Photocatalytic degradation of MB by lignin carbon quantum dot modified BiVO4 composites materials

Photocatalytic degradation of MB by lignin carbon quantum dot modified BiVO4 composites materials | 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 Photocatalytic degradation of MB by lignin carbon quantum dot modified BiVO 4 composites materials Chenghan Li, Shanshan Wang, Yiping Li, Oxana P. Taran, Fubao Sun, and 2 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7369134/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 20 Nov, 2025 Read the published version in Journal of Nanoparticle Research → Version 1 posted 9 You are reading this latest preprint version Abstract Methylene blue is a significant pollutant that seriously threatens the safety of water environments, and traditional methods have difficulty in removing it. In this study, lignin carbon quantum dots (CQDs) were prepared by hydrothermal method, and a BiVO 4 /CQDs composite materials was fabricated for the photocatalytic degradation of methylene blue. Through various testing methods, the optimal doping amount of carbon quantum dots was successfully studied. The doping of carbon quantum dots reduced the band gap of BiVO 4 and enhanced the light absorption and electron transfer ability of the BiVO 4 /CQDs composite materials. Among them, the BiVO 4 /CQDs-15 composite materials exhibited the best photodegradation effect. Under visible light irradiation, 99.7% of the methylene blue was degraded within just 120 min. The enhancement of photocatalytic activity is attributed to the highly dispersed nature of CQDs on the surface of BiVO 4 , their upconversion property, excellent electron transfer ability, and the synergistic effect on the photocatalytic performance of BiVO 4 . Alkali lignin Carbon quantum dots BiVO4 Photocatalysis Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 1. Introduction As industrialization advances globally, vast quantities of harmful pollutants are released into the environment. These emissions not only pose severe risks to public health but also inflict widespread damage on ecosystems [ 1 ] . As a textile industry dye, methylene blue (MB) frequently contaminates aquatic environments, where it bioaccumulates in marine life. Chronic human contact induces multiple health risks, including cutaneous hypersensitivity reactions, hormonal system disturbances and impaired cardiac function [ 2 ] . Given these risks, controlling anthropogenic organic pollutants becomes essential. Photocatalytic degradation emerges as an eco-conscious solution, enabling solar-powered purification while generating renewable energy. Photocatalytic processes typically employ semiconductor materials to degrade organic compounds into harmless CO₂ and H₂O through light absorption. Recent advances have yielded multiple efficient photocatalysts including TiO 2 , ZnO, CdS and ZrO 2 nanostructures. Current semiconductor photocatalysts predominantly exhibit wide energy bandgaps and rapid charge carrier recombination rates [ 3 ] . These inherent properties significantly limit their UV-visible spectrum responsiveness and overall photon conversion efficiency [ 4 ] . Effective photocatalysts require three key attributes: strong chemical inertness, affordable production costs and good suspension stability. These materials should additionally exhibit extensive spectral absorption and efficient electron-hole separation to optimize sunlight utilization [ 5 ] . Carbon quantum dots (CQDs), alternatively called carbon nanodots or fluorescent carbon nanoparticles, represent zero-dimensional carbon nanomaterials typically under 10 nanometers. These particles exhibit remarkable luminescent characteristics, superior aqueous dispersibility, exceptional chemical stability, minimal biological toxicity, outstanding biocompatibility and negligible ecological effects [ 6 , 7 ] . As a novel carbon-based material, carbon dots has a very excellent application range. After being first discovered by researchers in 2004, they have been extensive utilize in multifarious domain such as drug delivery [ 8 ] , photocatalysis [ 9 ] , biosensing [ 10 ] , the preparation of light-emitting devices [ 11 ] , and analytical testing [ 12 ] . The superior optoelectronic performance of CQDs enables substantial photocatalytic enhancement as they effectively harvest light energy and facilitate rapid electron-hole pair separation and transfer. The reaction conditions of CQDs will have a significant impact on their properties. The most common methods for preparing carbon quantum dots are two: the "top-down" method and the "bottom-up" method [ 13 ] . Compared with the top-down method, the fluorescent carbon quantum dots synthesized through the bottom-up approach [ 14 ] have the advantages of more precise preparation process and better controllability in the operation, for example, carbon quantum dots can rapidly and resultful compound by the hydrothermal method [ 15 ] . The compound of carbon quantum dots by the hydrothermal method is not only simple, but also the sources of carbon are very diverse, such as lignin, ascorbic acid and citric acid, etc. Although the sources of carbon for preparing carbon quantum dots are diverse, some of them are expensive and some are even toxic. Therefore, finding low-cost and carbon-rich substances as carbon sources has become a research hotspot in recent years [ 16 ] . As the second most abundant aromatic polymer in nature, lignin is composed of cross-linked phenylpropane units to form a three-dimensional network structure, and its content is second only to cellulose. The mass fractions of lignin in vascular plants were in the order of coniferous wood > broadleaf wood > herbaceous plants and the specific values ranged from 27–33%, 18–25% and 17–24% [ 17 ] . Structural analysis of lignin monomers reveals that its cross-linked network is rich in active functional groups such as phenolic hydroxyl and carboxyl groups. This distinctive chemical composition not only endows it with excellent biocompatibility [ 18 ] and high carbon content [ 19 ] , but also makes it an ideal precursor for the preparation of carbon quantum dots [ 20 ] . However, the global Industry for producing paper products generates over 50 million tons of lignose annually, with only about 2% being recovered and utilized. The indiscriminate massive emission of black liquor has led to severe ecological environment pollution while also resulting in significant resource waste. The valorization of lignin has emerged as a prominent research focus, with its application value in functional materials gradually being realized [ 21 ] . In recent years, semiconducting nanomaterials have shown excellent potential for applications in environmental catalysis and energy conversion. Due to their unique energy band structure and surface active sites, these materials have been shown to be highly efficient in degrading organic pollutants (including difficult-to-biodegrade textile dyes and antibiotics) in water and in removing heavy metal ions by adsorption. More strikingly, these catalysts have excellent performance in light-driven reactions, which can simultaneously realize the photocatalytic reduction of CO₂ and hydrogen production from water molecule cleavage, two key processes [ 22 ] . As a typical n-type semiconductor material, BiVO 4 shows unique advantages in visible photocatalysis. The material has the following remarkable features: firstly, its narrow bandgap of 2.4 eV enables it to utilize visible light efficiently; secondly, it exhibits excellent chemical stability and photocorrosion resistance; moreover, the easy availability of the raw materials and its environmentally friendly characteristics give it potential for practical applications [ 23 ] . These properties make BiVO 4 an ideal material system for photocatalysis research. To address the inherent shortcomings of BiVO 4 and further enhance its photocatalytic performance, researchers have effectively enhanced the light response capability of BiVO 4 by doping ions [ 24 ] . Research has shown that by constructing different semiconductor materials into heterojunctions, it is also possible to enhance the photocatalytic performance. The existing methods for modifying BiVO 4 have all shown limited improvement in the photocatalytic performance of BiVO 4 due to the large size of the material [ 25 ] . This might be due to the presence of surface defects in the material during the preparation process. At these defects, electrons and holes recombine, thereby limiting the photocatalytic effect of the material [ 26 ] . After research, it was found that using quantum dots, which are small-sized materials, can effectively solve the problem of defects formed during the synthesis process due to their uniform distribution characteristics [ 27 ] . Carbon quantum dots can effectively enhance the photocatalytic performance of composite photocatalysts [ 28 ] . Carbon quantum dots have attracted extensive attention in the field of photocatalysts due to their excellent optical properties. By incorporating carbon quantum dots to expand the light absorption range, this method can reduce the recombination of photoelectrons and holes. Carbon quantum dots can be combined with semiconductor photocatalysts to effectively enhance the performance of the photocatalyst [ 29 ] . Luo et al. [ 30 ] modified single-atom platinum on the surface of nitrogen-doped carbon dots by one-step photodeposition, and then combined it with TiO 2 , effectively enhancing the hydrogen production performance of the photocatalyst. Liu et al. [ 31 ] prepared a Ru@carbon dot composite catalyst by combining ruthenium with carbon quantum dots. They utilized the defects of the carbon dots to prevent the aggregation of ruthenium. Wang et al. [ 32 ] employed a simple pyrolysis reaction and successfully incorporated a single cobalt atom into carbon dots during the pyrolysis process of vitamin B 12 , thereby successfully preparing the CoSAS@CD composite photocatalytic material. In conclusion, the unique structure of carbon quantum dots and their abundant active sites enable them to effectively Quickly and effectively enhance the photocatalytic performance of the materials. By constructing a heterojunction using carbon quantum dots and BiVO 4 , the recombination of photogenerated electrons and holes in the photocatalyst can be effectively inhibited, thereby achieving the goal of improving the performance of the photocatalyst. This paper uses alkaline lignin as the carbon source and employs the hydrothermal method to manufacture carbon quantum dots. The carbon quantum dots are combined with BiVO 4 to prepare the BiVO 4 /CQDs composite materials. The structure of the BiVO 4 /CQDs composite materials are characterize by XRD, FT-IR, and TEM. The optical properties of the BiVO 4 /CQDs composite materials are characterized by UV-Vis. The BiVO 4 /CQDs composite materials with the optimal photocatalytic performance is prepared by changing the reaction conditions. It laid the research foundation for the efficient and high-value use of lignin. 2. Materials and Methods 2.1 Materials and reagents Alkali lignin (AL), Shanghai Yuanye Biotechnology Co., Ltd, analytically pure; ethylenediamine (EDA, 99%) and Methylene blue (MB, 96%) were purchased from Tianjin Zhiyuan Chemical Reagent Co. Hydrogen peroxide (H 2 O 2 30%), Xilong Chemical Co. Bismuth nitrate pentahydrate (Bi(NO 3 ) 3 -5H 2 O), ammonium isocyanate (NH 4 VO 3 ), urea (CO(NH 2 ) 2 ), ethanol (CH 3 CH 2 OH), dialysis bag, MD44-3500 Viskase Company, USA, concentrated nitric acid (H 2 SO 4 ) All chemicals were pure and could be used without weiter refinement. Deionised water was used for all experiments. 2.2 Preparation methods 2.2.1 Preparation of Lignin Carbon Quantum Dots Dissolve 0.5 g of alkaline lignin in 50 mL of distilled water, stir evenly at normal atmospheric temperature and ultrasonicate to ensure complete dissolution. Stir the uniformly dispersed solution continuously at normal atmospheric temperature, and add 30 mL of ethylenediamine and 10 mL H 2 O 2 . The miscible liquids was added into a 100 mL high-pressure reactor with a polytetrafluoroethylene inner lining, and then heated continuously at 190℃ for 12 h. After allowing the high-pressure reaction vessel to cool down to room temperature, the solution should be removed. Use a sand core funnel to filter the solution three times to remove the large particles of impurities in the solution. Then, add the 3000 Da dialysis bag and immerse it in deionized water for 48 h. Change the deionized water every 8 h. After 48 h of dialysis, the carbon dot solution can be obtained. 2.2.2 Preparation of BiVO 4 Dissolve 12 mmol of bismuth nitrate pentahydrate in 64 mL (1 mol/L) of nitric acid solution. Stir continuously at normal atmospheric temperature for 30 min. After the stirring is completed, add 12 mmol of ammonium vanoxide and continue stirring for 1 h. After the stirring is finished, add 6.0 g of urea and stir continuously for 30 min. Add the above stirred solution to a 100 mL high-pressure reactor with polytetrafluoroethylene inner lining. Heat continuously at 180℃ for 12 h. The kettle was naturally cooled down to room temperature at the end of the reaction. Centrifugation (9000 r/min, 10 min) to separate the bright yellow powder, rinse with deionized water for 5 times and oven-dry at 60℃ for 24 h. BiVO 4 can be obtained. 2.2.3 Preparation of BiVO 4 /CQDs Bi(NO 3 ) 3 -5H 2 O (12 mmol) was lysis in 64 mL of 1 mol/L HNO 3 and constant stirring for 30 min at normal atmospheric temperature and then 12 mmol of NH 4 VO 3 was added into the solution and constant stirring for 1 h. Add 6.0 grams of urea to the above solution and stir it rapidly at room temperature for 30 minutes. Different content of CQDs solutions were added to the aforementioned mixture, and then they were stirred continuously at a uniform speed for 30 min. After stirring, the solution was put into an ultrasonic machine (0℃) for ultrasonication. The aforementioned mixed solution was added to a 100 mL high-pressure reactor with a polytetrafluoroethylene inner lining, and was heated continuously at 10℃ for 12 h. The yellow solid powder was subjected to centrifugation, then washed with deionized water for 5 times, and dried at a constant temperature of 60℃ for 24 hours. In addition, BiVO 4 /CQDs composites materials materials with different CQDs co-catalyst contents could be gain by varying the volume of the CQDs solution (10, 15 and 20 mL) and the samples were noted as BiVO 4 /CQDs- x (x represents the volume of the CQDs solution). 2.3 Materials characterization test The surface shape of the composite materials was tested using transmission electron microscopes (TEM, HRTEM, JEOL and JEM-2100F). The crystal structure of the prepared samples was characterized by X-ray diffractometer (XRD, Brook D8 advanced X-ray diffractometer). The molecular structure of the prepared materials was characterized by FT-IR spectrometer (FT-IR, TENSOR II, Brook AXS GMBH, Germany). X-ray photoelectron spectroscopy (XPS, PHI-5702, Physical Electronics Company) was used to analyze the chemical composition of the prepared materials. Ultraviolet-visible diffuse reflectance spectroscopy (Shimazu UV-2450) was used to characterize the optical properties of the materials. Raman spectroscopy (DXR Micro Confocal Laser Raman Spectrometer, USA) was used to analyze the sample structure. The photocurrent and electrochemical impedance of the sample were measured by electrochemical workstation (CHI760E A17122). 2.4 Photocatalytic degradation of MB performance test Using a 300 W xenon lamp as the light source and filtering out the ultraviolet light component in the light through a 420 nm filter, with the photocatalytic degradation efficiency of methylene blue (MB) solution as the performance indicator, the photocatalytic performance of the BiVO 4 /CQDs composite catalysts was tested. Add 50 mg of the prepared photocatalyst to 50 mL of a MB solution with a concentration of 25 mg/L. Stir the solution for 30 min in the absence of light. During the period of no light stirring, the methylene blue solution and the photocatalyst reach an adsorption equilibrium state. After dark treatment for 30 min, the light source was turned on for photocatalytic degradation. Samples were collected every 20 min. To remove the catalyst from the samples, the collected samples were centrifuged, and the supernatant was taken for testing. After testing, it was found that MB had the maximum absorbance at 550 nm, which was used to Calculate the degradation rate. The calculation method of degradation rate is as shown in formula (1): A 0 represents the initial concentration of the MB dye, A is the concentration of the MB dye after being exposed to light, C 0 is the initial solubility of the MB dye, and C is the solubility of the MB dye after being exposed to light. To investigate the stability of the BiVO4/CQDs-15 composite material, the photocatalytic performance of the BiVO 4 /CQDs-15 composite materials was tested continuously for four cycles. 2.5 Photoelectrochemical test Electrochemical tests were conducted using the three-electrode method. Add the photocatalyst to 2 mL of ethanol and perform ultrasonic treatment for 30 min. 20 µL of 0.5% Nafion were added using a dropper. 50 µL of the dispersed solution were aspirated using a dropper and dropped onto the conductive surface of 2.0×1.0 cm 2 indium tin oxide (ITO) conductive glass. After dropping, the sample was dried in an oven and the above process was repeated 4–5 times. The prepared materials were then subjected to transient photocurrent tests and electrochemical impedance tests. 3. Results and Discussion The crystal structures of the CQDs, pure BiVO 4 and BiVO 4 /CQDs composite catalysts were characterized by X-ray diffraction (XRD). As shown in Fig. 1 (a) the diffraction pattern of CQDs shows typical amorphous carbon features: the broadened diffraction peak appearing at 20.3° corresponds to the (002) crystal plane of graphitic carbon, while the weak peak at 43.5° can be vest in to the (100) crystal plane [ 33 ] . This diffraction feature confirms that the lignin carbon source has been successfully converted into amorphous carbon quantum dots. XRD analysis of Fig. 1 (b) shows that all BiVO 4 -containing samples display characteristic diffraction peaks at 18.98° (011), 28.87° (112), and 30.51° (004), these peaks are in the same position as the standard card for monoclinic phase BiVO 4 (JCPDS 14–0688) in perfect agreement [ 34 ] . It is remarkable that: (1) The absence of stray peaks outside the crystal phase of BiVO 4 in the diffraction patterns of the composites suggests that the composite process did not change the intrinsic crystalline structure of BiVO 4 ; (2) No obvious carbon characteristic peaks were detected in all samples, which may be due to the fact that the low loading of carbon quantum dots poor crystallinity of carbon quantum dot. The carbon features are masked by the strong BiVO 4 diffraction peaks [ 35 ] . The FT-IR spectral analysis of BiVO 4 , CQDs and BiVO 4 /CQDs-15 composites materials is shown in Fig. 2 , where the main structural features of each component can be clearly observed. The absorption peak at 3258 cm − 1 is caused by the extension vibration of water molecules O-H radical on the surface of the CQDs and BiVO 4 /CQDs-15 composite materials. The characteristic O-H peak at 1614 cm − 1 and the C-O-C and C-O [ 36 ] . vibrational peaks at 1370 cm − 1 and 1240 cm − 1 together verify the modification of the CQD surface by oxygen-containing functional groups. In particular, the characteristic absorption peak of BiVO 4 (VO₄³-vibration at 736 cm − 1 ) and all the characteristic peaks of CQDs were retained in the composite. This result not only confirms the successful preparation of the composites, but also indicates that the surface functionalization properties of the CQDs were completely preserved during the composite process. These structural features provide an important molecular level basis for understanding the photocatalytic properties of the composites. Figure 3 (a) shows that the BiVO 4 crystal exhibits a one-sided single crystal shape with a regular decahedral appearance. Its structural characteristics are as follows: (1) The surface is smooth and defect-free; (2) Clear edges; (3) Uniform size distribution (0.9–1.5µm×0.38µm). After the introduction of CQDs Fig. 3 b) the composite material undergoes changes: (1) Passivation of crystal edges and corners; (2) The tendency of particle agglomeration is enhanced. This structural change mainly stems from: (1) The surface modification effect of CQDs; (2) Crystal growth regulation caused by interfacial interactions; (3) Spatial constraints at the nanoscale. Transmission electron microscopy analysis revealed the fine structural characteristics of carbon quantum dots (CQDs) and their composites Fig. (4). As shown in Fig. 4 (a, b) the CQDs are mainly spherical nanoparticles (1-5.5nm), concentrated in 2–4 nm with good dispersion and there are a few large particles present, resulting from high surface energy agglomeration caused by the small size effect. HRTEM confirmed that the 0.26 nm lattice spacing corresponds to the graphite (002) crystal plane, indicating a highly graphitized structure. Particle size statistics show that it is mainly distributed in the range of 2–4 nm (accounting for more than 60%). Analysis in Fig. 4 (c) reveals that pure BiVO 4 presents a bulk morphology, while in the composite material Fig. 4 (d), CQDs(< 10nm) are uniformly anchored on the surface of BiVO 4 , forming a tight interface contact [ 37 ] . These results confirm: (1) The successful preparation of highly crystalline CQDs; (2) Achieve uniform compounding at the nanoscale; (3) Provide a structural basis for performance optimization. The elements and chemical states contained in the BiVO 4 /CQDs-15 composite materials were analyzed in detail by X-ray photoelectron spectroscopy (XPS), as shown in Fig. 5 . Full-spectrum analysis confirmed that the material contained only Bi, V, C, O and trace N elements. As can be seen from Fig. 5 (a), the 284.6 eV value corresponds to the C1s peak. Fine spectral analysis showed that in Fig. 5 (b) the Bi 4f spectrum presented characteristic bipeaks at 158.5 eV (4f₇/₂), 163.9 eV (4f₅/₂) and the splitting at 5.3 eV confirmed the presence of the trivalent state Bi 3+ of monoclinic BiVO 4 . As shown in Fig. 5 (c), the peak positions of the V 2p spectrum at 516.0 eV (2p₃/₂) and 523.5 eV (2p₁/₂) are consistent with those of the standard BiVO4. Figure 5 (d) O-1s spectrum can be decomposed into lattice oxygen (529.5 eV) and adsorbed oxygen (531.5 eV). As can be seen from Fig. 5 (e), C1s consists of three types of chemical bonds, namely 284.6 electron volts (C-C), 285.9 electron volts (C-O-C), and 287.97 electron volts (O-C = O). The peak of 399.26 eV in the n-1s spectrogram Fig. 5 (f) belongs to pyrrole nitrogen [ 26 ] . These results fully confirm the successful preparation of the composite materials. Figure 6 reveals the structural characteristics of CQDs and BiVO 4 /CQDs-15 composites materials by Raman spectroscopy analysis. As shown in the small figure of Fig. 6 . The characteristic peaks of CQDs at 1350 cm − 1 (D band, unordered vibration of sp³ carbon) and 1581 cm − 1 (G band, sequential vibration of sp² carbon) are presented [ 28 ] and the ID/IG ratio is 0.85, indicating the presence of an appropriate amount of structural defects [ 38 ] . The Raman spectra of BiVO 4 show characteristic peaks such as 858 cm⁻¹ (symmetrical stretching of VO), 763 cm − 1 (asymmetrical stretching), 370 cm − 1 (symmetrical bending of δs) and 250 cm − 1 (external vibration mode). At the same time, the following points can be observed: (1) No peak position shift was observed in the composite material, confirming that CQDs only deposited on the surface and did not enter the lattice; (2) Because the presence of carbon quantum dots enhances the Raman scattering effect, the intensity of the peaks increases. These results verified the non-destructive modification of CQDs on the surface of BiVO 4 . As shown in Fig. 7 , the optical response characteristics of the BiVO 4 /CQDs composite materials were investigated by using the ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS) method. The test results show that pure BiVO 4 has a characteristic absorption edge at 515 nm, corresponding to its direct bandgap transition. Compared with single-component materials, BiVO 4 /CQDs composites materials exhibit significant improvements in optical properties: (1) Absorption edge redshift; (2) The absorbance in the visible light region has increased by 40%; (3) The absorption range has been significantly expanded. This enhancing effect mainly stems from the sensitization effect of CQDs. Spectral analysis confirmed that the composite material has a superior light capture ability, which is directly related to its enhanced photocatalytic activity. The band gap calculation is as shown in formula (2): As shown in Fig. 7 (b), the band gap widths of the BiVO4, BiVO 4 /CQDs-10, BiVO 4 /CQDs-15, and BiVO 4 /CQDs-20 composite materials were calculated to be 2.45, 2.36, 2.31 and 2.30 eV respectively. The calculation results indicate that by doping CQDs and preparing BiVO 4 /CQDs composite materials, the band gap width of the composite materials can be reduced. As shown in Fig. 8 (a), the degradation performance of BiVO4/CQDs composite materials for methyl blue varies with the amount of CQDs doped. After being exposed to light for 120 min, the degradation result of methylene blue by pure BiVO 4 composite materials are only 56.2%, while the degradation rates of BiVO 4 /CQDs-10, BiVO 4 /CQDs-15, and BiVO 4 /CQDs-20 composite materials for methyl blue are as high as 86.9%, 99.7%, and 74.6% respectively. Thus, it can be concluded that when the CQDs doping amount is 15 mL, the prepared BiVO 4 /CQDs-15 composite materials has the highest catalytic active degradation efficiency for methylene blue. In the composite material, CQDs act as an effective electron acceptor, promoting the efficient separation of photogenerated electrons and holes in BiVO 4 , achieving the purpose of improving the BiVO 4 /CQDs composite materials. In order to further study the reaction kinetics of BiVO 4 /CQDS composite materials for MB photocatalytic degradation, the zero-order model was used to simulate the experimental data: ln(C/C 0 ) = kt [ 39 ] . As shown in Fig. 8 (b), the apparent rate constant of BiVO 4 /CQDs-15 composite materials was 0.0178 min − 1 , which was 2.82, 9.36, 2.78 and 1.97 times higher than that of pure BiVO 4 , CQDs, BiVO 4 /CQDs-10 and BiVO 4 /CQDs-20 composite materials, respectively. The results indicate that the appropriate CQDs loading is conducive to improving the photocatalytic performance of BiVO 4 /CQDs composites materials. It is an important indicator for evaluating the stability of photocatalysis when it can be reused. As shown in Fig. 8 (c), to verify the stability of the BiVO 4 /CQDs-15 composite materials, four consecutive photocatalytic cycling tests were conducted. Four cycling experiments were conducted. The experimental results show that in the first two cycles of the experiments, the degradation rate of MB reached 90% each time. However, in the latter two tests, the degradation effect of MB decreased to varying degrees. The trend showed that MB completely degradation with increasing time. This indicates that the BiVO 4 /CQDs-15 composites materials have good photocatalytic stable, which is also favourable for their apply in photocatalysis. Figure 8 (d) shows the XRD comparison before and after the photocatalytic degradation of MB by BiVO 4 -CQDs-15. It can be found that there is no change, which proves that BiVO 4 -CQDs-15composite materials has excellent stability [ 40 ] . In Fig. 9 , the photoelectrochemical test reveals the excellent charge separation performance of the composite material. Figure 9 (a) shows that the photocurrent response test demonstrates that the photocurrent density of BiVO 4 / CQDs-15 composite materials are 3.2 times higher than that of pure BiVO 4 . This is mainly due to the introduction of CQDs significantly enhancing visible light absorption and the two-phase interface promoting the efficient separation of photogenerated carriers. Figure 9 (b) further confirms this conclusion through electrochemical impedance spectroscopy (EIS) analysis. The smaller arc radius in the figure indicates that BiVO 4 /CQDs-15 composite materials has a lower charge transfer resistance, a faster interinterface electron transport rate and a higher carrier separation efficiency. It is worth noting that the conclusions drawn from the two test methods are highly consistent jointly confirming that the loading of CQDs promotes the separation efficiency of photogenerated carriers in the material. Based on the above experimental results, a mechanism for the enhanced photocatalytic performance of BiVO 4 / CQDs-15 composites materials are proposed as shown in Fig. 10 . Under imitation solar illumination, CQDs can assimilat light beyond the absorption region of BiVO 4 and convert it to shorter wavelengths, which activates BiVO 4 to produce electron-hole pairs. As a result, solar energy is collected more efficiently. On the other hand, the CQDs act as electron storage layers that can trap electrons divergency by BiVO 4 , thereby inhibiting the compounding of BiVO 4 and prolonging the lifetime of the BiVO 4 photogenerated holes. CQDs can facilitate electron transfer and react with the O 2 adsorbed during the reaction process, thereby generating highly reactive superoxide radicals (·O 2 − ).The superoxide free radicals produced during the reaction are strong oxidants, which can effectively oxidize the pollutants adsorbed on the surface into small molecule substances [ 40 ] . 4. Conclusions In this paper, the photocatalysts of BiVO 4 /CQDs composites materials were prepared by one-step hydrothermal method, and the photocatalytic degradation test showed that the incorporation of CQDs improved the photodegradation performance of MB dyes. According to the results of PC and EIS measurements, the introduction of CQDS promotes charge separate and absorb of visible light, which increases the photocurrent and reduces the band gap of BiVO 4 /CQD composites materials. Among them, BiVO 4 /CQDs-15 composites materials had the best photodegradation effect, and the degradation rate was increased to 99.7%. Cycle testing shows that the material is reusable and stable. In addition, the photocatalytic degradation efficiency was improved by constructing BiVO 4 / CQDs-15 composites materials. The use of lignin to prepare CQDs has made lignin a high-value utilization. Declarations Acknowledgement This study was financially supported by the National Natural Science Foundation of China (22278099). Key project of Regional Innovation and Development Joint Fund of National Natural Science Foundation (U23A20135). Competing interests policy The authors declare no competing inter ests Ethics declaration Not applicable Author Contributions Statement In this article, Chenghan Li is responsible for analyzing the data generated by the experiment and making charts. Shanshan Wang is responsible for the operation of the experiment and the writing of the draft. Yiping Li is responsible for analyzing the data generated by the experiment and making charts. Oxana P. Taran is responsible for providing ideas for the article and solving some theoretical problems. Fubao Sun was responsible for providing research ideas for this article and resolving some writing grammar issues. 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(2022) Lignin-based carbon dots as high-performance support of Pt single atoms for photocatalytic H 2 evolution. Chem. Eng. J. 446: 136873. https://doi.org/10.1016/j.cej.2022.136873 Zhao Z, Li X, Dai K, et al. (2022) In-situ fabrication of Bi 2 S 3 /BiVO 4 /Mn 0.5 Cd 0.5 S-DETA ternary S-scheme heterostructure with effective interface charge separation and CO 2 reduction performance. Journal of Materials Science & Technology 117: 109-119. https://doi.org/10.1016/j.jmst.2021.11.046 Hao L, Yu Y, Liang Z, et al. (2023) Deciphering photocatalytic degradation of methylene blue by surface-tailored nitrogen-doped carbon quantum dots derived from Kraft lignin. International Journal of Biological Macromolecules 242: 124958. https://doi.org/10.1016/j.ijbiomac.2023.124958 Hu C, Chen Q, Tian M, et al. (2023) Efficient combination of carbon quantum dots and BiVO4 for significantly enhanced photocatalytic activities. Catalysts 13(3): 463. https://doi.org/10.3390/catal13030463 Yang Q, Li X, Tian Q, et al. (2023) Synergistic effect of adsorption and photocatalysis of BiOBr/lignin-biochar composites with oxygen vacancies under visible light irradiation. Journal of Industrial and Engineering Chemistry 117: 117-129. https://doi.org/10.1016/j.jiec.2022.09.044 Yu Y, Sun Y, Ge B, et al. (2023) Synergistic removal of organic pollutants from water by CTF/BiVO4 heterojunction photocatalysts. Environmental Science and Pollution Research 30(10): 27570-27582. https://doi.org/10.1007/s11356-022-24184-1 Additional Declarations No competing interests reported. Cite Share Download PDF Status: Published Journal Publication published 20 Nov, 2025 Read the published version in Journal of Nanoparticle Research → Version 1 posted Editorial decision: Revision requested 14 Sep, 2025 Reviews received at journal 11 Sep, 2025 Reviews received at journal 08 Sep, 2025 Reviewers agreed at journal 02 Sep, 2025 Reviewers agreed at journal 02 Sep, 2025 Reviewers invited by journal 02 Sep, 2025 Editor assigned by journal 18 Aug, 2025 Submission checks completed at journal 18 Aug, 2025 First submitted to journal 13 Aug, 2025 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. 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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-7369134","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":509345102,"identity":"33ce2fb0-1f82-4b78-bc3c-545c8aef9211","order_by":0,"name":"Chenghan Li","email":"","orcid":"","institution":"School of Materials Science and Chemical Engineering, Harbin University of Science and Technology, Harbin 150080, Heilongjiang, China","correspondingAuthor":false,"prefix":"","firstName":"Chenghan","middleName":"","lastName":"Li","suffix":""},{"id":509345103,"identity":"f85c3fc1-cadb-4a7b-999b-5df62f72c223","order_by":1,"name":"Shanshan Wang","email":"","orcid":"","institution":"School of Materials Science and Chemical Engineering, Harbin University of Science and Technology, Harbin 150080, Heilongjiang, China","correspondingAuthor":false,"prefix":"","firstName":"Shanshan","middleName":"","lastName":"Wang","suffix":""},{"id":509345104,"identity":"4425be07-639b-4ded-a141-a3511a0f6d36","order_by":2,"name":"Yiping Li","email":"","orcid":"","institution":"School of Materials Science and Chemical Engineering, Harbin University of Science and Technology, Harbin 150080, Heilongjiang, China","correspondingAuthor":false,"prefix":"","firstName":"Yiping","middleName":"","lastName":"Li","suffix":""},{"id":509345105,"identity":"b7d52735-8315-4505-a6a1-a05862cd41a1","order_by":3,"name":"Oxana P. 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Science and Technology, Harbin 150080, Heilongjiang, China","correspondingAuthor":false,"prefix":"","firstName":"Fen","middleName":"","lastName":"Li","suffix":""}],"badges":[],"createdAt":"2025-08-14 02:53:06","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-7369134/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-7369134/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s11051-025-06480-2","type":"published","date":"2025-11-20T15:57:35+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":90802716,"identity":"076a7131-e6fc-4b59-b17f-120ac92bf246","added_by":"auto","created_at":"2025-09-08 10:24:15","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":86470,"visible":true,"origin":"","legend":"\u003cp\u003eXRD: (a) CQDs, (b) BiVO\u003csub\u003e4\u003c/sub\u003e, BiVO\u003csub\u003e4\u003c/sub\u003e/CQDs composites materials\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-7369134/v1/a68e71b1932bb80af3a1a8e4.png"},{"id":90802675,"identity":"56671107-5b82-42f0-89d1-fac13391cb7c","added_by":"auto","created_at":"2025-09-08 10:24:13","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":59539,"visible":true,"origin":"","legend":"\u003cp\u003eFT-IR spectra: BiVO\u003csub\u003e4\u003c/sub\u003e, CQDs and BiVO\u003csub\u003e4\u003c/sub\u003e/CQDs-15 composite materials\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-7369134/v1/ea7a696ac52d3a871397f6e4.png"},{"id":90803425,"identity":"15812f56-ca4c-462d-981c-18ef5284f992","added_by":"auto","created_at":"2025-09-08 10:32:15","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":141908,"visible":true,"origin":"","legend":"\u003cp\u003eSEM images of (a) BiVO\u003csub\u003e4\u003c/sub\u003e, (b) BiVO4/CQDs-15 composite materials\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-7369134/v1/30a2524deaa1ae560900e679.png"},{"id":90802668,"identity":"d48bd1c8-d1bf-4215-9838-1ec40abbe745","added_by":"auto","created_at":"2025-09-08 10:24:12","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":267513,"visible":true,"origin":"","legend":"\u003cp\u003eTEM images of (a) CQDs (Inset: HRTEM of CQDs), (b) CQDs particle size distribution, (c) BiVO\u003csub\u003e4 \u003c/sub\u003e(Inset: HRTEM of BiVO\u003csub\u003e4\u003c/sub\u003e), (d) BiVO\u003csub\u003e4\u003c/sub\u003e/CQDs-15 composite materials\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-7369134/v1/faa60229d62a49dc7c22905b.png"},{"id":90802660,"identity":"8054dae7-5973-452c-af5f-722a63ea945f","added_by":"auto","created_at":"2025-09-08 10:24:11","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":112230,"visible":true,"origin":"","legend":"\u003cp\u003eXPS spectra of BiVO\u003csub\u003e4\u003c/sub\u003e/CQDs-15 composites materials: measured spectra (a), Bi 4f (b), V2p (c), O1s (d), C1s (e) , N1s (f)\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-7369134/v1/60e474af651d07ea2ec97567.png"},{"id":90802677,"identity":"f6bd9d83-3544-4feb-b8e1-4736552095b6","added_by":"auto","created_at":"2025-09-08 10:24:13","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":50537,"visible":true,"origin":"","legend":"\u003cp\u003eRaman spectra: CQDs, BiVO\u003csub\u003e4\u003c/sub\u003e and BiVO\u003csub\u003e4\u003c/sub\u003e/CQDs-15 composites materials\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-7369134/v1/b46357205be43e0ff836409c.png"},{"id":90802673,"identity":"6c956702-0fdd-452c-8010-2e084be3e7a2","added_by":"auto","created_at":"2025-09-08 10:24:12","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":82984,"visible":true,"origin":"","legend":"\u003cp\u003e(a) UV-Vis DRS spectra of prepared samples, (b) band gap widths\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-7369134/v1/51b459ebeddc930a03d12318.png"},{"id":90803424,"identity":"9f5587f3-8451-4953-aa2f-6fc22a8d481e","added_by":"auto","created_at":"2025-09-08 10:32:14","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":102248,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Photocatalytic degradation of MB by different photocatalysts under visible light, (b) quasi-primary rate constants of MB photodegradation over different photocatalysts, (c) results of the photocatalysts' recyclability test and (d) XRD pattern before and after reaction\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-7369134/v1/fd156154ef28e8b495a1d4de.png"},{"id":90802659,"identity":"dd677f7f-6d5b-4836-b1db-706655b1684e","added_by":"auto","created_at":"2025-09-08 10:24:11","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":63149,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Transient photocurrent response of CQDs, BiVO\u003csub\u003e4\u003c/sub\u003e and BiVO\u003csub\u003e4\u003c/sub\u003e/ CQDs-15 composites materials, (b) EIS Nyquist of CQDs, BiVO\u003csub\u003e4\u003c/sub\u003e and BiVO\u003csub\u003e4\u003c/sub\u003e/CQDs composites materials\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-7369134/v1/93d3635493766340922b2cbb.png"},{"id":90802674,"identity":"0cac5e1b-8323-4ce8-a5a9-63bed4a8a24d","added_by":"auto","created_at":"2025-09-08 10:24:12","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":34110,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic illustration for the photocatalytic\u003c/p\u003e\n\u003cp\u003emechanism of BiVO\u003csub\u003e4\u003c/sub\u003e/CQDs-15 composites materials\u003c/p\u003e","description":"","filename":"10.png","url":"https://assets-eu.researchsquare.com/files/rs-7369134/v1/7d1e4bfe9bad9f4413f1541e.png"},{"id":96650098,"identity":"7d605b38-c4bd-4d2e-b253-b61d00510c4e","added_by":"auto","created_at":"2025-11-24 16:07:27","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2433923,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7369134/v1/118c565f-47b9-40fa-8294-acf9a2c75e95.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"\u003cp\u003ePhotocatalytic degradation of MB by lignin carbon quantum dot modified BiVO\u003csub\u003e4\u003c/sub\u003e composites materials\u003c/p\u003e","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eAs industrialization advances globally, vast quantities of harmful pollutants are released into the environment. These emissions not only pose severe risks to public health but also inflict widespread damage on ecosystems\u003csup\u003e[\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]\u003c/sup\u003e. As a textile industry dye, methylene blue (MB) frequently contaminates aquatic environments, where it bioaccumulates in marine life. Chronic human contact induces multiple health risks, including cutaneous hypersensitivity reactions, hormonal system disturbances and impaired cardiac function\u003csup\u003e[\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]\u003c/sup\u003e. Given these risks, controlling anthropogenic organic pollutants becomes essential. Photocatalytic degradation emerges as an eco-conscious solution, enabling solar-powered purification while generating renewable energy. Photocatalytic processes typically employ semiconductor materials to degrade organic compounds into harmless CO₂ and H₂O through light absorption. Recent advances have yielded multiple efficient photocatalysts including TiO\u003csub\u003e2\u003c/sub\u003e, ZnO, CdS and ZrO\u003csub\u003e2\u003c/sub\u003e nanostructures. Current semiconductor photocatalysts predominantly exhibit wide energy bandgaps and rapid charge carrier recombination rates\u003csup\u003e[\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]\u003c/sup\u003e. These inherent properties significantly limit their UV-visible spectrum responsiveness and overall photon conversion efficiency\u003csup\u003e[\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]\u003c/sup\u003e. Effective photocatalysts require three key attributes: strong chemical inertness, affordable production costs and good suspension stability. These materials should additionally exhibit extensive spectral absorption and efficient electron-hole separation to optimize sunlight utilization\u003csup\u003e[\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eCarbon quantum dots (CQDs), alternatively called carbon nanodots or fluorescent carbon nanoparticles, represent zero-dimensional carbon nanomaterials typically under 10 nanometers. These particles exhibit remarkable luminescent characteristics, superior aqueous dispersibility, exceptional chemical stability, minimal biological toxicity, outstanding biocompatibility and negligible ecological effects\u003csup\u003e[\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]\u003c/sup\u003e. As a novel carbon-based material, carbon dots has a very excellent application range. After being first discovered by researchers in 2004, they have been extensive utilize in multifarious domain such as drug delivery\u003csup\u003e[\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]\u003c/sup\u003e, photocatalysis\u003csup\u003e[\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]\u003c/sup\u003e, biosensing\u003csup\u003e[\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]\u003c/sup\u003e, the preparation of light-emitting devices\u003csup\u003e[\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]\u003c/sup\u003e, and analytical testing\u003csup\u003e[\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eThe superior optoelectronic performance of CQDs enables substantial photocatalytic enhancement as they effectively harvest light energy and facilitate rapid electron-hole pair separation and transfer. The reaction conditions of CQDs will have a significant impact on their properties. The most common methods for preparing carbon quantum dots are two: the \"top-down\" method and the \"bottom-up\" method\u003csup\u003e[\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]\u003c/sup\u003e. Compared with the top-down method, the fluorescent carbon quantum dots synthesized through the bottom-up approach\u003csup\u003e[\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]\u003c/sup\u003e have the advantages of more precise preparation process and better controllability in the operation, for example, carbon quantum dots can rapidly and resultful compound by the hydrothermal method\u003csup\u003e[\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]\u003c/sup\u003e. The compound of carbon quantum dots by the hydrothermal method is not only simple, but also the sources of carbon are very diverse, such as lignin, ascorbic acid and citric acid, etc. Although the sources of carbon for preparing carbon quantum dots are diverse, some of them are expensive and some are even toxic. Therefore, finding low-cost and carbon-rich substances as carbon sources has become a research hotspot in recent years\u003csup\u003e[\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eAs the second most abundant aromatic polymer in nature, lignin is composed of cross-linked phenylpropane units to form a three-dimensional network structure, and its content is second only to cellulose. The mass fractions of lignin in vascular plants were in the order of coniferous wood\u0026thinsp;\u0026gt;\u0026thinsp;broadleaf wood\u0026thinsp;\u0026gt;\u0026thinsp;herbaceous plants and the specific values ranged from 27\u0026ndash;33%, 18\u0026ndash;25% and 17\u0026ndash;24%\u003csup\u003e[\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]\u003c/sup\u003e. Structural analysis of lignin monomers reveals that its cross-linked network is rich in active functional groups such as phenolic hydroxyl and carboxyl groups. This distinctive chemical composition not only endows it with excellent biocompatibility\u003csup\u003e[\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]\u003c/sup\u003e and high carbon content\u003csup\u003e[\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]\u003c/sup\u003e, but also makes it an ideal precursor for the preparation of carbon quantum dots\u003csup\u003e[\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]\u003c/sup\u003e. However, the global Industry for producing paper products generates over 50\u0026nbsp;million tons of lignose annually, with only about 2% being recovered and utilized. The indiscriminate massive emission of black liquor has led to severe ecological environment pollution while also resulting in significant resource waste. The valorization of lignin has emerged as a prominent research focus, with its application value in functional materials gradually being realized\u003csup\u003e[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eIn recent years, semiconducting nanomaterials have shown excellent potential for applications in environmental catalysis and energy conversion. Due to their unique energy band structure and surface active sites, these materials have been shown to be highly efficient in degrading organic pollutants (including difficult-to-biodegrade textile dyes and antibiotics) in water and in removing heavy metal ions by adsorption. More strikingly, these catalysts have excellent performance in light-driven reactions, which can simultaneously realize the photocatalytic reduction of CO₂ and hydrogen production from water molecule cleavage, two key processes\u003csup\u003e[\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]\u003c/sup\u003e. As a typical n-type semiconductor material, BiVO\u003csub\u003e4\u003c/sub\u003e shows unique advantages in visible photocatalysis. The material has the following remarkable features: firstly, its narrow bandgap of 2.4 eV enables it to utilize visible light efficiently; secondly, it exhibits excellent chemical stability and photocorrosion resistance; moreover, the easy availability of the raw materials and its environmentally friendly characteristics give it potential for practical applications\u003csup\u003e[\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]\u003c/sup\u003e. These properties make BiVO\u003csub\u003e4\u003c/sub\u003e an ideal material system for photocatalysis research. To address the inherent shortcomings of BiVO\u003csub\u003e4\u003c/sub\u003e and further enhance its photocatalytic performance, researchers have effectively enhanced the light response capability of BiVO\u003csub\u003e4\u003c/sub\u003e by doping ions \u003csup\u003e[\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]\u003c/sup\u003e. Research has shown that by constructing different semiconductor materials into heterojunctions, it is also possible to enhance the photocatalytic performance. The existing methods for modifying BiVO\u003csub\u003e4\u003c/sub\u003e have all shown limited improvement in the photocatalytic performance of BiVO\u003csub\u003e4\u003c/sub\u003e due to the large size of the material\u003csup\u003e[\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]\u003c/sup\u003e. This might be due to the presence of surface defects in the material during the preparation process. At these defects, electrons and holes recombine, thereby limiting the photocatalytic effect of the material\u003csup\u003e[\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]\u003c/sup\u003e. After research, it was found that using quantum dots, which are small-sized materials, can effectively solve the problem of defects formed during the synthesis process due to their uniform distribution characteristics\u003csup\u003e[\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]\u003c/sup\u003e. Carbon quantum dots can effectively enhance the photocatalytic performance of composite photocatalysts\u003csup\u003e[\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]\u003c/sup\u003e. Carbon quantum dots have attracted extensive attention in the field of photocatalysts due to their excellent optical properties. By incorporating carbon quantum dots to expand the light absorption range, this method can reduce the recombination of photoelectrons and holes. Carbon quantum dots can be combined with semiconductor photocatalysts to effectively enhance the performance of the photocatalyst\u003csup\u003e[\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]\u003c/sup\u003e. Luo et al.\u003csup\u003e[\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]\u003c/sup\u003e modified single-atom platinum on the surface of nitrogen-doped carbon dots by one-step photodeposition, and then combined it with TiO\u003csub\u003e2\u003c/sub\u003e, effectively enhancing the hydrogen production performance of the photocatalyst. Liu et al.\u003csup\u003e[\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]\u003c/sup\u003e prepared a Ru@carbon dot composite catalyst by combining ruthenium with carbon quantum dots. They utilized the defects of the carbon dots to prevent the aggregation of ruthenium. Wang et al.\u003csup\u003e[\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]\u003c/sup\u003e employed a simple pyrolysis reaction and successfully incorporated a single cobalt atom into carbon dots during the pyrolysis process of vitamin B\u003csub\u003e12\u003c/sub\u003e, thereby successfully preparing the CoSAS@CD composite photocatalytic material. In conclusion, the unique structure of carbon quantum dots and their abundant active sites enable them to effectively Quickly and effectively enhance the photocatalytic performance of the materials. By constructing a heterojunction using carbon quantum dots and BiVO\u003csub\u003e4\u003c/sub\u003e, the recombination of photogenerated electrons and holes in the photocatalyst can be effectively inhibited, thereby achieving the goal of improving the performance of the photocatalyst.\u003c/p\u003e\u003cp\u003eThis paper uses alkaline lignin as the carbon source and employs the hydrothermal method to manufacture carbon quantum dots. The carbon quantum dots are combined with BiVO\u003csub\u003e4\u003c/sub\u003e to prepare the BiVO\u003csub\u003e4\u003c/sub\u003e/CQDs composite materials. The structure of the BiVO\u003csub\u003e4\u003c/sub\u003e/CQDs composite materials are characterize by XRD, FT-IR, and TEM. The optical properties of the BiVO\u003csub\u003e4\u003c/sub\u003e/CQDs composite materials are characterized by UV-Vis. The BiVO\u003csub\u003e4\u003c/sub\u003e/CQDs composite materials with the optimal photocatalytic performance is prepared by changing the reaction conditions. It laid the research foundation for the efficient and high-value use of lignin.\u003c/p\u003e"},{"header":"2. Materials and Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\n \u003ch2\u003e2.1 Materials and reagents\u003c/h2\u003e\n \u003cp\u003eAlkali lignin (AL), Shanghai Yuanye Biotechnology Co., Ltd, analytically pure; ethylenediamine (EDA, 99%) and Methylene blue (MB, 96%) were purchased from Tianjin Zhiyuan Chemical Reagent Co. Hydrogen peroxide (H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e 30%), Xilong Chemical Co. Bismuth nitrate pentahydrate (Bi(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003e-5H\u003csub\u003e2\u003c/sub\u003eO), ammonium isocyanate (NH\u003csub\u003e4\u003c/sub\u003eVO\u003csub\u003e3\u003c/sub\u003e), urea (CO(NH\u003csub\u003e2\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e), ethanol (CH\u003csub\u003e3\u003c/sub\u003eCH\u003csub\u003e2\u003c/sub\u003eOH), dialysis bag, MD44-3500 Viskase Company, USA, concentrated nitric acid (H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e) All chemicals were pure and could be used without weiter refinement. Deionised water was used for all experiments.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\n \u003ch2\u003e2.2 Preparation methods\u003c/h2\u003e\n \u003cdiv id=\"Sec5\" class=\"Section3\"\u003e\n \u003ch2\u003e2.2.1 Preparation of Lignin Carbon Quantum Dots\u003c/h2\u003e\n \u003cp\u003eDissolve 0.5 g of alkaline lignin in 50 mL of distilled water, stir evenly at normal atmospheric temperature and ultrasonicate to ensure complete dissolution. Stir the uniformly dispersed solution continuously at normal atmospheric temperature, and add 30 mL of ethylenediamine and 10 mL H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e. The miscible liquids was added into a 100 mL high-pressure reactor with a polytetrafluoroethylene inner lining, and then heated continuously at 190℃ for 12 h. After allowing the high-pressure reaction vessel to cool down to room temperature, the solution should be removed. Use a sand core funnel to filter the solution three times to remove the large particles of impurities in the solution. Then, add the 3000 Da dialysis bag and immerse it in deionized water for 48 h. Change the deionized water every 8 h. After 48 h of dialysis, the carbon dot solution can be obtained.\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv id=\"Sec6\" class=\"Section3\"\u003e\n \u003ch2\u003e2.2.2 Preparation of BiVO\u003csub\u003e4\u003c/sub\u003e\u003c/h2\u003e\n \u003cp\u003eDissolve 12 mmol of bismuth nitrate pentahydrate in 64 mL (1 mol/L) of nitric acid solution. Stir continuously at normal atmospheric temperature for 30 min. After the stirring is completed, add 12 mmol of ammonium vanoxide and continue stirring for 1 h. After the stirring is finished, add 6.0 g of urea and stir continuously for 30 min. Add the above stirred solution to a 100 mL high-pressure reactor with polytetrafluoroethylene inner lining. Heat continuously at 180℃ for 12 h. The kettle was naturally cooled down to room temperature at the end of the reaction. Centrifugation (9000 r/min, 10 min) to separate the bright yellow powder, rinse with deionized water for 5 times and oven-dry at 60℃ for 24 h. BiVO\u003csub\u003e4\u003c/sub\u003e can be obtained.\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv id=\"Sec7\" class=\"Section3\"\u003e\n \u003ch2\u003e2.2.3 Preparation of BiVO\u003csub\u003e4\u003c/sub\u003e/CQDs\u003c/h2\u003e\n \u003cp\u003eBi(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003e-5H\u003csub\u003e2\u003c/sub\u003eO (12 mmol) was lysis in 64 mL of 1 mol/L HNO\u003csub\u003e3\u003c/sub\u003e and constant stirring for 30 min at normal atmospheric temperature and then 12 mmol of NH\u003csub\u003e4\u003c/sub\u003eVO\u003csub\u003e3\u003c/sub\u003e was added into the solution and constant stirring for 1 h. Add 6.0 grams of urea to the above solution and stir it rapidly at room temperature for 30 minutes. Different content of CQDs solutions were added to the aforementioned mixture, and then they were stirred continuously at a uniform speed for 30 min. After stirring, the solution was put into an ultrasonic machine (0℃) for ultrasonication. The aforementioned mixed solution was added to a 100 mL high-pressure reactor with a polytetrafluoroethylene inner lining, and was heated continuously at 10℃ for 12 h. The yellow solid powder was subjected to centrifugation, then washed with deionized water for 5 times, and dried at a constant temperature of 60℃ for 24 hours. In addition, BiVO\u003csub\u003e4\u003c/sub\u003e/CQDs composites materials materials with different CQDs co-catalyst contents could be gain by varying the volume of the CQDs solution (10, 15 and 20 mL) and the samples were noted as BiVO\u003csub\u003e4\u003c/sub\u003e/CQDs- x (x represents the volume of the CQDs solution).\u003c/p\u003e\n \u003c/div\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\n \u003ch2\u003e2.3 Materials characterization test\u003c/h2\u003e\n \u003cp\u003eThe surface shape of the composite materials was tested using transmission electron microscopes (TEM, HRTEM, JEOL and JEM-2100F). The crystal structure of the prepared samples was characterized by X-ray diffractometer (XRD, Brook D8 advanced X-ray diffractometer). The molecular structure of the prepared materials was characterized by FT-IR spectrometer (FT-IR, TENSOR II, Brook AXS GMBH, Germany). X-ray photoelectron spectroscopy (XPS, PHI-5702, Physical Electronics Company) was used to analyze the chemical composition of the prepared materials. Ultraviolet-visible diffuse reflectance spectroscopy (Shimazu UV-2450) was used to characterize the optical properties of the materials. Raman spectroscopy (DXR Micro Confocal Laser Raman Spectrometer, USA) was used to analyze the sample structure. The photocurrent and electrochemical impedance of the sample were measured by electrochemical workstation (CHI760E A17122).\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\n \u003ch2\u003e2.4 Photocatalytic degradation of MB performance test\u003c/h2\u003e\n \u003cp\u003eUsing a 300 W xenon lamp as the light source and filtering out the ultraviolet light component in the light through a 420 nm filter, with the photocatalytic degradation efficiency of methylene blue (MB) solution as the performance indicator, the photocatalytic performance of the BiVO\u003csub\u003e4\u003c/sub\u003e/CQDs composite catalysts was tested. Add 50 mg of the prepared photocatalyst to 50 mL of a MB solution with a concentration of 25 mg/L. Stir the solution for 30 min in the absence of light. During the period of no light stirring, the methylene blue solution and the photocatalyst reach an adsorption equilibrium state.\u003c/p\u003e\n \u003cp\u003eAfter dark treatment for 30 min, the light source was turned on for photocatalytic degradation. Samples were collected every 20 min. To remove the catalyst from the samples, the collected samples were centrifuged, and the supernatant was taken for testing. After testing, it was found that MB had the maximum absorbance at 550 nm, which was used to Calculate the degradation rate. The calculation method of degradation rate is as shown in formula (1):\u003c/p\u003e\n \u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\n \u003cdiv class=\"EquationNumber\"\u003e\u003cimg src=\"data:image/png;base64,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\"\u003e\u003c/div\u003e\n \u003c/div\u003e\n \u003cp\u003eA\u003csub\u003e0\u003c/sub\u003e represents the initial concentration of the MB dye, A is the concentration of the MB dye after being exposed to light, C\u003csub\u003e0\u003c/sub\u003e is the initial solubility of the MB dye, and C is the solubility of the MB dye after being exposed to light. To investigate the stability of the BiVO4/CQDs-15 composite material, the photocatalytic performance of the BiVO\u003csub\u003e4\u003c/sub\u003e/CQDs-15 composite materials was tested continuously for four cycles.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e\n \u003ch2\u003e2.5 Photoelectrochemical test\u003c/h2\u003e\n \u003cp\u003eElectrochemical tests were conducted using the three-electrode method. Add the photocatalyst to 2 mL of ethanol and perform ultrasonic treatment for 30 min. 20 \u0026micro;L of 0.5% Nafion were added using a dropper. 50 \u0026micro;L of the dispersed solution were aspirated using a dropper and dropped onto the conductive surface of 2.0\u0026times;1.0 cm\u003csup\u003e2\u003c/sup\u003e indium tin oxide (ITO) conductive glass. After dropping, the sample was dried in an oven and the above process was repeated 4\u0026ndash;5 times. The prepared materials were then subjected to transient photocurrent tests and electrochemical impedance tests.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"3. Results and Discussion","content":"\u003cp\u003eThe crystal structures of the CQDs, pure BiVO\u003csub\u003e4\u003c/sub\u003e and BiVO\u003csub\u003e4\u003c/sub\u003e/CQDs composite catalysts were characterized by X-ray diffraction (XRD). As shown in Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e(a) the diffraction pattern of CQDs shows typical amorphous carbon features: the broadened diffraction peak appearing at 20.3\u0026deg; corresponds to the (002) crystal plane of graphitic carbon, while the weak peak at 43.5\u0026deg; can be vest in to the (100) crystal plane\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e33\u003c/span\u003e]\u003c/sup\u003e. This diffraction feature confirms that the lignin carbon source has been successfully converted into amorphous carbon quantum dots. XRD analysis of Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e(b) shows that all BiVO\u003csub\u003e4\u003c/sub\u003e-containing samples display characteristic diffraction peaks at 18.98\u0026deg; (011), 28.87\u0026deg; (112), and 30.51\u0026deg; (004), these peaks are in the same position as the standard card for monoclinic phase BiVO\u003csub\u003e4\u003c/sub\u003e (JCPDS 14\u0026ndash;0688) in perfect agreement\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e34\u003c/span\u003e]\u003c/sup\u003e. It is remarkable that: (1) The absence of stray peaks outside the crystal phase of BiVO\u003csub\u003e4\u003c/sub\u003e in the diffraction patterns of the composites suggests that the composite process did not change the intrinsic crystalline structure of BiVO\u003csub\u003e4\u003c/sub\u003e; (2) No obvious carbon characteristic peaks were detected in all samples, which may be due to the fact that the low loading of carbon quantum dots poor crystallinity of carbon quantum dot. The carbon features are masked by the strong BiVO\u003csub\u003e4\u003c/sub\u003e diffraction peaks\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e35\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eThe FT-IR spectral analysis of BiVO\u003csub\u003e4\u003c/sub\u003e, CQDs and BiVO\u003csub\u003e4\u003c/sub\u003e/CQDs-15 composites materials is shown in Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e, where the main structural features of each component can be clearly observed. The absorption peak at 3258 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e is caused by the extension vibration of water molecules O-H radical on the surface of the CQDs and BiVO\u003csub\u003e4\u003c/sub\u003e/CQDs-15 composite materials. The characteristic O-H peak at 1614 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and the C-O-C and C-O\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e36\u003c/span\u003e]\u003c/sup\u003e. vibrational peaks at 1370 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 1240 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e together verify the modification of the CQD surface by oxygen-containing functional groups. In particular, the characteristic absorption peak of BiVO\u003csub\u003e4\u003c/sub\u003e (VO₄\u0026sup3;-vibration at 736 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and all the characteristic peaks of CQDs were retained in the composite. This result not only confirms the successful preparation of the composites, but also indicates that the surface functionalization properties of the CQDs were completely preserved during the composite process. These structural features provide an important molecular level basis for understanding the photocatalytic properties of the composites.\u003c/p\u003e\n\u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e(a) shows that the BiVO\u003csub\u003e4\u003c/sub\u003e crystal exhibits a one-sided single crystal shape with a regular decahedral appearance. Its structural characteristics are as follows: (1) The surface is smooth and defect-free; (2) Clear edges; (3) Uniform size distribution (0.9\u0026ndash;1.5\u0026micro;m\u0026times;0.38\u0026micro;m). After the introduction of CQDs Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eb) the composite material undergoes changes: (1) Passivation of crystal edges and corners; (2) The tendency of particle agglomeration is enhanced. This structural change mainly stems from: (1) The surface modification effect of CQDs; (2) Crystal growth regulation caused by interfacial interactions; (3) Spatial constraints at the nanoscale.\u003c/p\u003e\n\u003cp\u003eTransmission electron microscopy analysis revealed the fine structural characteristics of carbon quantum dots (CQDs) and their composites Fig. (4). As shown in Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e(a, b) the CQDs are mainly spherical nanoparticles (1-5.5nm), concentrated in 2\u0026ndash;4 nm with good dispersion and there are a few large particles present, resulting from high surface energy agglomeration caused by the small size effect. HRTEM confirmed that the 0.26 nm lattice spacing corresponds to the graphite (002) crystal plane, indicating a highly graphitized structure. Particle size statistics show that it is mainly distributed in the range of 2\u0026ndash;4 nm (accounting for more than 60%). Analysis in Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e (c) reveals that pure BiVO\u003csub\u003e4\u003c/sub\u003e presents a bulk morphology, while in the composite material Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e (d), CQDs(\u0026lt;\u0026thinsp;10nm) are uniformly anchored on the surface of BiVO\u003csub\u003e4\u003c/sub\u003e, forming a tight interface contact\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e37\u003c/span\u003e]\u003c/sup\u003e. These results confirm: (1) The successful preparation of highly crystalline CQDs; (2) Achieve uniform compounding at the nanoscale; (3) Provide a structural basis for performance optimization.\u003c/p\u003e\n\u003cp\u003eThe elements and chemical states contained in the BiVO\u003csub\u003e4\u003c/sub\u003e/CQDs-15 composite materials were analyzed in detail by X-ray photoelectron spectroscopy (XPS), as shown in Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e. Full-spectrum analysis confirmed that the material contained only Bi, V, C, O and trace N elements. As can be seen from Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e(a), the 284.6 eV value corresponds to the C1s peak. Fine spectral analysis showed that in Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e (b) the Bi 4f spectrum presented characteristic bipeaks at 158.5 eV (4f₇/₂), 163.9 eV (4f₅/₂) and the splitting at 5.3 eV confirmed the presence of the trivalent state Bi\u003csup\u003e3+\u003c/sup\u003e of monoclinic BiVO\u003csub\u003e4\u003c/sub\u003e. As shown in Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e(c), the peak positions of the V 2p spectrum at 516.0 eV (2p₃/₂) and 523.5 eV (2p₁/₂) are consistent with those of the standard BiVO4. Figure \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e(d) O-1s spectrum can be decomposed into lattice oxygen (529.5 eV) and adsorbed oxygen (531.5 eV). As can be seen from Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e(e), C1s consists of three types of chemical bonds, namely 284.6 electron volts (C-C), 285.9 electron volts (C-O-C), and 287.97 electron volts (O-C\u0026thinsp;=\u0026thinsp;O). The peak of 399.26 eV in the n-1s spectrogram Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e(f) belongs to pyrrole nitrogen\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e26\u003c/span\u003e]\u003c/sup\u003e. These results fully confirm the successful preparation of the composite materials.\u003c/p\u003e\n\u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e reveals the structural characteristics of CQDs and BiVO\u003csub\u003e4\u003c/sub\u003e/CQDs-15 composites materials by Raman spectroscopy analysis. As shown in the small figure of Fig. \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e. The characteristic peaks of CQDs at 1350 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (D band, unordered vibration of sp\u0026sup3; carbon) and 1581 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (G band, sequential vibration of sp\u0026sup2; carbon) are presented\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e28\u003c/span\u003e]\u003c/sup\u003e and the ID/IG ratio is 0.85, indicating the presence of an appropriate amount of structural defects\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e38\u003c/span\u003e]\u003c/sup\u003e. The Raman spectra of BiVO\u003csub\u003e4\u003c/sub\u003e show characteristic peaks such as 858 cm⁻\u0026sup1; (symmetrical stretching of VO), 763 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (asymmetrical stretching), 370 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (symmetrical bending of \u0026delta;s) and 250 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (external vibration mode). At the same time, the following points can be observed: (1) No peak position shift was observed in the composite material, confirming that CQDs only deposited on the surface and did not enter the lattice; (2) Because the presence of carbon quantum dots enhances the Raman scattering effect, the intensity of the peaks increases. These results verified the non-destructive modification of CQDs on the surface of BiVO\u003csub\u003e4\u003c/sub\u003e.\u003c/p\u003e\n\u003cp\u003eAs shown in Fig. \u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003e, the optical response characteristics of the BiVO\u003csub\u003e4\u003c/sub\u003e/CQDs composite materials were investigated by using the ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS) method. The test results show that pure BiVO\u003csub\u003e4\u003c/sub\u003e has a characteristic absorption edge at 515 nm, corresponding to its direct bandgap transition. Compared with single-component materials, BiVO\u003csub\u003e4\u003c/sub\u003e/CQDs composites materials exhibit significant improvements in optical properties: (1) Absorption edge redshift; (2) The absorbance in the visible light region has increased by 40%; (3) The absorption range has been significantly expanded. This enhancing effect mainly stems from the sensitization effect of CQDs. Spectral analysis confirmed that the composite material has a superior light capture ability, which is directly related to its enhanced photocatalytic activity.\u003c/p\u003e\n\u003cp\u003eThe band gap calculation is as shown in formula (2):\u003c/p\u003e\n\u003cp\u003e\u003cimg src=\"data:image/png;base64,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\"\u003e\u003c/p\u003e\n\u003cp\u003eAs shown in Fig. \u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003e(b), the band gap widths of the BiVO4, BiVO\u003csub\u003e4\u003c/sub\u003e/CQDs-10, BiVO\u003csub\u003e4\u003c/sub\u003e/CQDs-15, and BiVO\u003csub\u003e4\u003c/sub\u003e/CQDs-20 composite materials were calculated to be 2.45, 2.36, 2.31 and 2.30 eV respectively. The calculation results indicate that by doping CQDs and preparing BiVO\u003csub\u003e4\u003c/sub\u003e/CQDs composite materials, the band gap width of the composite materials can be reduced.\u003c/p\u003e\n\u003cp\u003eAs shown in Fig. \u003cspan class=\"InternalRef\"\u003e8\u003c/span\u003e(a), the degradation performance of BiVO4/CQDs composite materials for methyl blue varies with the amount of CQDs doped. After being exposed to light for 120 min, the degradation result of methylene blue by pure BiVO\u003csub\u003e4\u003c/sub\u003e composite materials are only 56.2%, while the degradation rates of BiVO\u003csub\u003e4\u003c/sub\u003e/CQDs-10, BiVO\u003csub\u003e4\u003c/sub\u003e/CQDs-15, and BiVO\u003csub\u003e4\u003c/sub\u003e/CQDs-20 composite materials for methyl blue are as high as 86.9%, 99.7%, and 74.6% respectively. Thus, it can be concluded that when the CQDs doping amount is 15 mL, the prepared BiVO\u003csub\u003e4\u003c/sub\u003e/CQDs-15 composite materials has the highest catalytic active degradation efficiency for methylene blue. In the composite material, CQDs act as an effective electron acceptor, promoting the efficient separation of photogenerated electrons and holes in BiVO\u003csub\u003e4\u003c/sub\u003e, achieving the purpose of improving the BiVO\u003csub\u003e4\u003c/sub\u003e/CQDs composite materials. In order to further study the reaction kinetics of BiVO\u003csub\u003e4\u003c/sub\u003e/CQDS composite materials for MB photocatalytic degradation, the zero-order model was used to simulate the experimental data: ln(C/C\u003csub\u003e0\u003c/sub\u003e)\u0026thinsp;=\u0026thinsp;kt\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e39\u003c/span\u003e]\u003c/sup\u003e. As shown in Fig. \u003cspan class=\"InternalRef\"\u003e8\u003c/span\u003e(b), the apparent rate constant of BiVO\u003csub\u003e4\u003c/sub\u003e/CQDs-15 composite materials was 0.0178 min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, which was 2.82, 9.36, 2.78 and 1.97 times higher than that of pure BiVO\u003csub\u003e4\u003c/sub\u003e, CQDs, BiVO\u003csub\u003e4\u003c/sub\u003e/CQDs-10 and BiVO\u003csub\u003e4\u003c/sub\u003e/CQDs-20 composite materials, respectively. The results indicate that the appropriate CQDs loading is conducive to improving the photocatalytic performance of BiVO\u003csub\u003e4\u003c/sub\u003e/CQDs composites materials. It is an important indicator for evaluating the stability of photocatalysis when it can be reused. As shown in Fig. \u003cspan class=\"InternalRef\"\u003e8\u003c/span\u003e(c), to verify the stability of the BiVO\u003csub\u003e4\u003c/sub\u003e/CQDs-15 composite materials, four consecutive photocatalytic cycling tests were conducted.\u003c/p\u003e\n\u003cp\u003eFour cycling experiments were conducted. The experimental results show that in the first two cycles of the experiments, the degradation rate of MB reached 90% each time. However, in the latter two tests, the degradation effect of MB decreased to varying degrees. The trend showed that MB completely degradation with increasing time. This indicates that the BiVO\u003csub\u003e4\u003c/sub\u003e/CQDs-15 composites materials have good photocatalytic stable, which is also favourable for their apply in photocatalysis. Figure \u003cspan class=\"InternalRef\"\u003e8\u003c/span\u003e(d) shows the XRD comparison before and after the photocatalytic degradation of MB by BiVO\u003csub\u003e4\u003c/sub\u003e-CQDs-15. It can be found that there is no change, which proves that BiVO\u003csub\u003e4\u003c/sub\u003e-CQDs-15composite materials has excellent stability\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e40\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eIn Fig. \u003cspan class=\"InternalRef\"\u003e9\u003c/span\u003e, the photoelectrochemical test reveals the excellent charge separation performance of the composite material. Figure \u003cspan class=\"InternalRef\"\u003e9\u003c/span\u003e (a) shows that the photocurrent response test demonstrates that the photocurrent density of BiVO\u003csub\u003e4\u003c/sub\u003e/ CQDs-15 composite materials are 3.2 times higher than that of pure BiVO\u003csub\u003e4\u003c/sub\u003e. This is mainly due to the introduction of CQDs significantly enhancing visible light absorption and the two-phase interface promoting the efficient separation of photogenerated carriers. Figure \u003cspan class=\"InternalRef\"\u003e9\u003c/span\u003e(b) further confirms this conclusion through electrochemical impedance spectroscopy (EIS) analysis. The smaller arc radius in the figure indicates that BiVO\u003csub\u003e4\u003c/sub\u003e/CQDs-15 composite materials has a lower charge transfer resistance, a faster interinterface electron transport rate and a higher carrier separation efficiency. It is worth noting that the conclusions drawn from the two test methods are highly consistent jointly confirming that the loading of CQDs promotes the separation efficiency of photogenerated carriers in the material.\u003c/p\u003e\n\u003cp\u003eBased on the above experimental results, a mechanism for the enhanced photocatalytic performance of BiVO\u003csub\u003e4\u003c/sub\u003e/ CQDs-15 composites materials are proposed as shown in Fig. \u003cspan class=\"InternalRef\"\u003e10\u003c/span\u003e. Under imitation solar illumination, CQDs can assimilat light beyond the absorption region of BiVO\u003csub\u003e4\u003c/sub\u003e and convert it to shorter wavelengths, which activates BiVO\u003csub\u003e4\u003c/sub\u003e to produce electron-hole pairs. As a result, solar energy is collected more efficiently. On the other hand, the CQDs act as electron storage layers that can trap electrons divergency by BiVO\u003csub\u003e4\u003c/sub\u003e, thereby inhibiting the compounding of BiVO\u003csub\u003e4\u003c/sub\u003e and prolonging the lifetime of the BiVO\u003csub\u003e4\u003c/sub\u003e photogenerated holes. CQDs can facilitate electron transfer and react with the O\u003csub\u003e2\u003c/sub\u003e adsorbed during the reaction process, thereby generating highly reactive superoxide radicals (\u0026middot;O\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e).The superoxide free radicals produced during the reaction are strong oxidants, which can effectively oxidize the pollutants adsorbed on the surface into small molecule substances\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e40\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e"},{"header":"4. Conclusions","content":"\u003cp\u003eIn this paper, the photocatalysts of BiVO\u003csub\u003e4\u003c/sub\u003e/CQDs composites materials were prepared by one-step hydrothermal method, and the photocatalytic degradation test showed that the incorporation of CQDs improved the photodegradation performance of MB dyes. According to the results of PC and EIS measurements, the introduction of CQDS promotes charge separate and absorb of visible light, which increases the photocurrent and reduces the band gap of BiVO\u003csub\u003e4\u003c/sub\u003e/CQD composites materials. Among them, BiVO\u003csub\u003e4\u003c/sub\u003e/CQDs-15 composites materials had the best photodegradation effect, and the degradation rate was increased to 99.7%. Cycle testing shows that the material is reusable and stable. In addition, the photocatalytic degradation efficiency was improved by constructing BiVO\u003csub\u003e4\u003c/sub\u003e/ CQDs-15 composites materials. The use of lignin to prepare CQDs has made lignin a high-value utilization.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis study was financially supported by the National Natural Science Foundation of China (22278099). Key project of Regional Innovation and Development Joint Fund of National Natural Science Foundation (U23A20135).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests policy\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing inter ests\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthics declaration\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Contributions Statement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eIn this article, Chenghan Li is responsible for analyzing the data generated by the experiment and making charts. Shanshan Wang is responsible for the operation of the experiment and the writing of the draft. Yiping Li is responsible for analyzing the data generated by the experiment and making charts. Oxana P. Taran is responsible for providing ideas for the article and solving some theoretical problems. Fubao Sun was responsible for providing research ideas for this article and resolving some writing grammar issues. Hong Yan is responsible for checking and revising articles, and providing financial support and daily supervision. Fen Li is responsible for daily supervision and supervision, and checking and revising articles. All the authors participated in the review of the manuscript.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eAbd Rani U, Ng LY, Ng YS, Ng CY, et al. (2022) Photocatalytic degradation of methyl green dye by nitrogen-doped carbon quantum dots: optimisation study by Taguchi approach. Mater. Sci. Eng, B 283: 115820. \u003cstrong\u003ehttps://doi.org/10.1016/j.mseb.2022.115820\u003c/strong\u003e\u003c/li\u003e\n \u003cli\u003eNajjar M, Nasserib 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. 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Environmental Science and Pollution Research 30(10): 27570-27582. \u003cstrong\u003ehttps://doi.org/10.1007/s11356-022-24184-1\u003c/strong\u003e\u003c/li\u003e\n\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":"journal-of-nanoparticle-research","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"nano","sideBox":"Learn more about [Journal of Nanoparticle Research](http://link.springer.com/journal/11051)","snPcode":"11051","submissionUrl":"https://submission.nature.com/new-submission/11051/3","title":"Journal of Nanoparticle Research","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Alkali lignin, Carbon quantum dots, BiVO4, Photocatalysis","lastPublishedDoi":"10.21203/rs.3.rs-7369134/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7369134/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eMethylene blue is a significant pollutant that seriously threatens the safety of water environments, and traditional methods have difficulty in removing it. In this study, lignin carbon quantum dots (CQDs) were prepared by hydrothermal method, and a BiVO\u003csub\u003e4\u003c/sub\u003e/CQDs composite materials was fabricated for the photocatalytic degradation of methylene blue. Through various testing methods, the optimal doping amount of carbon quantum dots was successfully studied. The doping of carbon quantum dots reduced the band gap of BiVO\u003csub\u003e4\u003c/sub\u003e and enhanced the light absorption and electron transfer ability of the BiVO\u003csub\u003e4\u003c/sub\u003e/CQDs composite materials. Among them, the BiVO\u003csub\u003e4\u003c/sub\u003e/CQDs-15 composite materials exhibited the best photodegradation effect. Under visible light irradiation, 99.7% of the methylene blue was degraded within just 120 min. The enhancement of photocatalytic activity is attributed to the highly dispersed nature of CQDs on the surface of BiVO\u003csub\u003e4\u003c/sub\u003e, their upconversion property, excellent electron transfer ability, and the synergistic effect on the photocatalytic performance of BiVO\u003csub\u003e4\u003c/sub\u003e.\u003c/p\u003e","manuscriptTitle":"Photocatalytic degradation of MB by lignin carbon quantum dot modified BiVO4 composites materials","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-09-08 10:23:30","doi":"10.21203/rs.3.rs-7369134/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-09-14T15:12:37+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-09-11T11:48:38+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-09-08T10:37:03+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"167683153266336735218032076342039123089","date":"2025-09-03T00:16:53+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"10874408440108371475110109007093743142","date":"2025-09-02T16:32:12+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-09-02T16:18:22+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-08-18T20:34:59+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-08-18T12:20:22+00:00","index":"","fulltext":""},{"type":"submitted","content":"Journal of Nanoparticle Research","date":"2025-08-14T02:37:48+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"journal-of-nanoparticle-research","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"nano","sideBox":"Learn more about [Journal of Nanoparticle Research](http://link.springer.com/journal/11051)","snPcode":"11051","submissionUrl":"https://submission.nature.com/new-submission/11051/3","title":"Journal of Nanoparticle Research","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"b5764d14-bba4-435c-bf99-fa674f95aaf7","owner":[],"postedDate":"September 8th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2025-11-24T16:01:00+00:00","versionOfRecord":{"articleIdentity":"rs-7369134","link":"https://doi.org/10.1007/s11051-025-06480-2","journal":{"identity":"journal-of-nanoparticle-research","isVorOnly":false,"title":"Journal of Nanoparticle Research"},"publishedOn":"2025-11-20 15:57:35","publishedOnDateReadable":"November 20th, 2025"},"versionCreatedAt":"2025-09-08 10:23:30","video":"","vorDoi":"10.1007/s11051-025-06480-2","vorDoiUrl":"https://doi.org/10.1007/s11051-025-06480-2","workflowStages":[]},"version":"v1","identity":"rs-7369134","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7369134","identity":"rs-7369134","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}