, composition, phase structure, surface area, and reduced band gap energy were determined using TEM, FESEM, EDX, XRD, BET, and UV-Vis DRS. The results revealed that the ZnO/CQD composite was able to degrade 84% MG for 90 minutes with a reaction rate constant of 0.01137 when exposed to visible light. The results were excellent, more than those of ZnO commercial and ZnO nanoflower photocatalysts. Furthermore, ZnO/CQD can be recyclable and reused for four photocatalytic cycles without significantly reducing their degrading performance.\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e \u003ch2\u003eDeclaration of interests\u003c/h2\u003e \u003cp\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eH.W: Writing \u0026ndash; original draft, Writing \u0026ndash; review \u0026amp; editing, Validation, Supervision, Project administration, Methodology, Conceptualization. P.L: Writing \u0026ndash; original draft, experimental and data curation. A.A.N.U: experimental and data curation. A.A.A.M: experimental and data curation. H.P: Writing \u0026ndash; original draft, Writing \u0026ndash; review \u0026amp; editing. O.A: TEM and FE-SEM characterization.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eThe authors (PL, AANU, and AAAM) thank to The Directorate General of Higher Education, Research, and Technology (DGHERT) of the Ministry of Education, Culture, Research, and Technology for funding this research through Program Kreativitas Mahasiswa (PKM) 2023. Also, the author (HW) thanks to Universitas Sebelas Maret through Penelitian Unggulan Terapan (PUT-UNS) contract number: 194.2/UN27.22/PT.01.03/2024\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eDjebbari C, Zouaoui E, Ammouchi N, Nakib C, Zouied D, Dob K (2021) Degradation of Malachite green using heterogeneous nanophotocatalysts (NiO/TiO\u003csub\u003e2\u003c/sub\u003e, CuO/TiO\u003csub\u003e2\u003c/sub\u003e) under solar and microwave irradiation. SN Appl Sci 255:1\u0026ndash;11\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBazazi S, Jodeyri S, Hosseini SP, Arsalani N, Rashidzadeh B, Fathalipour S, Seidi F, Hashemi E (2023) Ball mill-hydrothermal method for one-step synthesis of zinc oxide/carbon quantum dot (ZnO-CQD) nanocomposites as photocatalyst for degradation of organic pollutants. J Photochem Photobiology A: Chem 445:1\u0026ndash;11\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSekar A, Yadav R (2021) Green fabrication of zinc oxide supported carbon dots for visible light-responsive photocatalytic decolourization of Malachite Green dye: Optimization and kinetic studies. Optik 242:1\u0026ndash;20\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eMontazer M, Pebdeni AB, Sheikholeslami MN, Abkenar SD, Firoozbakhtian A, Hosseini M, Dragoi EN (2023) Synthesis of cuttlebone/carbon quantum dots/nickel oxide nanocomposite for visible light photodegradation of malachite green used for environmental remediation. Chemosphere 333:1\u0026ndash;10\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSaad AM, Abukhadra MR, Ahmed SA, Elzanaty AM, Mady AH, Betiha MA, Shim J, Rabie AM (2020) Photocatalytic degradation of malachite green dye using chitosan supported ZnO and Ce-ZnO nano-flowers under visible light. J Environ Manage 258:1\u0026ndash;11\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eTeymourinia H, Alshamsi HA, Al-nayili A, Sohouli E, Gholami M (2023) Synthesis of new photocatalyst based on g-C\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e/N, P CQD/ZIF-67 nanocomposite for ciprofloxacin degradation under visible light irradiation. J Ind Eng Chem 125:259\u0026ndash;268\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eIyyappan J, Gaddala B, Gnanasekaran R, Gopinath M, Yuvaraj D, Kumar V (2024) Critical review on wastewater treatment using photo catalytic advanced oxidation process: Role of photocatalytic materials, reactor design and kinetics. Case Stud Chem Environ Eng 9:1\u0026ndash;16\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSoni V, Sonu, Sudhaik A, Singh P, Thakur S, Ahamad T, Nguyen VH, Thi LP, Quang HHP, Chaudhary V, Raizada P (2024) Visible-light-driven photodegradation of methylene blue and doxycycline hydrochloride by waste-based S-scheme heterojunction photocatalyst Bi\u003csub\u003e5\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003eI/PCN/tea waste biochar. Chemosphere 347:1\u0026ndash;19\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003ePivert ML, Poupart R, Gnambodoe MC, Martin N, Wang YL (2019) Direct growth of ZnO nanowires on civil engineering materials: smart materials for supported photodegradation. Microsystems Nanoengineering 57:1\u0026ndash;7\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eVelumani A, Sengodan P, Arumugam P, Rajendran R, Santhanam S, Palasinamy M (2020) Carbon Quantum dots supported ZnO sphere based photocatalyst for dye degradation application. Curr Appl Phys 20:1176\u0026ndash;1184\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZouhier M, Tanji K, Navio JA, Hidalgo MC, Paez J, Kherbeche A (2020) Preparation of ZnFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e/ZnO composite: Effect of perational parameters for photocatalytic degradation of dyes under UV and visible illumination. J Photochem Photobiology A: Chem 390:1\u0026ndash;12\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eNguyen HP, Cao TM, Nguyen TT, Pham VV (2023) Improving photocatalytic oxidation of semiconductor (TiO\u003csub\u003e2\u003c/sub\u003e, SnO\u003csub\u003e2\u003c/sub\u003e, ZnO)/CNTs for NO\u003csub\u003ex\u003c/sub\u003e removal. J Ind Eng Chem 127:321\u0026ndash;330\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eChanu LA, Singh KJ, Devi KN (2022) UV light illuminated photodegradation of Malachite green dye using ZnO/CuO nanocomposites. Materials Today: Proceedings. ;65:2865\u0026ndash;2870\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eJa\u0026rsquo;farawy MSA, Kusumandari, Purwanto A, Widiyandari H (2022) Carbon quantum dots supported zinc oxide (ZnO/CQD) efficient photocatalyst for organic poluutant degradation \u0026ndash; A systemic review. Environ Nanatechnol Monit Manage 18:1\u0026ndash;23\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBehnood R, Sodeifian G (2020) Synthesis of N doped-CQD/Ni doped-ZnO nanocomposites for visible light photodegradation of organic pollutants. J Environ Chem Eng 8:1\u0026ndash;11\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHojaghan HS, Niasari MS, Amiri O, Rashki S, Ashrafi M (2021) Green synthesis, characterization and antimicrobial activity of carbon quantum dots-decorated ZnO nanoparticles. Ceram Int 47:5187\u0026ndash;5197\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSong S, Wu K, Wu H, Guo J, Zhang L (2019) Multi-shelled ZnO decorated with nitrogen and phosphorus co-doped carbon quantum dots: synthesis and enhanced photodegradation activity of methylene blue in aqueous solutions. RSC Adv 9:7362\u0026ndash;7374\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHu X, Li Y, Xu Y, Gan Z, Zou X, Shi J, Huang X, Li Z, Li Y (2021) Green one-step synthesis of carbon quantum dots from orange peel for fluorescent detection of \u003cem\u003eEscherichia coli\u003c/em\u003e in milk. Food Chem 339:1\u0026ndash;8\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSariga, Kolaprath MKA, Benny L, Varghese A (2023) A facile, green synthesis of carbon quantum dots from \u003cem\u003ePolyalthia longifolia\u003c/em\u003e and its application for the selective detection of cadmium. Dyes Pigm 210:1\u0026ndash;11\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003ePatra S, Singh M, Subudhi S, Mandal M, Nayak AK, Sahu BB, Mahanandia P (2023) One-step green synthesis of in-situ functionalized carbon quantum dots from \u003cem\u003eTagetes patula\u003c/em\u003e flowers: Applications as a fluorescent probe for detecting Fe\u003csup\u003e3+\u003c/sup\u003e ions and as an antifungal agent. J Photochem Photobiology A: Chem 442:1\u0026ndash;3\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eTyagi A, Tripathi KM, Singh N, Choudhary S, Gupta RK (2016) Green synthesis of carbon quantum dots from lemon peel waste: applications in sensing and photocatalysis. RSC Adv 6:72423\u0026ndash;72432\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eGedda G, Sankaranarayanan SA, Putta CL, Gudimella KK, Rengan AK, Girma WK Green synthesis of multi-functional carbon dots from medicinal plant leaves for antimicrobial, antioxidant, and bioimaging applications. Sci Rep ;6371:1\u0026ndash;9\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eKurian M, Paul A Recent trends in the use of green sources for carbon dot synthesis-A short review. Carbon Trends ;3:1\u0026ndash;11\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWang L, Zhou HS (2014) Green Synthesis of Luminescent Nitrogen-Doped Carbon Dots from Milk and Its Imaging Application. ACS Publications 86:8902\u0026ndash;8905\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSaxena M, Sarkar S (2012) Synthesis of carbogenic nanosphere from peanut skin. Diam Relat Mater 24:11\u0026ndash;14\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003ePrasannan A, Imae T (2013) One-Pot Synthesis of Fluorescent Carbon Dots from Orange Waste Peels. ACS Publications 52:15673\u0026ndash;15678\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eDubey P, Tripathi KM, Mishra R, Bhati A, Singh A, Sonkar SK (2015) A simple one-step hydrothermal route towards water solubilization of carbon quantum dots from soya-nuggets for imaging applications. RSC Adv 5:87528\u0026ndash;87534\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eMalavika JP, Shobana C, Ragupathi M, Kumar P, Lee YS, Govarthanan M, Selvan RK (2021) A sustainable green synthesis of functionalized biocompatible carbon quantum dots from \u003cem\u003eAloe barbandensis\u003c/em\u003e Miller and its multifunctional applications. Environ Res 200:1\u0026ndash;10\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBas\u0026cedil;lak C, Ozt\u0026uuml;rk G, Demirel S, Kocyigit A, Dogu S, Yildirim M (2023) Green synthesis of carbon quantum dots from \u003cem\u003eSideritis vuralii\u003c/em\u003e and its application in supercapasitors. Inorg Chem Commun 153:1\u0026ndash;7\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eNajjar M, Nasseri MA, Allahresani A, Darroudi M (2022) Green and efficient synthesis of carbon quantum dots from \u003cem\u003ecordia myxa\u003c/em\u003e L. and their application in photocatalytic degradation of organic dyes. J Mol Struct 1266:1\u0026ndash;10\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eValencia OGR, Carrasco MR, Fuentes JH, German CMRS, Flores ME, Chavez EV (2021) Synthesis of blue emissive carbon quantum dots from \u003cem\u003eHibiscus Sabdariffa\u003c/em\u003e flower: Surface functionalization analysis by FT-IR spectroscopy. Materialia 19:1\u0026ndash;8\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLi H, Cheng B, Xu J, Yu J, Cao S (2024) Crystalline carbon nitrides for photocatalysis. Royal Soc Chem 2:411\u0026ndash;447\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBarati A, Shamsipur M, Arkan E, Hosseinzadeh L, Abdollahi H (2015) Synthesis of biocompatible and highly photoluminescent nitrogen doped carbon dots from lime: Analytical applications and optimization using response surface methodology. Mater Sci Eng C 47:325\u0026ndash;332\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWidiyandari H, Prilita O, Ja\u0026rsquo;farawy MSA, Nurosyid F, Arutanti O, Astuti Y, Mufti N (2023) Nitrogen-doped carbon quantum dots supported zinc oxide (ZnO/N-CQD) nanoflower photocatalyst for methylene blue photodegradation. Results Eng 17:1\u0026ndash;7\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHidayat RN, Widiyandari H, Parasdila H, Prilita O, Astuti Y, Mufti N, Ogi T (2024) Green synthesis of ZnO photocatalyst composited carbon quantum dots (CQD) from lime (Citrus aurantifolia). Catal Commun 187:1\u0026ndash;8\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSonu DV, Sudhaik A, Khan AAP, Ahamad T, Raizada P, Thakur S, Asiri AM, Singh P (2023) GCN/CuFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e/SiO\u003csub\u003e2\u003c/sub\u003e photocatalyst for photo-Fenton assisted degradation of organic dyes. Mater Res Bull 164:1\u0026ndash;13\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSong S, Wu K, Wu H, Guo J, Zhang L (2019) Multi-shelled ZnO decorated with nitrogen and phosphorus co-doped carbon quantum dots: synthesis and enhanced photodegradation activity of methylene blue in aqueous solutions. RSC Adv 9:7362\u0026ndash;7374\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eKumar R, Sudhaik A, Sonu, Nguyen VH, Le QV, Ahamad T, Thakur S, Kumar N, Hussain CM, Singh P, Raizada P (2023) Graphene oxide modified K, P co-doped g-C\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e and CoFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e composite for photocatalytic degradation of antibiotics. J Taiwan Inst Chem Eng 150:1\u0026ndash;13\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nanotechnology-for-environmental-engineering","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"ntee","sideBox":"Learn more about [Nanotechnology for Environmental Engineering](http://link.springer.com/journal/41204)","snPcode":"41204","submissionUrl":"https://submission.springernature.com/new-submission/41204/3","title":"Nanotechnology for Environmental Engineering","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"CQD, ZnO/CQD, Photodegradation, Malachite Green, Ciprofloxacin, Stale soy milk","lastPublishedDoi":"10.21203/rs.3.rs-5372134/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5372134/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eVisible light-driven photocatalysts are widely investigated to produce high removal efficiency in removing organic pollutants. Carbon quantum dots (CQD) are a plausible candidate for enhancing photocatalytic activity and play an essential role in malachite green (MG) degradation. Biomass waste, stale soy milk, contains lactic acid, which is utilized as a carbon precursor to prepare CQD. ZnO photocatalysts were composited with CQD derived from stale soy milk by green synthesis for the first time. The presence of CQD and their effect on morphology, surface area, decrease in band gap energy, and reduced electron-hole recombination. Indicating that the photocatalytic activity of ZnO/CQD in MG degradation was confirmed after 90 minutes, reaching 84% with a reaction rate constant of 0.01137 k/min\u003csup\u003e-1\u003c/sup\u003e. Furthermore, the reusability study after four reaction cycles revealed that ZnO/CQD were stable, and scavenger tests were performed to identify the active sites. As a result, we believe that CQD from stale soy milk composited with ZnO is an excellent photocatalyst candidate for removing organic pollutants.\u003c/p\u003e","manuscriptTitle":"Green Synthesis of Carbon Quantum Dots from Stale Soy Milk Composited Zinc Oxide (ZnO/CQD) for Photodegradation of Malachite Green","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-11-13 07:03:07","doi":"10.21203/rs.3.rs-5372134/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Accepted","date":"2025-03-05T03:35:06+00:00","index":"","fulltext":""},{"type":"reviewerAgreed","content":"120074301498391120701972893694307557257","date":"2024-12-08T05:01:13+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"45649155915614218853235800724381735367","date":"2024-12-08T02:04:22+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-11-08T18:30:52+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-11-01T09:42:17+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2024-11-01T09:41:50+00:00","index":"","fulltext":""},{"type":"submitted","content":"Nanotechnology for Environmental Engineering","date":"2024-11-01T09:00:05+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nanotechnology-for-environmental-engineering","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"ntee","sideBox":"Learn more about [Nanotechnology for Environmental Engineering](http://link.springer.com/journal/41204)","snPcode":"41204","submissionUrl":"https://submission.springernature.com/new-submission/41204/3","title":"Nanotechnology for Environmental Engineering","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"974aec03-98f6-4d21-b11f-367205454a50","owner":[],"postedDate":"November 13th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2025-03-24T16:06:27+00:00","versionOfRecord":{"articleIdentity":"rs-5372134","link":"https://doi.org/10.1007/s41204-025-00425-9","journal":{"identity":"nanotechnology-for-environmental-engineering","isVorOnly":false,"title":"Nanotechnology for Environmental Engineering"},"publishedOn":"2025-03-21 15:57:54","publishedOnDateReadable":"March 21st, 2025"},"versionCreatedAt":"2024-11-13 07:03:07","video":"","vorDoi":"10.1007/s41204-025-00425-9","vorDoiUrl":"https://doi.org/10.1007/s41204-025-00425-9","workflowStages":[]},"version":"v1","identity":"rs-5372134","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5372134","identity":"rs-5372134","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}