
==== Front
Sci Rep
Sci Rep
Scientific Reports
2045-2322
Nature Publishing Group UK London

39266647
72186
10.1038/s41598-024-72186-5
Article
Solar light driven enhanced in photocatalytic activity of novel Gd incorporated ZnO/SnO2 heterogeneous nanocomposites
Panwar Sagar 1
Kumar Vinod 2
Purohit L. P. proflppurohitphys@gmail.com
lppurohit@gkv.ac.in

1
1 grid.411895.0 0000 0001 0790 0819 Semiconductor Research Lab, Department of Physics, Gurukula Kangri (Deemed University), Haridwar, India
2 https://ror.org/003kgv736 grid.430529.9 Department of Physics, University of the West Indies, St. Augustine Campus, St. Augustine, Trinidad and Tobago
12 9 2024
12 9 2024
2024
14 213419 5 2024
4 9 2024
© The Author(s) 2024
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ Open Access This article is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License, which permits any non-commercial use, sharing, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if you modified the licensed material. You do not have permission under this licence to share adapted material derived from this article or parts of it. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by-nc-nd/4.0/.
The Gd-doped ZnO/SnO2 nanocomposites with various atomic percentages (0, 0.5, 0.8, and 1.2 at%) of gadolinium (coded as GdZS0, GdZS1, GdZS2, and GdZS3) was synthesis via the sol–gel method and explored for photodegradation against dye solutions exposing solar light irradiation. The synthesized nanocomposites were characterized employing the XRD, FTIR, FE-SEM, Raman spectroscopy, BET analysis and UV–Vis spectrophotometer. The FE-SEM results indicated that the formation of nanoparticles to nanoflowers covered with Gd ions was observed with an increased doping concentration of Gd. The optical bandgap was evaluated and found in the range of 3.21–3.27 eV for GdZS nanocomposites. The GdZS nano-photocatalysts were investigated against the degradation of different organic dyes and GdZS3 shows the highest degradation efficiencies of 99.3%, 98.3% and 99.4% towards MO, MB and RhB dyes, respectively at neutral pH in aqueous media. Before and after photodegradation. Biological oxygen demand and chemical oxygen demand tests to make estimations of mineralization. The investigations are very promising for the degradation process in rare earth doped metal oxide nanocomposites. A plausible photodegradation mechanism of synthesized nanocomposites under investigation has also been proposed.

Keywords

Gd ion
ZnO/SnO2
Nanocomposites
Sol–gel
Photocatalyst
BOD and COD
Subject terms

Materials science
Nanoscience and technology
issue-copyright-statement© Springer Nature Limited 2024
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pmcIntroduction

At present, wastewater effluents from the up growing textile and dye industries contain a large variety of organic compounds which creates a large number of serious environmental problems and affect human beings also. Dyes, which are primarily released into the environment from the textile and printing industries, is a type of harmful organic pollutant1. These organic dyes produce lethal carcinogenic byproducts via hydrolyze as well as reacting with other organic pollutants contaminated effluents. These dyes and their byproducts at the concentration at which they are being discharged directly to receiving bodies of water, not only threaten the aquatic life but also the health issues of human beings2. Therefore, the environmental and remediation field has focused on the elimination of high concentrations of dyes from industrial wastewater outlets. As a result, the search for a cost-effective and efficient treatment approach for wastewater that contains dyes has become a crucial field of study.

The scientific community has implemented various techniques to treat wastewater containing organic contaminants, including but not limited to adsorption, oxidation, biosorption, membrane filtration, floatation, and photocatalytic oxidation. However, most of the above-mentioned methods generate hazardous byproducts and require a high-cost treatment process to eradicate the organic contaminants. Hence, finding cost-effective, sustainable, efficient, and environmentally-friendly methods to efficiently break down pollutants is a matter of utmost importance. Considering this fact, photocatalytic oxidation (PCO) has received much attention for wastewater treatment. PCO utilizes solar light energy to degrade organic pollutants in a cost-effective manner, while avoiding the production of any carcinogenic byproducts3. The process of PCO was employed to remove diverse organic or microorganism pollutants from the water, which can be evaluated by conducting tests such as biological oxygen demand (BOD) and chemical oxygen demand (COD)4. Primarily, BOD and COD are crucial factors in determining the quality of water. In the process, photons interact with H2O or OH molecules adsorbed on a catalytic surface, creating holes in the valence band. These holes subsequently develop into extremely reactive and indiscriminate oxidants referred to as O2− radicals. Furthermore, the interaction between adsorbed O2 and excited electrons in the conduction band results in the formation of hydroxyl radicals (OH−)5. Therefore, organic pollutants undergo decomposition through the photocatalytic oxidation process, facilitated by wide bandgap semiconductors that enable reactions under UV light without being consumed in the process, resulting in a significantly longer reaction time6. As a result, the scientific community has been concentrating on photocatalytic oxidation as a means of maximizing beneficial results while minimizing power and time usage.

Recently, semiconductor metal oxides nanocomposites based photocatalyst such as ZnO, TiO2, SnO2, MgO, CdO, etc. have shown significant potential in effectively eliminating organic dye pollutants from water effluents7–10. Semiconductors are gaining popularity as efficient photocatalyst for wastewater treatment due to their low-cost production, high thermal stability, non-toxic nature, and excellent chemical and physical stability. Despite their simple processing, mild reaction conditions, and low energy consumption, a single semiconductor may not produce efficient photocatalytic oxidation results due to rapid electron–hole pair recombination. Two major challenges facing current photocatalyst are poor absorption of visible light and fast recombination of charge carriers. Therefore, it is necessary to fine-tune these semiconductors to improve their ability to absorb visible light, suppress recombination rates, and effectively degrade organic compounds and pollutants11. Therefore, enhancing the performance and preventing electron/hole pair recombination in photocatalysts can be achieved by modifying individual photocatalysts with various semiconductors, making it a highly effective approach. Several types of coupled semiconductor-based heterostructures, including ZnO/SnO212, ZnO/TiO213, CdO/TiO214, NiO:nCdO15 , MgO/ZnO16, and RGO/ZnO17 have been developed and utilized in various photocatalytic processes with the aim of reducing the recombination rate of electron–hole pairs and enhancing the degradation of organic components. Bandara et al.18 reported that the recombination of fast electron–hole pairs in ZnO can be mitigated by pairing it with SnO2. The study also suggests that coupling ZnO with TiO2 can similarly reduce the fast electron–hole pair recombination in ZnO. Regularly, various semiconductor materials with different bandgaps are chosen for coupling to tune the absorption band edges. This is considered crucial for achieving efficient degradation results under visible light, and it is often viewed as “drowning man catches at a straw” by the scientific community. By coupling these semiconductors, researchers were able to achieve optimal results in photocatalytic oxidation (PCO) due to the presence of sufficient oxygen vacancies in the semiconductor. However, even though photocatalysts have demonstrated impressive performance in degrading organic pollutants, several factors such as the dosage of the catalyst, energy of the light source, reaction time, and stability can impact their photocatalytic performance. The reaction time of PCO was determined to be insufficient for the degradation of contaminants with the coupled semiconductors. However, when thermal treatment temperature exceeds a certain point, a surface coupling process takes place, causing the material to become two-phase and reducing the thermal stability of powders.

However, some researchers introduced a new modification technique for PCO by introducing some foreign atoms i.e. doping of different transition metals (Cr, Co, Mn, Ni, etc.)19–21 and rare earth metals (Eu, Gd, Y, Dy, Sm, Nd, etc.)8,22 with single metal oxide semiconductors like ZnO, SnO2, TiO2, etc., to hindering the recombination frequency of electron–hole pairs. According to Akhtar et al., the efficiency of ZnO in degrading Rhodamine 6G was observed to be enhanced by the introduction of Nd doping23. Oranuch et al.24 and Bhararti et al.25 claimed that the photocatalytic efficacy towards MB of ZnO nanoparticles doped with Gd was found to be superior in comparison to the undoped ZnO nanoparticles. Alnazi et al. found that the methyl orange (MO) degradation rate increased upon introduce N and Gd in ZnO photocatalyst in comparison of pure ZnO26. Moreover, Rana et al.27 observed a significant improvement in degradation of RhB for Gd doped SnO2 nanostructures with respect to SnO2. Nevertheless, the composition of coupled and doped metal oxide gives the bounteous outcomes for decontaminations of organic pollutants, but the decontaminations process requires high energy, high catalyst dose and large degradation time.

Therefore, introducing a new approach with coupled metal oxide semiconductor and rare earth ion dopants could be an advantageous technique to obtain highly efficient heterogeneous photocatalyst. Coupled (ZnO/SnO2) modified with Gd ions exhibits a new heterogeneous photocatalyst with high photocatalytic activity and was found suitable. As rare earth dopant, Gd ions show the best catalytic activity with metal oxide semiconductor having half-filled (4f.) and empty 5d electronic configuration which is beneficial to trapping the electrons and suppressed the recombination time25,28. Moreover, Gd ions doped metal oxides hold good optical and surface properties which provide the enhanced catalytic activity in presences of visible light photons. The introduction of rare earth ions through doping affects the electronic configuration of the material, leading to the absorption of oxygen species, which generates particles with a high specific surface area and smaller size. Because, band gap of both ZnO and SnO2 are found in the UV region having the values 3.37 and 3.56 eV, respectively, so there is a need for tailoring the band gap of coupled ZnO/SnO2 from UV region to visible region to reducing the energy consumption29,30. Therefore, Gd ions could be used as band gap regulator for the coupled metal oxide semiconductors and increase the surface area for catalytic reaction to obtain the high yielding.

The current investigation entails the synthesis of Gd doped ZnO/SnO2 nanocomposites using a facile sol–gel method, followed by an assessment of their photodegradation performance on organic dyes in the presence of visible light (solar light) irradiation. The elimination of MB, MO, and RhB via the photodegradation process using GdZS nanocomposites as a photocatalyst was evaluated using a UV–vis spectrophotometer. The synthesis of Gd doped ZnO/SnO2 as photocatalyst has not been carried out previously as far as we know on the basis of the literature. This report presents an in-situ examination on the photocatalytic efficacy of synthesized GdZS nanocomposites.

Experimental information

Used materials

Tin dichloride monohydrate [SnCl2.H2O, 98.6%] zinc acetate dihydrate [Zn(CH3COO)2.2H2O, 99%] and gadolinium acetate hydrate [Gd(CH3CO2)3.xH2O, 99.9%] were purchased from Alfa Aesar (Thermofisher), USA. Sulphuric acid [H2SO4, 99.99%], sodium hydroxide [NaOH, ≥ 98%], methyl blue (99.9 + % USP Grade), methyl orange, rhodamine-B and ethanol (HPLC grade) were arranged from Sigma Aldrich, and were used without further refinement. Double deionized water (DI) used as washing and solvent agent throughout the experiment.

Synthesis of Gd doped ZnO/SnO2 nanocomposites

Sol–gel precipitation method was employed to obtain pristine and Gd doped ZnO/SnO2 nanocomposites, for this firstly, about 70 mL aqueous solution (0.5 M) of [Zn(CH3COO)2.2H2O] and 20 mL of [SnCl4.H2O] were prepared separately in borosilicate beaker and stirred for 60 min. Afterwards, both solutions mixed well in dropwise fashion of [SnCl4.H2O] into [Zn(CH3COO)2.2H2O] solution then stirred magnetically for 180 min31. Next, the reaction mixture was stirred continuously for 120 min while a 0.5 M aqueous solution of NaOH was dropped until the pH reached approximately 10.0. The resulting white precipitate was then allowed to settle overnight. In the subsequent step, the white precipitate was separated by centrifugation at 5000 rpm for 5 min, and washed repeatedly with alternating rinses of DI water and ethanol, for a total of 7–8 cycles32. Further, obtained white precipitate dried overnight at 80 °C in the hot air oven then calcined at 550 °C for 120 min which was used for characterization later and application purpose12. Moreover, to obtain gadolinium (Gd) doped nanocomposites, same process was applied and its concentration range of 0 atomic% to 1.2 atomic%. The suitable quantity of Gd precursors was dissolved individually in ZnO and SnO2 solutions separately and then mixed as mentioned previously. Numerous batches of GdZS nanocomposites were synthesized in the same approach with different at% (0, 0.5, 0.8 and 1.2) of Gd coded as GdZS0, GdZS1, GdZS2 and GdZS3.

Characterizations for GdZS nanocomposites

Using a powder X-ray diffractometer (PXRD) equipped with a CuKα X-ray source of 1.5406 Å wavelength, operated at 40 kV and a 2θ scale range of 20°–65°, the crystalline phase and structure of ZnO/SnO2 (GdZS0) and various GdZS nanocomposites (GdZS1, GdZS2, and GdZS3) were determined. Raman spectrophotometer (Renishaw Centrus), which utilizes an excitation wavelength of approximately 532 nm, was employed to analyze the crystal structure order, crystal structure disorder, and crystal defects of GdZS nanocomposites. A Shimadzu 8400 S FT-IR spectrophotometer was utilized to analyze the surface functional groups and presence of metal oxygen M–O bonds in various nanocomposites. The Field Emission Scanning Electron Microscope (FE-SEM), specifically the Zeiss Gemini-1 model, in combination with Energy Dispersive X-ray (EDX) analysis, was employed for tracing the surface morphology and elemental composition of the sample. Using a UV–Vis–NIR spectrophotometer (Shimadzu, UV-3600), diffused reflectance spectroscopy (DRS) analysis was carried out with BaSO4 as baseline material to investigate the optical characteristics, specifically the optical band gap for different GdZS nanocomposites.

Photocatalytic degradation procedure

To evaluate the photocatalytic performance of various GdZS nanocomposites, Methyl orange (MO), methyl blue (MB), and rhodamine-B (RhB) were selected as representative organic pollutants in water. The photodegradation of various GdZS nanocomposites on MB, MO, and Rh-B using solar light illumination was conducted in a borosilicate glass reactor equipped with a magnetic stirrer, which has a capacity of 250 mL, an internal diameter of 7 cm, and a depth of 9 cm. About 20-ppm aqueous solution of organic dyes (MO, MB and RhB) were prepared in 1000 mL graduated volumetric flask of borosilicate glass for the investigation of photodegradation. To initiate photodegradation, a test solution (50 mL) of organic contaminants (MO, MB, and RhB) was treated with different doses of GdZS nanocomposite photocatalysts (GdZS0, GdZS1, GdZS2, and GdZS3) and stirred magnetically for 30 min in the dark until adsorption–desorption equilibrium was achieved. After achievement of equilibrium between photocatalyst and test solution, all contaminants test solution (MB, MO and RhB) were exposed under solar light for 60 min in a petri dish of 100 mm (~ 55 cm2 base area and 21 mm height) and the UV–vis spectrophotometer (Shimadzu, UV-3600) was used to measure the concentration of contaminants that remained in aqueous solution. All photocatalytic experiments were done under natural sunlight investigations, in an open atmosphere at lab conditions between 11.00 am and 2.00 pm in the months of March and April with a sunlight intensity of 150 mW/cm2 33. The kinetics of photodegradation of MO, MB and RhB was investigate using GdZS nanocomposites and GdZS3 nanocomposites with variable dose under exposure of solar light and 2 mL aliquot from the test solution of each organic dye was withdrawn within a specific interval of time 30 min then photocatalyst was separated using centrifuge machine (Eltek) for 5 min at 4000 rpm. After that, the concentration of obtained test solutions of MO, MB and RhB was examined through analyzing the respective absorbance at specific wavelength maxima (λmax) of 453 nm, 665 nm, and 556 nm using a spectrophotometer. Correspondingly, blank experiments for the photodegradation of MO, MB and RhB were carried out without using photocatalyst under exposure of solar light with similar conditions. All investigations related to the photocatalytic degradation study of contaminants were carried out under neutral pH (~ 7) and ambient aerated conditions. To ensure stability and reusability, the photocatalyst was collected via centrifugation and reused for the photodegradation of MO, MB, and RhB over five cycles in an organized manner. After each photodegradation cycle, the photocatalyst was retrieved by centrifugation for 10 min at 5000 rpm. The collected nanocomposites were then washed with warm DI water, followed by drying in a hot air oven at 80 °C for 60 min before being used in the next photodegradation cycle. Subsequently, COD and BOD tests were conducted using the open reflux method (Scientech Technologies, India) and a standard BOD incubator (model 1011) (Scientech, India), respectively.

Results and discussion

Characterization of GdZS nanocomposites

Powder X-ray diffraction

The diffraction pattern of powder x-ray diffraction (XRD) was used to investigate the purity and crystalline arrangement of the as-prepared pristine ZnO/SnO2 (GdZS0) phase, as well as various nanocomposites of Gd-doped ZnO/SnO2 (GdZS) including GdZS1, GdZS2, and GdZS3. The XRD pattern of these GdZS nanocomposites demonstrates peaks at 31.83°, 34.57°, 36.36°, 47.70°, 56.63°, and 62.94° on the 2θ scale, which correspond to the reflection planes (100), (002), (101), (102), (110), and (103), respectively, as depicted in Fig. 1 for ZnO. The XRD patterns obtained for various GdZS nanocomposites were found to be in good agreement with JCPDS card #36-1451, indicating the presence of a hexagonal wurtzite structure of ZnO34. Similarly, the diffraction peaks observed at 26.58°, 33.96°, 37.91°, 51.82°, 55.04°, and 61.95° in the XRD pattern of SnO2 nanocomposites correspond to the plane (110), (101), (200), (211), (220), and (310), respectively, corresponding to JCPDS card #72-114735. The purity and coexistence of separate phases of ZnO and SnO2 in GdZS nanocomposites were confirmed and the additional diffraction peaks related to other phases, such as ZnxSnOy or any oxide phase of Gd not exhibits in XRD patterns. Furthermore, the substitution of Gd for Zn and Sn lattice sites did not alter the physical properties of the bare material, as evidenced by the lack of diffraction peaks by introducing the Gd in ZnO/SnO2 lattice. However, an observation was made that the diffraction peaks of ZnO decreased as the Gd content in GdZS nanocomposites was increased while the growth of SnO2 crystal was augmented. Palanichamy et al.36 was also observed such augmentation in the XRD peaks as Gd ions incorporation in SnO2 lattice increased. The Debye Scherrer’s formula was used to evaluate the crystallite size of the various GdZS nanocomposites that were synthesized as given in Eq. (1)37,1 D=0.94λβCosθ

Fig. 1 XRD patterns of Gd doped ZnO:SnO2 nanocomposites as GdZS0, GdZS1,GdZS2 and GdZS3 which are matched with JCPDS card no. 36–1451 and 72–1147 for ZnO and SnO2, respectively which represented with sign of $ and ∆.

In this context, β refers to the full width at half maximum (FWHM) of X-ray diffraction peaks (in radians), with θ representing the Bragg diffraction angle, λ denoting the wavelength (0.1504 nm) of the X-ray source, 0.94 indicating the shape factor constant, and D representing the crystallite size (in nm). The most intense peaks were utilized to estimate the size of the crystallites in different GdZS nanocomposites. It was observed that increasing the concentration of Gd resulted in a reduction in the average crystallite size of the GdZS nanocomposites. This might be due to the replacement of the interstitial sites of Zn2+/Sn4+ by Gd3+ ions having different effective ionic radii as 74, 69 and 93.5 pm for Zn, Sn, and Gd ions, respectively. The lattice parameters ‘a’ and ‘c’ for different GdZS nanocomposites were determined by employing the XRD diffraction peaks that correspond to the (002) reflection plane for the hexagonal structure of ZnO and the (110) plane for the tetragonal structure of SnO2, through applying of the following equation38,2 1d2=43a2(h2+hk+k2)+l2c2

3 1d2=h2+k2a2+l2c2

4 ε=βcosθ4

where, ‘hkl’ refers the miller indices of the corresponding reflection plane while ‘d’ signifies the interplanar spacing among reflection planes and ε refer as strain in nanomaterials. The lattice parameters corresponding to each GdZS nanocomposites can be seen from Table 1.Table 1 Structural parameters and optical parameters of all obtained GdZS nanocomposites.

	ZnO (Space group: P63mc)	SnO2 (Space group: P42/mnm)	
Sample (Band gap (eV))	FWHM (deg)	Crystallite size (nm)	Strain (ε × 10–3)	Lattice constant	FWHM (deg)	Crystallite size (nm)	Strain (ε × 10–3)	Lattice constant	
	(a) (Å)	(c) (Å)		(a) (Å)	(c) (Å)	
GdZS0 (3.22)

GdZS1 (3.26)

GdZS2 (3.25)

GdZS3 (3.21)

	0.3423

0.5344

0.4802

0.4345

	24.30

15.57

17.32

19.14

	5.24

7.10

6.38

5.77

	3.3203

3.3161

3.3189

3.3196

	5.3216

5.3243

5.3206

5.3238

	0.3784

0.2998

0.4367

0.4906

	21.56

27.22

18.68

16.63

	6.99

5.54

8.07

9.06

	4.8643

4.8601

4.8624

4.8624

	3.2423

3.2234

3.2389

3.2412

	

FT-IR spectroscopy analysis

The presence of functional groups on the surface of pure and doped nanocomposites obtained can have a significant impact on the degradation of organic contaminants. Therefore, FT-IR spectroscopy measurements were used to examine the surface chemical bonding structure and various functional groups present on the surfaces of the nanocomposites (GdZS0 and GdZS3), thereby allowing for their identification. FT-IR spectra within the wavenumber 500–4000 cm-1 range to identify the functional groups and vibrational bonds of nanocomposites ZnO/SnO2 (GdZS0) and 1.2 at% Gd doped ZnO/SnO2 (GdZS3) nanocomposites are depicted in Fig. 2. Obtained FT-IR spectrum of the synthesized GdZS nanostructure exhibits significant absorption peaks at 619 and 915 cm−1 corresponds to M–O bonds (Zn–O)39. The stretching vibration mode corresponding to another M–O bond for Sn–O observed at 640 cm−1. The peaks observed in the wavenumber range of 661–734 cm−1 could potentially be attributed to the influence of the Gd3+ ion on the modification40. The C–H functional group exhibits spectral bands in the range of 1000–1100 cm−1, while the chemical bonds of C = C–C, C = C, and C = O are characterized by tensile vibrations that produce spectral peaks at 1420 cm−1, 1610 cm−1, and 2355 cm−1, respectively41. These could be associated with organic residues from the synthesis process, or they could be part of organic ligands attached to the nanocomposites because the used precursors were in the form of acetates. The band in the range 3100–3550 cm−1 is corresponding to the existence of hydroxyl groups (−OH) due to presence of −OH bending and stretching vibration peaks. It could be due to the adsorption of moisture at the time of sample preparing for FT-IR in an open air atmosphere condition42. The results of FT-IR analysis showed that the hybridization of ZnO nanostructure with SnO2 was occurred in presence of Gd ions and results as expansion in wavenumber towards high values43. Additionally, the absence of impurities in the GdZS nanocomposites after annealing was confirmed by the absence of any corresponds to alkoxy residue.Fig. 2 FTIR spectra of GdZS0 and GdZS3 nanocomposites.

Surface morphology analysis

FE-SEM and EDX analysis were used to investigate the superficial morphology and composition of elements in different GdZS nanocomposites, labeled as GdZS0, GdZS1, GdZS2, and GdZS3, with varying concentrations of Gd. FE-SEM images of GdZS0 (ZnO/SnO2) nanocomposites showed an irregular stone like shape arrangement44 with rugged agglomeration of SnO2 over the ZnO that could be seen in the Fig. 3a–d. Therefore, the successful synthesis of the nanocomposites indicates that the sol–gel technique has been improved, resulting in consistent levels of Zn, Sn, and O within the ZnO/SnO2 crystal structure45,46. However, augmented amount of the Gd eliminates the stone like shape as generating flower like structural arrangement all over the morphology for GdZS3 photocatalyst as can be seen in Fig. 4a-d. The SEM image of GdZS3 nanocomposites showed that a higher concentration of Gd improves the electron donation compared to SnO2 nanoflowers that were evenly distributed on the ZnO surface and had a uniform size of 40–50 nm25,47. The particle size of ZnO/SnO2 doped with Gd nanocomposites was significantly decreased as a result of Gd doping, as evidenced by the notable reduction in size compared to the pristine ZnO/SnO2.Fig. 3 FE-SEM images of undoped ZnO:SnO2(GdZS0) nanocomposites at different scales (a) 20 μm, (b) 10 μm, (c) 3 μm and (c) 1 μm.

Fig. 4 FE-SEM images of Gd doped ZnO:SnO2(GdZS3)nanocomposites at different scales (a) 10 μm, (b) 5μm, (c) 2μm and (c) 1 μm.

The EDX analysis verified that the synthesized nanocomposites contained Zn, Sn, O and Gd. The atomic percentage of Gd in the Gd-doped ZnO/SnO2 nanocomposites was found to be 0.51%, 0.81%, and 1.10% for the three different nominal compositions, which closely matched the actual doping concentration of Gd. The elemental color mapping depicted in Fig. 5 demonstrated that Gd doped ZnO/SnO2 nanocomposites contain Zn, Sn, O, and Gd elements in an evenly distributed manner, indicating the presence of Gd, ZnO, and SnO2 within the nanocomposites. The successful fabrication of Gd-doped ZnO/SnO2 nanocomposites was confirmed by means of FE-SEM, elemental mapping, and EDX.Fig. 5 EDX spectrum and color mapping for as intensity counts Vs X-ray energy (a) GdZS0 elemental composition and color mapping for Zn, Sn and O by individual color, (b) GdZS3 elemental composition and color mapping for Zn, Sn, Gd and O by individual color.

Raman spectroscopy study

Raman spectroscopy is a persuasive technique used to identify the crystal structure and impurity phases, whether ordered or disordered, in metal oxide nanocomposites, including doped and undoped ones. It can also reveal the presence of ZnO and SnO2, as well as the interactions between ZnO/SnO2 and Gd ions in various GdZS nanocomposites. The Raman spectra of GdZS nanocomposites consistently show that certain peaks, specifically those at 333, 437, 536, 582, 666, and 1086 cm−1, correspond to the hexagonal wurtzite structure of ZnO. Additionally, other peaks at 476, 638, 691, and 752 cm−1 are provided the evidence of the tetragonal rutile structure of SnO2, as demonstrated in Fig. 648,49. The ZnO crystal lattice’s hexagonal wurtzite structure at the center of the Brillouin zone demonstrates C6v symmetry, as per group theory. This symmetry corresponds to eight irreducible representations (IRR) of phonon normal modes at the Γ point, which can be represented as 2A1 + 2B1 + 2E1 + 2E2. Some of the phonon modes in set IRR of A1 and E1 correspond to acoustic modes, while the rest indicate optical modes (represented by Γopt = A1 + 2B1 + E1 + 2E2). Among these modes, only E2 is Raman active. Both IR and Raman activity are observed in A1 and E1 modes, whereas the B1 mode is inactive (also known as a silent mode)50. IRR A1 and E1 represent the polar modes and could fragment into longitudinal optical (LO) and transverse optical (TO) modes (assigned by 582 cm−1). The E2 mode comprises two sets of high-low (E2High–E2Low) frequency phonons (assigned at 365 cm−1), which correspond to vibrations of the oxygen atoms and the heavy ZnO sub-lattice, respectively. A sharp peak at 458 cm−1 assigns the E2High mode of oxygen atom vibration that attached with Zn atom in tetrahedral coordination. The optical overtones 2LO (2E1 and 2A1) have been identified as the cause of the small and wide band observed at 927 cm−1 and 1086 cm−151. Similarly, The D4h space group of the rutile tetragonal structure in the SnO2 lattice is characterized by eleven IRR normal modes phonon at Γ point, which are represented as 1A1g, 1A2g, 1A2u, 1B1g, 1B2g, 2B1u, 1Eg, and 3Eu. Within this context, the Raman active modes are demonstrated by A1g, B1g, B2g, and Eg IRR, while the IR active modes are indicated by A2u and Eu IRR. The strong peak at 626 cm−1 is identified as the A1g mode of the tetragonal crystal lattice of SnO2, while the peak at 722 cm−1 is recognized as the B2g mode of IRR. The emergence of SnO2 clusters or surface defects could be the possible cause of the peak observed at 294 cm−135,52.Fig. 6 Raman spectrum of GdZS0 and GdZS3 nanocomposites within 300–1200 cm−1.

Optical bandgap

UV–visible spectra are extensively employed to evaluate the optical energy band gap of nanocomposites for prior information about the suitable type of absorption light for photocatalytic experiment. Therefore, the optical properties of synthesized various GdZS nanocomposites were examined by UV–Vis diffused reflectance spectroscopy (DRS) and the reflectance of nanocomposites measured in the 300–800 nm range and is presented in Fig. 7a, which was employed to determine the optical band gap of the samples. In addition, the Kubelka–Munk equation provides a means of determining the optical band gap of nanocomposites through the use of dispersion and absorption coefficients, with reference to a specific reflectance value of the sample, as illustrated by Eq. (5)53.5 fR=KS=1-R22R

where, the symbols K and S represent the Kubelka–Munk absorption and dispersion coefficients, respectively, while R represents reflectance and F(R) denotes the function of reflectance. K/S is not an exact measure of absorbance for nanocomposite samples, but rather an estimation. Latterly, the optical band gap (Eg) of various GdZS nanocomposites was determined employing of Tauc’s plot as ((F(R)hν)n vs photon energy (hν)), as depicted in Fig. 7b, and Eq. (6) was employed in the calculation54 ;6 FRhn=Ah-Eg

Fig. 7 (a) Diffused reflectance spectra of GdZS nanocomposites with Gd content, (b) Tauc’s plot of GdZS nanocomposites using Kubelka–Munk function for varying Gd concentration.

In this context, the symbol ν represents the frequency of photon radiation, the constant A depends on the material’s properties, and h represents the Planck’s constant. The absorption constant F(R) relates to reflectance, while the values of n = 2 and 1/2 respectively denote direct and indirect allowed transitions. The nanocomposites of GdZS1, which contained 0.5 at% of Gd, had a larger optical band gap of 3.27 eV, compared to GdZS0 nanocomposites with a band gap of 3.21 eV. While in case of GdZS2 and GdZS3, band gap found decreased as with 3.25 and 3.22 eV, respectively. The reduction in the optical bandgap can be attributed to the downward displacement of the Fermi energy level and the concentration of free carriers of Gd3+ ions55,56. The blue and red shift in the absorption edge of GdZS nanocomposites were observed for all values of optical band gap after decorating Gd3+ ions with homogeneous ZnO/SnO2. This decoration confirmed the existence of surface defects and electronic association among the nanocomposites, which led to an easier electro transfer process. As a result, the photocatalytic degradation of MB, MO, and RhB was enhanced under solar light irradiation.

Optical properties analysis

The dominant factors which can curtail the activity of photoactive materials such as transfer and separation of charge, light absorption, adsorption of molecules of pollutant etc.57. Therefore, the degradation process is significantly impacted by the absorption capability of photoactive nanocomposites. The absorbance spectra presented in Fig. 8 were used to assess the optical characteristics of both undoped ZnO/SnO2 and Gd doped ZnO/SnO2 nanocomposites through UV–Vis spectroscopy. Compared to individual ZnO and SnO2, the absorption edge of pure ZnO/SnO2 showed a red shift because of its quantum confinement effect58. The addition of Gd ions to the ZnO/SnO2 nanostructure resulted in a blue shift of the absorption edge, which was attributed to the presence of Gd ions that facilitated charge separation, thus reducing the rate of recombination. The incorporation of Gd3+ ion onto the ZnO/SnO2 nanocomposites significantly improved their photo-activity. Consequently, Gd doping causes a reduction in the energy bandgap, resulting in the creation of a sub-bandgap in both ZnO and SnO2, thereby increasing the visible light absorption edge of the nanocomposites59,60. Further, Gd doped ZnO/SnO2 (1.2 at% Gd) nanocomposites showed a stronger response to visible light as compared to the nanocomposites of ZnO/SnO2, Gd doped ZnO/SnO2 (0.5 at% Gd) and Gd doped ZnO/SnO2 (0.8 at% Gd) nanocomposites. This enhancement in response is attributed to the creation of a sub-bandgap energy state within the SnO2 and ZnO nanocomposites by the Gd3+ ion. The phenomenon enables the stimulation of electrons from the valence band (VB) of SnO2 towards the newly formed intermediate states of ZnO61. Therefore, the synergistic effect of Gd3+ ion and ZnO/SnO2 is the source of this enhancement.Fig. 8 Absorbance spectra of the all Gd doped ZnO:SnO2 nanocomposites with different doping amount of Gd.

BET surface area analysis

To determine the exact surface area necessary for the photocatalyst to carry out its photocatalytic function, an analysis was conducted on the GdZS nanocomposites using the BET method for specific surface area measurement. Typically, a photocatalyst with larger surface area induces a higher photocatalytic activity on its surface as well as absorbability due to surface phenomenon which are impacted by the quantity of active sites present on the surface of the photocatalyst. Accordingly, the influence of Gd content on the BET surface area, pore size, and volume of ZnO/SnO2 nanocomposites was examined using the Barrett–Joyner–Halenda (BJH) technique to analyze N2 adsorption–desorption isotherm results62 as can be seen from Fig. 9a–c and Table 2. Undoped ZnO/SnO2 nanocomposites exhibited a BET surface area of 129.62 m2/g, surpassing that of individual ZnO or SnO2 nanoparticles63,64. A broad range of pore sizes was observed in the Gd-doped ZnO/SnO2 nanocomposites. The maximum pore size found for the GdZS3 with 30.48 nm which can provide the more sites to the dye molecules to create free radicals to enhance the photocatalytic activity. The specific surface area of different GdZS nanocomposites was found 1.31, 1.47 and 1.60 times higher compared with pure ZnO/SnO2. As a result of its greater pore volume (0.253 cm3/g) in comparison to the other GdZS nanocomposites, Gd-ZnO/SnO2 (containing 1.2% Gd) possesses the highest specific surface area. This increased surface area might be due to the incorporation of Gd ions as it provides more site defects in nanomaterials. Which indicates that increasing the concentration of Gd in ZnO/SnO2 up to 1.2% leads to a larger surface area, which could enhance the absorbability of GdZS nanocomposites and facilitate their photocatalysis, as indicated by the BET results.Fig. 9 N2 isotherms adsorption–desorption of GdZS nanocomposites (a) GdZS0, (b) GdZS1 and (c) GdZS3.

Table 2 BET surface area, pore volume and size of synthesised GdZS nanocomposites.

Photocatalyst	BET surface area (m2/g)	Mean pore size (nm)	Pore volume (cm3/g)	
GdZS0	129.62	23.33	0.137	
GdZS1	115.87	26.86	0.148	
GdZS2	160.20	27.56	0.235	
GdZS3	208.31	30.48	0.253	

Photocatalysis activity of MB, MO and RhB dyes

The estimation of photocatalytic activity of the as synthesized GdZS nanocomposites coded as GdZS0, GdZS1, GdZS2, and GdZS3 was explored for the photocatalytic activity towards organic dyes such as MB, MO and RhB from water in solar light exposure. Further, the test solutions containing organic contaminants (MO, MB, and RhB dyes) were stirred with photocatalyst (GdZS) for 30 min under dark conditions to assess the adsorption–desorption activity of the photocatalyst, prior to exposure to solar irradiation. The solutions were then illuminated with solar light and the degradation of contaminants was assessed by analyzing the corresponding absorbance spectrum using a UV–vis spectrophotometer. The photodegradation efficiency (η) of all GdZS nanocomposites towards photodegradation of MO, MB and RhB dyes were determined applying the given equation as follows65;7 Photodegradationefficiencyη=1-CtCtCoCo×100

In this context, “Co” denotes the original concentration of contaminants (MO, MB, and RhB dyes) in water, while “Ct” refers to the concentration of contaminants at various time intervals measured in minutes, denoted by ‘t’. Therefore, the values of photodegradation efficiency for all GdZS nanocomposites (GdZS0, GdZS1, GdZS2, and GdZS3) were observed when considering the photodegradation of MO. The observed values were 73.86%, 53.78%, 59.53%, and 85.70% using a photocatalyst dose of 0.5 g/L and 60 min of solar light exposure, respectively, as depicted in Fig. 10a,g. Afterwards, the sample GdZS3 was considered as best photocatalyst for MO dye and applied for next examination with altering the loading dose such 1, 1.5, and 2 g/L in test solution of MO dye and obtained degradation curves shown in Fig. 10b. The GdZS3 nanocomposites attained the maximum degradation 99.3% for MO dye with loading dose 1 g/L within 60 min (irradiation time). Furthermore, increasing the loading dose to 1.5 and 2 g/L of the photocatalyst results in less degradation of the organic contaminants in the aqueous solution, as compared to a loading dose of 1 g/L of GdZS3. This could happen due to the shadow effect of the nanocomposites as an increased dose of the photocatalyst66. Generally, photocatalytic activity is a surface phenomenon that particularly occurs at the surface of the photocatalyst. Therefore, an increased amount of the photocatalyst provides less surface area for the reaction, resulting in decreased degradation32. In contrast, the complete nanocomposites present a barrier to irradiated light photons, resulting in a lower generation of electron hole pairs compared to the small amount of photocatalyst. Thus, optimizing the quantity of photocatalyst can lead to an ideal number of electron hole pairs and enhance the degradation of organic pollutants.Fig. 10 (a) MO dyes degradation performance as intensity ratio (Ct/Co) of all GdZS photocatalyst with a loading dose of 0.5 g/L as in dark and light with irradiation time, (b) Photodegradation performance of GdZS3 with varying loading dose of photocatalyst against MO dye, (c) Photodegradation performance of GdZS3 with catalyst dose 1 g/L for MO, MB and RhB dyes in light irradiation, (d) First orderkinetics of MB photodegradation, (e) First order kinetics of RhB photodegradation, (f) First order kinetics of MO photodegradation with GdZS3 photocatalyst, (g) Degradation efficiency of all GdZS photocatalyst with a loading dose of 0.5 g/L as in dark and light with irradiation time for MO dye and (h) Degradation efficiency of GdZS3 photocatalystwith a loading dose of 1 g/L as in dark and light for MO, MB and RhB dyes.

Furthermore, to evaluate the degradation of other organic dyes such as MB and RhB, optimized photocatalyst GdZS3 with appropriate amount was examined under solar irradiation for 60 min and 50 min duration and the maximum degradation was achieved 98.5% and 99.4% for MB and RhB dyes, respectively. The obtained results from the photodegradation of organic dyes using GdZS3 nanocomposites reveal the enhanced photocatalytic performance and which shown in Fig. 10c. The results of the photodegradation study validate the exceptional performance of GdZS3 nanocomposites when used with an appropriate amount of 1 g/L in degradation of MO, MB, and RhB dyes in solar light irradiation. These results were compared with previously reported findings in Table 3.Table 3 Optimized results of photocatalytic degradation of present study and comparison with previous reported results.

Photocatalyst	Pollutants	Irradiation source	pH	Irradiation time (Minutes)	Degradation efficiency (%)	References	
SnO2@ZnO hierarchical	MB	10W white light LED lamp		180	97.6	67	
SnO2@ZnO heterojunctions	RhB	300W Xe lamp	7	80	97	68	
ZnO/SnO2 nanocomposites	MO	UV light	7	210	90	69	
Green synthesised ZnO-SnO2	MO	Visible light	7	90	60	70	
ZnO-SnO2/nano clinoptilolite	4-methylbenzoic acid@2-chloro-5-nitrobenzoic acid	2Hg lamps (35W, Philips)	–	600	85

90

	71	
ZnO/Sn1−xZnxO2−x nanocomposites	MO	4W Hg-lamp (UV 4W, 254 nm)	7	130	98	72	
ZnO/SnO2 heterojunction	Tetracycline Hydrochloride (TCH)	Xenon lamp (300 W)	7	50	90	73	
SnO2 − ZnO/g‑C3N4 Nanosheets	RhB	300 W xenon lamp	7	60	96	74	
ZnO/CuO/ SnO2	Fast green (FG) dye	(500 W tungsten)	7	100	93.6	75	
SnO2 − ZnO	MB	UV lamp 125 W	7	30	98	76	
ZnO-SnO2 Hollow Spheres	RhB	UV lamp (CHF-XM- 300W)	7	50	90.34	77	
ZnO/SnO2 photocatalysts	MB	UV light	12	60	96	78	
SnO2/ZnO heterostructured nanorods	Ethyl 4- hydroxybenzoate (EHB) and bisphenol A (BPA)	UV light 300 W Xenon lamp	7	240	90	52	
ZnO-SnO2 nanocomposites	4-NP	UV lamp 125 W	7	120	98	79	
Gd doped ZnO:SnO2 nanocomposites	MO

MB

RhB

	Visible light (Natural Solar light)	7	60

60

50

	99.3

98.3

99.4

	Present study	

The accomplish outcomes also revealed that GdZS3 nanocomposites demonstrate enhanced photocatalytic activity towards photodegradation of MO, MB and RhB dyes with 1 g/L under solar light irradiation with 60 min, 60 min and 50 min, respectively from all GdZS nanocomposites. Subsequently, these nanocomposites were further studied for photodegradation efficiency at different doses and were optimized for the complete removal of MO, MB, and RhB dyes from water. GdZS3 nanocomposites, which contains the 1.2 at% of Gadolinium in ZnO/SnO2 coupled framework showed the enhanced photodegradation of organic dyes in presence of light. The maximum degradation was achieved as 99.35, 98.30 and 99.4% for MO, MB and RhB dyes (Fig. 10h), respectively with appropriate catalyst dose 1 gm/L of GdZS3 photocatalyst Gd ions provide the electrons to trap the radicals generated during the reaction and suppressed the electron hole recombination frequency as well as providing large surface area of nanocomposites. Therefore, enhanced photodegradation activity of Gd doped ZnO/SnO2 was achieved as compare with bare ZnO/SnO2 nanocomposites29.

Kinetic study of photocatalytic degradation

The photocatalytic degradation kinetic study of organic pollutants such as MB, MO and RhB was examined using different GdZS nanocomposites in presence of solar light irradiation employing UV–Vis spectrophotometer. The wavelength maxima (λmax) for MO, MB, and RhB were observed at 665 nm, 453 nm, and 556 nm, respectively, indicating absorbance peaks. A decreasing pattern was observed in the absorbance peaks of MO, MB, and RhB, indicating real-time measurement of the degradation of organic pollutants.

After being treated with a loading dose of 1 g/L of GdZS3 nanocomposites and exposed to solar light for 60 min, the consistent peaks for MO, MB, and RhB exhibited a significant decrease in absorbance, with MO and MB almost disappearing, and RhB taking 50 min to reduce. It was observed that the experimental results obtained from the photodegradation of MO, MB, and RhB followed the Langmuir–Hinshelwood model’s reaction kinetics, which can be defined by the following equation80;8 lnCoCt=kappt

where, Co indicates original concentration of dye (MB, MO and RhB) after dark stirring, while Ct refers as degraded concentration after different time interval ‘t’ min. The symbol ′k′app denotes the constant rate of reaction, which conforms to a model of pseudo first-order kinetics. This value is obtained by calculating the slope of a linear plot of the natural logarithm of the initial concentration to the current concentration [ln (Co/Ct)] against time ‘t’. The corresponding graphs for MO, MB, and RhB are shown in Figs. 10d,e,f, respectively. The experimental rate equation derived from the photodegradation of organic dyes, as mentioned below;9 rate=-dorganiccontiminantsdt=kapp

From the photodegradation experiments of organic contaminants, using GdZS3 sample as a photocatalyst with a loading dose, the rate constant values (kapp) were determined, which yielded 0.1951, 0.0216, and 0.29291 min−1 for MO, MB, and RhB dyes, respectively. The experimental kinetics data revealed the significant photocatalytic degradation efficiency of GdZS3 nanocomposites in regards to the degradation of MO, MB, and RhB dyes. Consequently, further GdZS3 nanocomposites were considered as best photocatalytic candidate and used for next examinations.

Plausible Photodegradation mechanism

Scheme 1 presents the proposed photodegradation mechanism, which is based on the results of the photodegradation of MO, MB, and RhB dyes using various GdZS photocatalyst and advanced oxidation processes (AOP) under solar light exposure. The proposed mechanism has been demonstrating about the functionality of excess electron generation and trapping by Gd3+ ions from Gd doped ZnO/SnO2 nanocomposites. The proposed mechanism entails the adsorption of dye molecules onto active sites present on the surface of photocatalyst (GdZS), which leads to the degradation of dye molecules in the presence of light photons. In contrast to other rare earths, Gd supports electrons trapped which are produced through photocatalysis. Beside this, Gd decorated ZnO/SnO2 provides the large surface area to accomplish photocatalytic reaction having small crystallite size over all pristine ZnO/SnO2 nanocomposites.Scheme 1 The plausible photodegradation mechanism of MO dye Gd doped ZnO: SnO2 photocatalyst under light irradiation.

The most significant photodegradation performance was exhibited by GdZS3, which resulted in an enlargement of the interaction region between the active sites situated on the photocatalyst surface and organic dyes. As a result, the recombination frequency of charge carriers, i.e., electron–hole pairs, was inhibited81. When visible light photon starts interacting with the dyes molecules and photocatalyst during the photocatalytic reaction, photogenerated charge carriers established the oxidation and reduction reactions at conduction band (CB) and valance band (VB) of the photocatalyst. Moreover, the formation of h+ holes occurred in the valence band, which then reacted with either the hydroxyl groups present on the photocatalyst’s surface or those found in the water. As a result, an increased amount of hydroxyl radicals was produced, leading to the rapid degradation of contaminants82–84. On next, superoxide ion formation is facilitated by the trapping of electrons (e−) in the conduction band, which occurs in the presence of dissolved oxygen. Further, the presence of Gd3+ promotes the captured electrons and convert it to Gd2+ which inhibited the electron hole recombination. The Gd2+ reacts with oxygen and generates superoxide anion. In mid of this, h+ generated by the light photons react with water molecule and develop OH• and H+. Additionally, when superoxide ions react with water, they generate hydroxyl ions and hydrogen peroxide, providing ample opportunity for both the valence band holes and conduction band electrons of Gd-ZnO/SnO2 to participate in redox reactions, which ultimately contribute to the breakdown of organic dye molecules. During the process Gd ions reduces the electron hole recombination as electron participates for large time. After degradation, CO2 and H2O were produced as degraded products having non-toxic nature. The presence of an optimal amount of Gd in GdZS resulted in improved absorption of visible light and reduced recombination rates of electron–hole pairs, which led to the enhancement of the photocatalytic degradation of organic dyes by trapping electrons.

The series of chemical reactions are found to be elaborated during the photocatalysis procedure;10 Gd-ZnO:SnO2+hϑVBh++CBe-

11 h++MO/MB/RhB Dyes Oxidation

12 Gd3++ElectronsCBe-Gd2+Doubledonor

13 Gd2++Oxygen O2-superoxide anion

14 GdZSh++H2O→GdZS+OH∙+H+

15 GdZSh++OH-→GdZS+OH∙

16 Gd2+(e-)+O2→Gd3++O2∙-

17 O2∙-+H2O→HOO∙+OH-

18 Gd2+e-+H2O2+H+→OH∙+H2O+Gd3+

19 H2O2+h→2OH∙

20 Organic dyesMO,MB,RhB+OH∙→Oxidised Degradation productsH2O,CO2etc.

Consequently, a hybrid nanocomposite that achieves high photodegradation can be engineered by appropriately incorporating Gd3+ ion, leveraging the enhanced charge carrier mobility and specific surface area of GdZS nanocomposites.

Stability test of the photocatalyst

Demonstrating the stability and suitability of GdZS3 nanocomposites for large-scale photocatalytic remediation of contaminated water necessitates taking into account the typical performance of these materials in terms of stability and recyclability85. The photocatalyst was collected via centrifugation and reused for the photodegradation of MO, MB, and RhB in an organized mode up to five-photodegradation cycles. After completing of every photodegradation cycle of GdZS3 photocatalyst was retrieved through centrifugation for 10 min at 5000 rpm. Consequently, collected nanocomposites after photodegradation test washed with warm DI water followed by the drying process for 60 min in a hot air oven at 80 °C before being utilized in the next photodegradation cycle. About 95.64, 97.60 and 95.13% degradation of MB, MO and RhB were observed after fifth cycle. The stability test cycles for MO dye treated with GdZS3 photocatalyst as shown in the Fig. 11a. Therefore, the photocatalyst’s stability and reusability for treating contaminated water may be indicated by a lack of significant reduction in photodegradation efficiency. Further the obtained GdZS3 nanocomposites were recovered and the characterized for XRD as shown in Fig. 11b and there is no any change in the XRD patterns which prove the stability of the nanocomposites.Fig. 11 (a) Stability studies of GdZS3 photocatalyst, (b) XRD patterns of the recovered GdZS3 nanocomposites and (c) COD and BOD removal from experimental organic dyes.

Mineralization study: COD and BOD

Mostly, wastewater after used in textile industry or from other sources directly flows in the rivers without purification or any prior treatment. This waste water contains various types of organic contaminants which are hazardous for all aqueous life. Therefore, after degradation of organic dyes, degraded products require to examine for dissolved oxygen level as chemically and biologically also. After the photodegradation process, all test solutions underwent examinations for their chemical and biological properties in aqueous medium, including tests for chemical oxygen demand (COD) and biological oxygen demand (BOD)2. The COD and BOD values found decreased below the recommended values (250 mg per liter (mg/L) and 30 mg of oxygen per liter (mg/L))86,87 for all organic dyes (MO, MB, RhB) after highest degradation as shown in Fig. 11c.

Conclusion

The present work demonstrates about the successful synthesis of Gd doped ZnO/SnO2 photocatalyst and photodegradation activity towards organic dyes. The photodegradation performance of the obtained nanocomposites was considered both in the dark and in presence of light with MO, MB and RhB dyes. The tetragonal rutile and hexagonal wurtzite structures of SnO2 and ZnO was confirmed in different GdZS nanocomposites. The Gd doped ZnO/SnO2 nanocomposites showed enhanced degradation performance with enhanced degradation as 99.35, 98.30 and 99.4% for MO, MB and RhB dyes. The obtained nanocomposites showed high stability towards photodegradation of organic dye. Grabbed together, these outcomes provide further insight into the combined effects of Gd ion doping and coupling of ZnO/SnO2 on photocatalytic activity enhancement. The plausible photodegradation mechanism of GdZS photocatalyst towards organic dyes including MO, MB and RhB was also proposed in appropriate pathway. Therefore, this study provides the insights into the new outcomes for waste water remediation and for industry also.

Author contributions

S.P., First author Sample preparation, data collection, analysis, and interpretation. Drafted the content of the paper including results and discussion part of the paper V.K., Co-author. Discussed the intellectual content of the work. Contributed to the results and discussion part of the paper. Proof reading and finalizing the draft of the manuscript L.P.P., Corresponding author. Discussed the intellectual content of the work. Proof reading and finalising the draft of the manuscript. Accountable for the research work, ensuring that queries related to the accuracy or integrity of the research. Apart from this, communicate with the journal during manuscript submission, peer review, and approval of the final version of the manuscript for publication.

Data availability

The datasets used and/or analysed during the current study available from the corresponding author on reasonable request.

Competing interests

The authors declare no competing interests.

Publisher's note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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