Introduction

In recent decades, the world has witnessed an unprecedented increase in environmental pollution due to rapid industrialization, urban development, and unsustainable waste disposal practices. Pollutants, especially toxic organic dyes and chemicals, discharged from textile, pharmaceutical, and agricultural industries, pose a severe threat to aquatic ecosystems, biodiversity, and human health. These compounds are not only carcinogenic and mutagenic but also resistant to natural degradation processes, necessitating the development of efficient, cost-effective, and eco-friendly strategies for their removal1. Among the various technologies employed for environmental remediation, photocatalysis has emerged as one of the most promising and sustainable approaches. It leverages light energy typically from sunlight to activate semiconductor materials that degrade pollutants into harmless end products such as CO₂ and H₂O2,3,4.

Various semiconductor-based photocatalysts, including Co3O4, TiO₂, ZnO, WO₃, SnO₂, and g-C₃N₄, have been extensively explored for photocatalytic degradation of pollutants5,6,7,8,9. Among these, ZnO is particularly attractive due to its high photosensitivity, non-toxicity, chemical stability, abundance, and ease of synthesis10,11. However, its wide bandgap (~ 3.3 eV) restricts light absorption primarily to the UV region, which constitutes only ~ 5% of the solar spectrum12,13. Additionally, the rapid recombination of photogenerated electron–hole pairs reduce its photocatalytic efficiency. To address these limitations, researchers have explored the design of heterostructured NCs, especially those incorporating carbon-based materials such as graphene oxide (GO) or reduced graphene oxide (rGO), which act as effective electron sinks and charge transporters14,15,16,17.

Graphene oxide in particular, has attracted attention as a valuable cocatalyst owing to its excellent electrical conductivity, large specific surface area, and tunable surface chemistry18,19,20. When coupled with ZnO, GO not only enhances the separation of photogenerated charges but also broadens the light absorption spectrum and facilitates pollutant adsorption. The synergistic interaction between ZnO and GO improves the photocatalytic performance of the composite under visible light, making ZnO/rGO NCs ideal candidates for wastewater treatment and environmental remediation21,22,23.

Traditionally, ZnO/rGO NCs have been synthesized using physical and chemical techniques that often involve toxic solvents, harsh reducing agents, and complex processes24,25. In contrast, bioinspired or green synthesis approaches provide a sustainable, cost-effective, and environmentally benign alternative26,27. These techniques use natural reducing and stabilizing agents from biological resources such as plants, algae, bacteria, fungi, and waste biomass to synthesize nanomaterials (NMs) under mild conditions28,29. In particular, plant-mediated synthesis has gained momentum due to its simplicity, scalability, and the rich diversity of phytochemicals including flavonoids, alkaloids, terpenoids, saponins, phenolic acids, and glycosides which can reduce metal ions and stabilize the resulting nanoparticles (NPs)30,31.

Among the diverse medicinal plants, Conyza bonariensis, a member of the Asteraceae family, stands out for its pharmacological properties and wide geographic distribution. It is a fast-growing annual to biennial herb found in tropical and temperate regions across Asia, Africa, Europe, and the Americas32,33. Traditionally, it has been used for treating ailments such as rheumatism, nephritis, and arthritis. Phytochemical analyses reveal that this plant contains a rich profile of secondary metabolites that can act as natural reducing, capping, and stabilizing agents, making it an ideal candidate for bioinspired NPs synthesis34,35,36. Despite its widespread availability and medicinal importance, no studies to date have reported the use of Conyza bonariensis for the synthesis of ZnO/rGO NCs, marking a significant gap in the literature.

The present study aims to bridge this gap by reporting, for the first time, the green synthesis of ZnO/rGO NCs using Conyza bonariensis leaf extract. The plant extract serves a dual function as both a reducing and stabilizing agent facilitating the conversion of zinc salts to ZnO and simultaneously reducing GO to rGO. This eco-friendly approach eliminates the need for harmful chemicals, aligns with green chemistry principles, and promotes sustainable nanotechnology. The synthesized NCs were comprehensively characterized using advanced analytical techniques, including XRD, FT-IR, EDS, UV-DRS, XPS, BET, SEM, TEM, and AFM, to investigate their structural, morphological, and optical properties. The photocatalytic performance of the bioinspired ZnO/rGO NCs was evaluated against CR, MB and TB dye under visible-light irradiation. The results demonstrate an excellent degradation efficiency, outperforming bare ZnO, which is attributed to enhanced light harvesting, reduced bandgap, and improved charge separation facilitated by rGO. This work not only introduces a novel green route for NCs fabrication but also reinforces the potential of bioinspired ZnO/rGO NCs for practical environmental applications.

Materials and methods

Materials

Zinc acetate (Zn (CH3CO2)2) was purchased from Himedia Laboratories Ltd. India (purity ≥ 98%). Leaves of Conyza bonariensis plant was collected from the Nagarjun Forest, Ramtek (Nagpur), Maharashtra, India. The methylene blue, congo red, and thymol blue were purchased from Himedia Laboratories Ltd. India (purity ≥ 98%), and Fischer Scientific India Pvt. Ltd. (purity ≥ 98%) respectively.

Preparation of plant extracts

Fresh and healthy leaves of Conyza bonariensis were systematically collected from diverse locations within the ecologically rich Nagarjun Forest situated in Ramtek, Nagpur District, Maharashtra, India. To preserve their phytochemical integrity, the harvested leaves were carefully shade-dried for a period of three days under ambient conditions, away from direct sunlight to prevent the degradation of sensitive bioactive compounds. Once adequately dried, the leaves were mechanically ground using an electric grinder into a uniform, fine powder to increase the surface area for efficient extraction of phytoconstituents. For the preparation of the aqueous leaf extract, approximately 10 g of the powdered leaf material was accurately measured and transferred into a 250 mL borosilicate conical flask containing 150 mL of deionized water. The mixture was subjected to gentle refluxing on a magnetic stirrer hot plate for 10 min to facilitate the extraction of various bioactive compounds such as flavonoids, terpenoids, saponins, and polyphenols from the plant matrix into the aqueous medium (Fig. 1). After the heating process, the resulting extract was allowed to cool to room temperature, and then centrifuged at 4000 rpm for 10 min to effectively separate the coarse plant residues and solid debris from the clear supernatant. The clarified supernatant, rich in phytochemicals, was carefully decanted and stored in a clean, airtight container at 4 °C under refrigeration conditions to preserve its activity and prevent microbial contamination. This freshly prepared plant extract was later used as a natural reducing and stabilizing agent in the green synthesis of ZnO-rGO NCs.

Conyza bonariensis-mediated ZnO-rGO NCs

The ZnO-rGO NCs were synthesized via a green, eco-friendly route using the leaf extract of Conyza bonariensis as a natural reducing and stabilizing agent (Fig. 1). Initially, 15 mg of GO powder was accurately weighed and dispersed in 20 mL of deionized water contained in a 250 mL borosilicate glass beaker. The mixture was subjected to magnetic stirring for 1 h to achieve a homogenous exfoliated graphene oxide (EGO) suspension, which ensured better dispersion and prevented aggregation of GO sheets. Following this, 100 mL of a 0.1 M zinc acetate solution (equivalent to 1.834 g of zinc acetate dihydrate) was gradually added to the EGO suspension under constant stirring to promote uniform mixing of the precursors. Subsequently, 30 mL of freshly prepared Conyza bonariensis leaf extract was introduced dropwise into the reaction mixture, providing natural phytochemicals such as flavonoids, phenolics, and terpenoids that functioned as reducing and capping agents. The resultant mixture was stirred continuously for an additional 20 min to ensure proper interaction between the extract and metal precursors. The entire reaction mixture was then transferred to a 100 mL Teflon-lined stainless-steel autoclave reactor and subjected to hydrothermal treatment at 200 °C for 8 h. This step facilitated the in-situ reduction of GO to rGO and the nucleation and growth of ZnO NPs on the rGO surface, resulting in the formation of ZnO-rGO NCs with improved crystallinity and stability. After completion of the hydrothermal process, the system was allowed to cool naturally to room temperature. The resulting solid precipitate was collected by centrifugation and thoroughly washed several times with distilled water, absolute ethanol, and acetone to remove unreacted precursors, excess plant extract, and other organic residues. The cleaned product was then dried in a vacuum oven at 120 °C for 24 h to obtain the final ZnO-rGO NCs. The synthesized NCs were subsequently subjected to a comprehensive set of characterization techniques. These analyses were performed to authenticate and evaluate the crystalline phase, functional groups, oxidation states, morphology, porosity, surface area, particle size, and distribution, as well as to confirm the successful formation and integration of ZnO NPs with the rGO sheets.

Fig. 1
figure 1

Conyza bonariensis-mediated phytosynthesis of ZnO-rGO NCs.

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Mechanism for formation of ZnO–rGO NCs

The possible mechanism for the formation of ZnO–rGO NCs using Conyza bonariensis leaf extract involves a green synthesis approach in which the plant extract plays a dual role as a reducing and stabilizing agent (Fig. 2). Initially, GO is exfoliated in deionized water through ultrasonication to obtain a uniform suspension of EGO sheets. Zinc acetate is then introduced into this suspension, providing Zn²⁺ ions. Upon the addition of Conyza bonariensis leaf extract, which is rich in phytochemicals such as flavonoids (FL), phenolic (PH) compounds, terpenoids (TR), and saponins (SP), two key processes occur simultaneously: the reduction of GO to rGO and the nucleation and growth of ZnO NPs from Zn²⁺ ions. The oxygen-containing functional groups on the rGO sheets facilitate the anchoring of ZnO NPs through electrostatic and coordination interactions. The entire reaction mixture is then subjected to hydrothermal treatment at 200 °C for 8 h in a stainless-steel autoclave, which enhances the crystallinity of ZnO and promotes strong interfacial contact between ZnO and the rGO matrix. After cooling, the resulting product is thoroughly washed and dried to yield the ZnO–rGO NCs, characterized by well-dispersed ZnO NPs uniformly distributed across the rGO sheets. This bio-inspired synthesis method not only eliminates the need for hazardous chemicals but also offers an eco-friendly and efficient route for preparing high-performance photocatalytic NCs.

Fig. 2
figure 2

Mechanism of formation of the ZnO-rGO NCs using Conyza bonariensis leaf extract.

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Characterization techniques

X-ray diffraction (XRD) performed on Bruker (AXS D8 Advance) diffractometer using Cu Kα radiation at λ = 0.154 nm, while Fourier transform infrared (FT-IR) were recorded in the range 400–4000 cm−1 on a Bruker (IFS 66v) spectrophotometer. The qualitative elemental analysis was performed using energy dispersive spectroscopy (EDS; Oxford Instrument), as well as X-ray photoelectron spectroscopy (ESCALAB 250 XPS, Al Ka (150 W)) system was used for chemical state identification. UV-visible diffuse reflectance spectra were recorded on (UV-DRS; Cary-100UV) spectrophotometer, while Raman spectroscopy was performed using a JY Horiba (HR-800) spectrophotometer. Transmission electron microscopy (TEM: JEOL-JEM 100SX) were used to study the surface morphology. Micromeritic ASAP 2010 BET analyser was used for surface area and porosity determination.

Results and discussion

Structural investigation of ZnO-rGO NCs

Initially, the samples were analysed by X-ray diffraction (XRD) to determine their crystal structure and phase composition. The XRD pattern of the pure ZnO nanoparticles displayed sharp and intense diffraction peaks at 2θ values of 31.6°, 34.5°, 36.1°, 47.5°, 56.5°, 62.9°, and 67.8°, corresponding to the (100), (002), (101), (102), (110), (103), and (112) planes, respectively (Fig. 3a). These peaks are in good agreement with the standard hexagonal wurtzite structure of ZnO (JCPDS card no. 36–1451), confirming the high crystallinity of the synthesized ZnO NPs37. The lattice parameters ‘a’ and ‘c’ were computed as 3.278 Å and 5.2103 Å, respectively, which matched with the literature38. The ratio of the lattice parameters “c/a” was equal to 1.589 which matched well with the ZnO NPs synthesized using the flower extract of Aspalathus linearis39. The lattice constants were determined using the standard equations for hexagonal systems.

$$\:\frac{1}{{d}^{2}}=\frac{4}{3}\:\left(\frac{{h}^{2}+hk+\:{k}^{2}}{{a}^{2}}\right)+\:\frac{{l}^{2}}{{c}^{2}}$$

The XRD pattern of the rGO exhibited broad and low-intensity peaks around 25.2° and 28.1°, which can be attributed to the (002) and (004) diffraction planes of graphitic carbon. These features indicate the partial reduction and restacking of graphene oxide sheets during the synthesis process. In the case of the ZnO/rGO NCs, the XRD pattern retained the prominent peaks of ZnO, indicating that its crystalline structure remained intact after compositing with rGO. However, a noticeable decrease in the intensity and broadening of the rGO peaks was observed, suggesting good dispersion of ZnO NPs over the rGO sheets and a relatively lower content of rGO in the composite. The absence of any extra peaks further confirms the purity of the ZnO/rGO NCs and the successful formation of a two-phase composite without the formation of any secondary phases. The crystallite size of ZnO in the NCs was estimated using the Debye–Scherrer equation:

$$\:D=\:\frac{K\lambda\:}{\beta\:cos\theta\:}$$

where D is the crystallite size, λ is the X-ray wavelength (0.154 nm for Cu Kα), β is the full width at half maximum (FWHM) of the most intense peak (in radians), and θ is the corresponding Bragg angle. Based on the (101) plane at 2θ = 36.1°, the average crystallite size was calculated to be approximately 17.22 nm. This relatively small crystallite size favours enhanced surface reactivity and charge transfer, which is advantageous for photocatalytic applications.

To investigate the chemical bonding and functional groups present in the synthesized materials, FT-IR spectroscopy was performed on ZnO/rGO NCs. As shown in Fig. 3b, the characteristic absorption peaks at 642 cm⁻¹ and 867 cm⁻¹ are attributed to the stretching vibrational modes of Zn–O bonds, confirming the presence of ZnO NPs within the composite structure. A broad absorption band observed around 3372 cm⁻¹ corresponds to the O–H stretching vibrations of hydroxyl groups, which may originate from adsorbed water molecules or surface –OH groups on rGO or ZnO surfaces. Furthermore, the peak at 2717 cm⁻¹ is associated with the C–H stretching vibrations, while the band at 1387 cm⁻¹ can be assigned to the bending vibrations of H–O–H, indicating residual moisture or water molecules trapped within the NCs matrix. The sharp peak at 1155 cm⁻¹ is due to the C–O stretching vibrations, which are characteristic of oxygenated functional groups on the rGO sheets40.

Compared to the spectra of pure rGO (not shown here), the intensity of oxygen-containing functional group peaks was found to decrease in the ZnO/rGO NCs, indicating partial reduction of GO and the effective interaction between ZnO NPs and rGO sheets. This interaction may arise from electrostatic attractions or chemical bonding between Zn–O groups and oxygen-containing groups on rGO. The combined presence of Zn–O vibrations and residual oxygenated groups on rGO supports the successful formation of ZnO/rGO NCs. These interactions not only enhance the stability of the composite but also facilitate improved charge carrier separation, which is beneficial for photocatalytic applications.

Fig. 3
figure 3

a XRD, b FT-IR spectra, c Raman and d EDX of ZnO-rGO NCs.

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To further confirm the structural features and chemical interactions in the ZnO/rGO NCs, Raman spectroscopy was performed, as shown in Fig. 3c. The spectrum reveals two prominent peaks located at approximately 1355 cm⁻¹ (D-band) and 1561 cm⁻¹ (G-band), which are characteristic of carbon-based materials. The D-band arises from the breathing modes of κ-point phonons of A1g symmetry, indicating the presence of structural defects and disordered sp²-hybridized carbon domains. In contrast, the G-band corresponds to the E2g phonon mode of sp²-bonded carbon atoms in a graphitic lattice, representing the degree of graphitization and crystalline order.

The intensity ratio of the D to G bands (ID/IG) was calculated to be 0.66, which is higher than typically reported values for pristine GO, suggesting a partial reduction of GO to rGO. This increase in ID/IG implies a higher density of defects and smaller sp² domains, which is likely a result of the reduction process mediated by the zinc precursor during composite formation. The introduction of ZnO NPs likely induced localized strain and defects in the rGO sheets, leading to increased disorder in the graphitic structure. In addition to the D and G bands, a weak peak was observed in the low-wavenumber region (~ 437 cm⁻¹), which can be attributed to the E₂ (high) optical phonon mode of ZnO. This further confirms the presence of ZnO in the composite and its interaction with rGO. Overall, the Raman analysis supports the successful incorporation of ZnO nanoparticles onto the rGO matrix, with evidence of defect generation and structural distortion in rGO due to the interaction with ZnO. These defects can play a significant role in enhancing charge separation and catalytic activity by providing more active sites within the composite structure41.

The elemental composition of the biosynthesized ZnO/rGO NCs was further confirmed using Energy Dispersive X-ray Spectroscopy (EDS), as shown in Fig. 3d, confirms the elemental composition of the ZnO/rGO NCs. The spectrum exhibits distinct peaks corresponding to zinc (Zn), oxygen (O), and carbon (C), which are the main constituents of the material. Quantitative analysis reveals that Zn is the major element with a weight% of 75.61% and an atomic percentage of 59.88%. Oxygen is present with 11.66 wt% and 14.21 at%, while carbon contributes 12.73 wt% and 25.91 at%. The high Zn and O content confirms the successful formation of ZnO nanoparticles, whereas the presence of carbon indicates the incorporation of rGO sheets within the composite. No significant peaks corresponding to extraneous elements or contaminants were detected in the spectrum, indicating the high purity of the synthesized material and the effectiveness of the green synthesis route using Conyza bonariensis extract. The absence of residual metallic or organic impurities also suggests efficient capping and reduction during the synthesis process.

Further, the powder sample was studied by UV-DRS spectroscopy, and obtained spectrum has been shown in Fig. 4a, b. The ZnO-rGO NCs exhibited absorption maxima band around 378 nm that indicates the formation of NCs. Moreover, the absorbance increased around 350–450 nm indicating the nucleation, growth, and reduction of particles sizes. These bands assigned to an intrinsic bandgap absorption of ZnO-rGO NCs, due to the electron transitions from the valence band to the conduction band. Additionally, the bandgap of NCs was calculated about 3.46 eV using K-M plot function by the extrapolation of a linear regression on X-axis (Fig. 4b)42. The large bandgap of as-synthesized NCs indicates superior quality and an efficient photocatalytic performance towards degradation of organic dyes43,44,45.

Fig. 4
figure 4

a UV-DRS spectra, and b bandgap (by K-M plot) of ZnO-rGO NCs.

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After EDS study, the chemical composition and oxidation states of the elements present in the biosynthesized mesoporous ZnO/rGO NCs were analysed using X-ray photoelectron spectroscopy (Fig. 5a-d). The survey spectrum (Fig. 5a) revealed the presence of Zn, O, and C elements46, indicating the successful synthesis of ZnO/rGO NCs. The binding energy peaks corresponding to Zn 2p, O 1s, and C 1s were clearly identified. The high-resolution XPS spectrum of Zn 2p (Fig. 5b) displayed two prominent peaks at 1021.6 eV and 1044.7 eV, which are attributed to Zn 2p3/2 and Zn 2p1/2, respectively. These binding energy values confirm the presence of Zn in the Zn2+ oxidation state, characteristic of ZnO46. The O 1s spectrum (Fig. 5c) exhibited two distinct peaks at 530.1 eV and 531.5 eV. The peak at 530.1 eV is assigned to lattice oxygen in the ZnO matrix, while the peak at 531.5 eV corresponds to oxygen species associated with surface hydroxyl groups and chemisorbed oxygen47. These surface oxygen species play a crucial role in enhancing the photocatalytic activity of the NCs by promoting efficient charge separation and reactive oxygen species generation. The C 1s spectrum (Fig. 5d) showed a dominant peak at 284.6 eV, attributed to sp²-hybridized carbon in the graphene structure, confirming the successful incorporation of rGO into the NCs. The presence of rGO provides a conductive network, enhancing charge carrier mobility and improving the photocatalytic performance of the material. Overall, the XPS analysis demonstrates the successful synthesis of ZnO/rGO NCs with well-defined elemental composition and oxidation states, which are essential for their superior photocatalytic properties.

Fig. 5
figure 5

XPS spectrum a survey, b Zn-2p, c O1s, and d C1s of ZnO-rGO NCs.

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Morphological exploration of ZnO-rGO NCs

The morphological aspects like surface, shape and particle size of ZnO-rGO NCs was examined by SEM and TEM (Fig. 6 a–d). The SEM image clearly indicates ‘Tuberose-flower’ or Nishigandha like morphology of ZnO NPs, which is anchored on the surface of graphene nanosheet (Fig. 6c, d). An average diameter of Tuberose-flower’ is ∼1–2 μm, while petal size diameter is 40–70 nm. The resulting ZnO-rGO NCs showed a 3D porous structure (Fig. 6c). The particles size distribution of ZnO on the graphene nanosheet is in the range of ∼45–70 nm48,49,50. The distance between neighbour flowers is about ∼80–100 nm. Moreover, Fig. 6a-b revealed few nanoholes formation on the graphene sheets. The nanoflowers and nanohole formation on the surface of graphene may be beneficial for an effective adsorption or catalytic performances51,52,53,54,55.

Fig. 6
figure 6

a–d SEM image of ZnO-rGO NCs.

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The transmission electron microscopy (TEM) analysis provides comprehensive insights into the morphological and structural features of the biosynthesized ZnO-rGO NCs (Fig. 7a–f). In Fig. 7a, agglomerated ZnO NPs are observed, indicative of a high surface area and porous nature, which is beneficial for surface reactions during photocatalysis. Figure 7B and C clearly display well-dispersed ZnO NPs distributed across the wrinkled and layered structure of rGO, confirming the effective integration of ZnO with the rGO matrix. This intimate interaction not only stabilizes NPs but also facilitates efficient charge transfer between ZnO and rGO, which is essential for suppressing electron-hole recombination and enhancing photocatalytic efficiency. Figure 7b particularly reveals the individual nanoparticles to be nearly uniform in shape and distribution. This estimation is supported by the histogram shown in Fig. 7f, with average particle size ranging from 10 to 15 nm, which presents the particle size distribution with a narrow range, confirming the uniformity in nanoparticle synthesis. Such controlled particle size is a critical factor that influences surface energy, light absorption, and photocatalytic activity. Additionally, Fig. 7d highlights distinct lattice fringes within the ZnO particles, indicating their crystalline structure and confirming the formation of well-ordered crystal planes. The observed lattice spacing aligns with the characteristic planes of hexagonal wurtzite ZnO56. Furthermore, the selected area electron diffraction (SAED) pattern shown in Fig. 7e exhibits concentric diffraction rings composed of numerous bright spots, clearly confirming the polycrystalline nature of the ZnO-rGO NCs. Also, the d-spacing value calculate from SAED pattern was found to be 0.57 nm corresponding to (1 0 1) plane (Fig. 7e). This crystallinity, combined with the mesoporous structure observed in the TEM images, is favourable for improving light harvesting and charge carrier mobility. The mesoporous architecture allows for the diffusion and adsorption of dye molecules onto the catalyst surface, while the conductive rGO network ensures rapid electron transport and reduced recombination rates.

Together, these morphological and structural characteristics such as nanoscale particle size, high crystallinity, polycrystalline structure, and strong interfacial contact between ZnO and rGO—play a pivotal role in boosting the photocatalytic performance of NCs. The successful green synthesis of this ZnO-rGO NC using Conyza bonariensis leaf extract thus results in a highly efficient, eco-friendly photocatalyst with strong potential for environmental remediation, particularly in the degradation of hazardous organic dyes under visible light irradiation.

Fig. 7
figure 7

a–d TEM image, e SAED, and f particle size histogram of ZnO-rGO NCs.

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Next, the sample was examined by BET measurement for adsorption-desorption, as shown in Fig. 8a. The specific surface area of the material was found to be approximately 159 m²/g, while BJH plot estimated the pore size around 11.6 nm, hence, the study recommended a mesoporous nature of NCs. Additionally, the surface texture or roughness of NCs was measured using AFM analysis. The AFM study revealed a rough surface of NCs around 1.7 nm with nearly monodispersed nature (Fig. 8b).

Fig. 8
figure 8

a Nitrogen adsorption-desorption isotherm, and b AFM micrograph of ZnO–rGO NCs.

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Photocatalytic degradation of toxic dyes using ZnO-rGO NCs

The present study illuminates on photodegradation of Congo red (CR), Methyl blue (MB) and Thymol blue (TB) using ZnO-rGO NCs in visible-light irradiation. In a typical photocatalytic experiment, 25 mg of the synthesized ZnO-rGO NCs catalyst was dispersed in 50 mL of an aqueous dye solution and stirred magnetically to ensure uniform suspension. The photocatalytic activity was evaluated under visible light irradiation using a 100 W incandescent electric bulb as the light source. The bulb was positioned at a fixed vertical distance of 15 cm above the reaction vessel to ensure consistent and effective illumination across the surface of the reaction mixture. The incandescent bulb emits broad-spectrum light within the visible range (approximately 400–700 nm), simulating natural sunlight conditions. The reaction setup was maintained at room temperature, and the suspension was stirred continuously throughout the experiment to avoid sedimentation of the photocatalyst. The present biosynthesized NCs were utilized for the first time for photocatalytic degradation of the said dyes in visible-light irradiation at pH 7 for 90 min at room temperature. The larger bandgap energy (3.46 eV) and crystallite particle size of NCs helps for the charge separation and photo-excitation of electron from VB to CB, and generate an electron hole57. Moreover, large specific surface area (159 m2/gram) with small pore size (11.6 nm) would enhance the adsorption of toxic dyes on the surface of NCs58,59,60,61,62. Henceforth, the phytosynthesized NCs was employed to understand its photodegradation efficiency. The photodegradation of dyes were studied by measuring the change in the absorbance value (λmax = 495, 665 and 590 nm) as a function of irradiation time for CR, MB and TB respectively in Fig. 9 a, c and e). The absorption bands were observed in the range of 400 to 700 nm. The colors of the respective dye solutions faded as the irradiation time increases due to the gradual decomposition of chromophoric groups present in the dye as well as fragmentation of bulky molecules in to small radicals (Fig. 12a–c). These observation clears, the steady decrease in absorbance values corresponds to adsorbed dye on the surface of NCs.

Table 1 Degradation efficiencies, rate constants and R2 of dyes over ZnO-rGO NCs.
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Fig. 9
figure 9

Absorbance spectra of dyes a CR, c MB and e TB, and optimization of dyes b CR, d MB, (f TB using ZnO-rGO NCs in visible-light irradiation.

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Fig. 10
figure 10

Effects (optimization) of ZnO-rGO NCs loading (a, c and e) on photodegradation, and (b, d and f) reaction kinetics for CR, MB and TB.

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The optimizations of dyes concentrations were carried out by maintaining a constant NCs loading of 10 mg at room temperature (25 ± 2 °C) and pH 7. The results, as shown in Fig. 9 b, d, and f), demonstrated that out of the concentrations 5, 10, 15, 20 and 25 ppm, the 20 ppm of concentration exhibited the maximum efficiency of degradation for CR and TB (Fig. 9b and f). Thus, 20 ppm concentration was selected as the stock solution for further experiments. However, MB showed maximum efficiency of degradation at 25 ppm (Fig. 9d). From Fig. 9b, it is clearly seen that 90.10% CR degraded at 20 ppm within 90 min using the said NCs. Likewise, significant photocatalytic efficacy was perceived for TB ~ 91.43% using same NCs in 20 ppm of concentration (Fig. 10f). The NCs shows a slower rate of degradation for MB of about ~ 88.08% degradation in 90 min for 25 ppm of solution compared with CR and TB (Fig. 10d). Overall, NCs show remarkable photocatalytic efficiency towards toxic dyes mitigation in 90 min of light irradiation.

$${\text{Degradation }}\left( \% \right) = \left\{ {\left( {{\text{C}}_{0} - {\text{C}}_{{\text{t}}} } \right){\text{ }}/{\text{C}}_{0} } \right\}/*100.$$

To evaluate the outcome of NCs dosage on the photocatalytic efficacy, the catalyst loading varied between (5 to 80 mg) for all dyes in Fig. 10(a, c and e). In heterogeneous photocatalysis, increasing the catalyst loading that creates more reactive sites, leads to higher degradation efficiencies63,64,65,66,67. Figure 10 (c and e) shows the degradation profile with different catalyst doses for MB and TB. Increasing degradation efficiency with successive catalyst loadings until almost complete degradation with 40 mg of catalyst is achieved. Whereas, it was found that 20 mg of NCs for CR, while 40 mg for MB and TB is an optimum dose under visible-light irradiation (Fig. 10a). Moreover, it was checked that if further increases the catalyst weight, decreased efficiency of the reaction occurs. The rate kinetics of ZnO-rGO NCs was studied and found pseudo-first order reaction using following equation-.

$$- {\text{ln }}\left( {{\text{C}}_{{\text{t}}} /{\text{C}}_{0} } \right) = {\text{ kt}}{-}\left( {\text{2}} \right).$$

The difference of –ln (Ct/C0) verses time for the degradation of different dyes are plotted in Fig. 10b, d and f). The reaction rate constant of ZnO-rGO NCs for the CR, MB, and TB concentration of 5 mg/L, 10 mg/L, 20 mg/L, 40 mg/L and 80 mg/L were calculated, and concentration of 20 mg/L of NCs were displaying higher degradation efficiency 93.62% (Fig. 11a) with reaction rate constant at 0.0.12741 min− 1 towards CR dye as shown in Fig. 11b. Whereas, 40 mg/L dose of ZnO-rGO NCs reveals 91.44% (Fig. 11c) and 91.36% (Fig. 11e) photodegradation efficiency for MB and TB which attributed to rate constant 0.15797 and 0.13675 min− 1 respectively in Fig. 11d and f. The time dependent dyes degradation under visible-light irradiation is shown in Fig. 11 (a, c, e). In contrast to very rapid degradation in the presence of ZnO-rGO NCs, an insignificant photodegradation occurred in dark condition as well as in blank experiment (absence of NCs) under visible-light. The degradation efficiency, reaction rate kinetics and regression coefficient values of ZnO-rGO NCs under visible-light irradiation for the corresponding CR, MB, and TB dyes are provided (Table 1). The NCs catalysed to CR, MB, and TB with 91.67, 91.98 and 87.17% photodegradation in 90 min respectively. Among all the.

Fig. 11
figure 11

a, c and e Comparison of degradation efficiency of ZnO-rGO NCs, and (b, d and f) reaction kinetics for CR, MB and TB dyes.

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Corresponding different dyes, MB shows enhanced photo-degradation activity, and photodegradation follows the order: MB > CR > TB. The kinetics study of photodegradation activity of NCs for different dyes were calculated using Langmuir–Hinshelwood kinetic model.

$$\:\varvec{r}=\frac{\varvec{d}\varvec{C}}{\varvec{d}\varvec{t}}=\frac{\varvec{k}\varvec{K}\varvec{C}}{1+\varvec{K}\varvec{C}}$$

From Fig. 10 b, d, f, shows a plot of a linear regression between -ln Ct/C0 versus reaction time. The plot is well fitted with a straight line, this is well signified by the Langmuir–Hinshelwood model. The slope matches to the pseudo-first order reaction kinetics, verified with the closeness of the correlation constants to unity, indicating that the degradation process of the NCs. Additionally, the real-time images of CR, MB and TB decolouration in 90 min during photodegradation are shown in Fig. 11a–c. The calculated reaction rate constants and resultant correlation coefficients are given in Table 1.

The ZnO/rGO NCs synthesized in this study exhibited catalytic performance comparable to other photocatalysts, as shown in Table 2. The comparative data presented in Table 2 highlights the photocatalytic efficiency of various photocatalysts in comparision with ZnO/rGO NCs synthesized through green and conventional methods under visible-light irradiation. The table demonstrates that the ZnO/rGO NCs synthesized using Conyza bonariensis extract in this study exhibits excellent degradation performance (> 95%) against multiple toxic dyes. Key factors influencing the performance include surface area, morphology, heterojunction formation, and improved charge separation. The comparison underscores the effectiveness of green synthesis strategies and the role of composite design in enhancing visible-light-driven photocatalysis.

Fig. 12
figure 12

Decolouration photographs of a CR, b MB, and c TB in 90 min of degradation.

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Table 2 The comparative catalytic performance of ZnO/rGO NCs with other photocatalysts.
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Photocatalytic mechanism of ZnO-rGO NCs

The rapid industrialization and widespread use of synthetic dyes in textiles and other industries have led to severe water pollution. Among these dyes, CR, MB, and TB pose significant environmental and health risks due to their carcinogenic, mutagenic, and recalcitrant nature. Photocatalysis, an advanced oxidation process, has emerged as a promising solution to degrade these dyes efficiently. This study explores biosynthesized ZnO/rGO NCs performed an excellent photodegradation of CR, MB, and TB under visible light irradiation. The overall photocatalytic mechanism is shown in Fig. 13. The integration of ZnO and rGO creates a mesoporous structure with a large surface area, facilitating enhanced light absorption, charge separation, and pollutant adsorption. The estimated bandgap energy (Eg = 3.46 eV) indicated visible light activity, as illustrated in the mechanism. Under visible-light irradiation, ZnO absorbs photons (hv) to excite electrons (e⁻) from the valence band to the conduction band, leaving behind holes (h⁺). The rGO acts as an electron acceptor, suppressing recombination and prolonging charge carrier lifetime. The photogenerated electrons react with dissolved oxygen to form superoxide radicals (•O₂⁻), while the holes oxidize water molecules to hydroxyl radicals (•OH). These reactive oxygen species (ROS) degrade dye molecules into non-toxic byproducts. The schematic mechanism depicted in the image highlights the stepwise degradation process.

Fig. 13
figure 13

Possible photocatalytic mechanism of the dye degradation by ZnO-rGO NCs.

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The photocatalytic efficiency of ZnO/rGO was evaluated by degrading CR, MB, and TB under visible light. The results demonstrated over 90% degradation within 90 min, with ZnO/rGO outperforming bare ZnO. The synergistic effect of ZnO and rGO, coupled with the mesoporous structure, ensured high adsorption capacity, efficient ROS generation, and superior degradation rates. Furthermore, the nanocomposite exhibited excellent reusability and stability over multiple cycles, highlighting its practical applicability. This green synthesis method aligns with environmental goals by utilizing plant-based resources and minimizing chemical waste. The remarkable photocatalytic performance against CR, MB, and TB under visible light underscores the potential of ZnO/rGO as a robust photocatalyst for wastewater treatment, paving the way for future advancements in green nanotechnology and environmental remediation.

Conclusions

This study demonstrated a green, cost-effective synthesis of hexagonal wurtzite ZnO–rGO NCs using Conyza bonariensis leaf extract. The NCs exhibited a tuberose-flower morphology with ZnO NPs (20–30 nm) anchored on rGO sheets, confirmed by SEM and TEM. XRD analysis revealed a crystallite size of 17.22 nm, while AFM and BET showed a mesoporous, nearly monodispersed structure with a high surface area (159 m²/g). These structural features contributed to the NCs’ excellent photocatalytic degradation of CR, MB, and TB dyes under visible light, with over 87% efficiency within 90 min. Optimization revealed 20 mg and 40 mg as effective doses for CR and MB/TB, respectively. Minimal degradation occurred in the absence of light, confirming the photocatalytic nature of the process. Overall, these biosynthesized NCs show strong potential as eco-friendly nanophotocatalyst for wastewater treatment and could be extended to degrade other environmental pollutants such as pesticides and pharmaceuticals.