Photocatalytic activity of red emission PVA-GdVO4:Eu3+ nanocomposite towards the degradation of Eosin Y in water

ABSTRACT The availability of clean and safe water is a fundamental necessity for the sustenance of life and the well-being of our planet. Photocatalytic water treatment using nanomaterials has emerged as a promising and cutting-edge approach to address this pressing environmental challenge. In this work, a co-precipitation method was employed to synthesize europium (Eu3+) doped gadolinium metavanadate (GdVO4). The GdVO4:Eu3+ was encapsulated in polyvinyl alcohol (PVA). The GdVO4:Eu3+ and PVA-GdVO4:Eu3+ were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), Fourier-transform infrared (FTIR) spectroscopy, Raman spectroscopy, thermogravimetric analysis (TGA), photoluminescence (PL), and UV–visible spectroscopy (UV-Vis). The XRD, FTIR, and Raman spectroscopy results confirmed the formation of both GdVO4:Eu3+ crystals and PVA-GdVO4:Eu3+ nanocomposite. Both compounds’ photoluminescence spectra demonstrated effective energy transfer from the GdVO4 host to the Eu3+. Results obtained from the TGA indicate that PVA-GdVO4:Eu3+ nanocomposite is stable at a temperature of 330°C. Under UV irradiation, materials’ photocatalytic efficiency was examined regarding their ability to degrade Eosin Y dye in water. Results from the photocatalytic studies of the synthesized PVA-GdVO4:Eu3+ showed improved photocatalytic activity compared to GdVO4:Eu3+ under the same experimental conditions. When operating under ideal conditions of pH = 2, initial dye concentration of 30 ppm, a catalyst dosage of 200 mg, the degradation of Eosin Y surpassed 95% within 150 min of exposure to light.


Introduction
When it comes to dying, printing, leather, paper products, and luminous pigments, Eosin Y, a heterocyclic dye with bromine atoms, is frequently utilized (Yadav et al., 2022).Due to its vibrant hue, Eosin Y is utilized in painting and colouring companies (Sabnis, 2017).Anthropogenic activities that lead to the direct discharge of wastewater that contains Eosin Y can pose serious environmental effects due to its toxicity and carcinogenicity and its resistance to conventional chemical and biological treatment methods (Zhu et al., 2016).It is therefore important to eliminate this dye in water using a simple and cost-effective technique.
Most techniques such as coagulation, flocculation, membrane separation and adsorption on activated carbon, which are only based on the phase transfer mechanism of pollutants, are employed for the removal of dyes from water (Afroze & Kanti Sen, 2018).However, these methods have a lot of drawbacks such as high operational costs and the generation of high amounts of sludges.Photocatalysis includes the degradation of contaminants from complex compounds into simple and safer chemicals, contrary to other traditional techniques of environmental remediation (Ameta et al., 2018).As a result, there is no sediment or waste left behind from the operation, and the catalyst was unaltered continuously, allowing it to be reused (Karthik et al., 2017).Rare earth orthovanadates RVO 4 (R = Y, Nd, Gd) have dominated recent studies due to their versatile applications (Jing et al., 2016).Trivalent lanthanide ions have been used as hosts in many lightemitting materials because of their greater spectral properties including narrow-band emission and long lifetime (Singh et al., 2022).There has been a wide range of applications of Eu 3+ because of the increased luminescent efficiency it possesses (Pravinraj et al., 2017).Luminescent BiVO 4 :Eu 3+ has been reported to be a promising photocatalyst for water treatment.Highly luminescent red emitting GdVO 4 :Eu 3+ has also been applied in many fields such as cathode ray tubes, X-Ray detectors, and lamps (Thakur et al., 2021).Its great ultraviolet light absorption, efficient energy transfer from light VO 4 3-to Eu 3+ , and efficient stimulation of Eu 3+ by the 450 nm intense emission of VO 4 3-are what cause the material to glow red (Huang et al., 2013).Similar to YVO 4 :Eu 3+ and LuVO 4 :Eu 3+ , GdVO 4 :Eu 3+ has luminous intensity.Of all the rare earth-doped vanadates, it is said that GdVO 4 :Eu 3+ has the highest luminescence intensity (González-Penguelly et al., 2022).Composites of intrinsically conducting polymers have drawn considerable interest for their worldwide applications (Xu et al., 2021).Composites of conducting polymers are made of conjugated polymers and at least one inorganic or organic material (Sardana et al., 2022).Alternatives to traditionally filled polymers include polymer nanocomposites.The filler's size leads to better distribution, which enhances the synergistic effect of the different components' features in the nanocomposite material.
Several efforts have been made to find ways to disseminate the nanomaterials in water and make them mobile since they must be in a non-agglomerated state to be employed in water contamination cleanup.Several biopolymer-based composites such as starch (Rao et al., 2014), polylactic acid delivery vehicles (Bhattarai et al., 2017) and modified cellulose and various polymers have been tested and proven to be extremely effective.PVA is a biopolymer that has been used extensively as a host for many nanosized materials due to its high mechanical and thermal properties, conductivity, and biocompatibility (Jiang & Dawei, 2022;Rol et al., 2019).
In this study, we report an easy and environmentally friendly method for synthesizing GdVO 4 :Eu 3+ nanoparticles.The synthesized GdVO 4 :Eu 3+ nanoparticles were then encapsulated in a PVA polymer matrix to form PVA-GdVO 4 :Eu 3+ .Moreover, the photocatalytic activities of GdVO 4 :Eu 3+ and PVA-GdVO 4 :Eu 3+ in the degradation of Eosin Y were also discussed.

Experimental
None of the chemicals used for the experiment were subjected to additional processing; they were all of analytical grade.Gadolinium nitrate hexahydrate (Gd(NO 3 ) 3 •6 H 2 O), ammonia solution, Eosin Y, Vanadium pentoxide(V 2 O 5 ), europium nitrate hexahydrate (Eu(NO 3 ) 3 •6 H 2 O), and polyvinyl alcohol (PVA) were obtained from Sigma Aldrich, Germany.
After stirring for 30 mins, this solution was added to the V 2 O 5 -ammonia solution, and the pH of the mixture was raised to 8.0 using NH 4 OH.The yellow precipitate formed was centrifuged and washed sequentially with ethanol and distilled water.The resultant precipitates were heated at 150°C for 6 hrs.

Synthesis of PVA-GdVO 4 :Eu 3+
PVA (0.9 g) was slowly dispersed into distilled water (10 mL) whilst on a hot plate magnetic stirrer.The preprepared GdVO 4 :Eu 3+ (0.1 g) was added to the reaction mixture and was heated for 30 mins and sonicated for 45 minutes.The mixture was then cast on a cleaned glass plate, dried at 50°C and pulverized.

Testing photocatalytic activity of materials
Using a Camag UV-Cabinet II (= 366 nm) at room temperature, a suspension of 100 mg of the catalyst in 100 mL of an aqueous solution of Eosin Y (30 mg/L) was put under mechanical stirring in order to study the photocatalytic activity of the materials.The lamp is operated at 12 V which is internally converted to 25-30 kHz at a high UV lamp long-wave at 2 × 8 W. The pH of the dye solution was adjusted in the range of 2.0 to 7.0 using dilute HNO 3 or NaOH to obtain the optimum pH for the degradation reaction of the samples.One hour before light illumination, the materials were magnetically stirred in the dark to enable adsorption equilibrium.A disposable syringe was used to remove aliquots of the suspension (5 mL) at regular time intervals.Centrifugation was used to separate the mixture from the supernatant for 15 minutes at 3000 rpm.Using a Shimadzu UV-2450 spectrophotometer set to a wavelength of 515 nm, the amount of Eosin Y left in the supernatant solution after irradiation was measured.Dye degradation efficiency was determined as stated in Equation 1 (Chen et al., 2020).

Removal Efficiency of Dye
Where C o and C represent, respectively, the initial and remaining dye concentrations at a particular time.

Sample characterization
The samples' X-ray diffractograms were captured using a Rigaku Ultima IV X-ray diffractometer with Cu K radiation, while their TEM images were captured using a field emission electron microscope (JEM-2100F).The SEM analysis was performed using a TESCAN (Vega 3 XMU) to determine the surface morphology and particle size.FT-IR spectra were obtained using a PerkinElmer Spectrum 100 FT-IR spectrometer, the Raman spectra were taken using a PerkinElmer Raman microscope (Raman Micro 200), optical absorption spectra were recorded using a Shimadzu UV-Visible spectrophotometer (UV-2450), photoluminescence studies were performed using PerkinElmer fluorescence spectrometer (LS:45), and TGA measurements were undertaken using PerkinElmer thermogravimetric analyzer (TGA 4000).

XRD analysis
Figure 1 shows the XRD pattern of the as-prepared Eu 3+ doped GdVO 4 .All the peaks matched with the standard diffraction data for the bulk GdVO 4 with a tetragonal gadolinium structure (JCPDS, No. 17-0260).There were no further peaks, indicating that the tetragonal gadolinium structure was formed in a single crystal and that the Eu 3+ ions had been incorporated into the GdVO 4 host crystal lattice.The values measured from the XRD patterns for the as-prepared GdVO 4 :Eu 3+ powder and those given in JCPDS No. 17-0260 are listed in Table 1.The enlarged d values of the strongest peaks show that the Eu 3+ ions in the host material partially replaced Gd 3+ ions.This is due to the relatively large ionic radius of Eu 3+ ions (94.7 pm) compared to that of Gd 3+ ions (93.8 pm).A similar report was reported by other researchers (Szczeszak et al., 2014) while studying the structural and spectroscopic properties of Eu 3+ doped GdVO 4 .

FTIR analysis
The FTIR data of materials provide good information about the functional groups and surface chemistry.The FTIR spectra of GdVO 4 :Eu 3+ and PVA-GdVO 4 :Eu 3+ nanocomposite are shown in Figure 2.There were four major peaks observed at 792, 1024, 1387, and 3650 cm −1 for GdVO 4 :Eu 3+ (Figure 2(a)).The bulk information about lattice vibration of GdVO 4 nanoparticle is indicated by its strong absorption at ~800 cm −1 , which is assigned to the V-O vibration stretch of the host lattice (Szczeszak et al., 2014).Because nanocomposites are typically impacted by adsorbed water molecules, which have various distinctive IR bands as established by several researchers, the IR spectrum also reveals the surface chemistry of the nanocomposites.The characteristic broad peaks of the GdVO 4 :Eu 3+ at 3650 cm −1 and the sharp peak at 1024 cm −1 may be attributed to O-H and C-O stretch of ethanol used for washing the samples, respectively.The peaks observed at 792 and 1387 cm−1 correspond to V-O and bending vibrations of CH2.
The FT-IR spectrum of Figure 2 the formation of the nanocomposite.The V-O stretch shifted from 792 to 842 cm −1 and in addition, the stretching vibration of O-H at 3650 cm −1 shifted to 3673 cm −1 , indicating that the GdVO4:Eu 3+ nanoparticle was bound to the OH-functional groups of PVA (Mansur et al., 2004;Sudhamani et al., 2003).The absorption band at 2919 cm −1 is due to the C-H symmetric stretch.The bands at 1733 and 1427 cm −1 can be assigned to C=O and C=C stretches, respectively.The weak peak at ~2161 cm −1 corresponds to the combination frequency of (C-H + C-C), whereas the strong peak at ~1343 cm −1 was due to the combination frequency of (C-H+O-H) group (Abral et al., 2019).

Raman spectroscopy
The measured Raman spectra of zircon-type GdVO 4 : Eu 3+ and PVA-GdVO 4 :Eu 3+ nanocomposite are shown in Figure 3.The Raman spectrum for the GdVO 4 :Eu 3+  in Figure 3(a) showed only at 8 of 11 Raman active modes, which were in accord with other studies (Pudkon et al., 2016).With the zircon-type  The absorption bands at 825 cm −1 correspond to a combined frequency of (E g and B 1 g ) modes, which can be assigned to VO 4 stretches.The absorption peak at 883 cm −1 (υ 1 ) corresponds to VO 4 asymmetric stretch (Bespalova et al., 2017).The weak peak 2807 is assigned to C-H stretches.
Figure 3(b) shows the Raman spectrum of PVA-GdVO 4 :Eu 3+ nanocomposite.The absorption bands at 1366 and 2827 cm −1 are ascribed to C-H stretches, and the broad and strong peak at 1440 cm −1 is due to -CH 2 deformation.The peak at 1150 cm −1 corresponds to the C=C stretch.The results from the Raman spectrum of the nanocomposite also confirm the incorporation of the GdVO 4 :Eu 3+ nanoparticle into the PVA.The weak absorption peak at 883 cm −1 can be assigned to A 1 g (υ 1 ) band (Mansur et al., 2008), which corresponds to VO 4 asymmetric stretch.The results from Raman studies therefore collaborate with the FTIR results, showing the presence of GdVO 4 :Eu 3+ nanocomposite in PVA.

TEM and SEM analysis
The morphology of the prepared GdVO 4 :Eu 3+ powder and PVA-GdVO 4 :Eu 3+ nanocomposites was observed by TEM and SEM. Figure 4(a) shows the TEM images of GdVO 4 :Eu 3+ grains.The particles are rice-like in structure and uniform in shape with an average size of 37 nm in width and 96 nm in length with good dispersity.The crystals were not tightly aggregated.Li et al. (2016), reported a similar effect while studying the photoluminescence properties of Dy 3+ , Eu 3+ , and doped GdVO 4 .SEM images of the PVA-GdVO 4 :Eu 3+ nanocomposite are shown in Figure 4(b).PVA coatings are responsible for the rise in particle diameter shown in the photograph.The enlarged size of the PVA coated GdVO 4 :Eu 3 + showed no agglomeration which will enhance its photocatalytic performance and the recovery of the material after photocatalysis.The elemental composition of the GdVO 4 :Eu 3+ nanoparticles was investigated by EDX analysis.The signals observed in Figure 4(c) show that the main elemental components are Gd, V, O, and Eu, confirming the formation of GdVO 4 :Eu 3+ .

Thermogravimetric analysis
The representative TGA spectra for PVA-GdVO 4 :Eu 3+  and GdVO 4 :Eu 3+ in a nitrogen-free environment are shown in Figure 5.According to Figure 5(a), the weight loss did not significantly change across the full temperature range of 50°C to about 800°C.The evaporation of the surface that absorbed moisture is what causes the weight loss between 50°C and 100°C.The unstable nature of the VO 4 subunits in the complex is what causes the steady decline in weight loss at elevated temperatures (Sudhamani et al., 2003).The PVA-GdVO 4 :Eu 3+ nanocomposite's TGA spectrum is depicted in Figure 5(b).Water loss, caused by the loss of moisture and perhaps other volatile organic molecules in the composites, is the first step of weight loss between 50°C and 120°C.At approximately 120°C and 240°C, where active pyrolysis began, little weight loss was seen.PVA is decomposing at temperatures between 120°C and 330°C, which causes a weight loss of roughly 67%.
The temperature range between 400°C and 540°C shows the last stage of pyrolysis of all organic compounds in the PVA-GdVO 4 :Eu 3+ nanocomposite and the gradual reduction in the weight at higher temperatures is due to the instability of VO 4 groups (Huang et al., 2013(Huang et al., , 2012;;Sudhamani et al., 2003).This shows that PVA-GdVO 4 : Eu 3+ is stable up to approximately 330°C, whereas pure PVA is known to be stable at 340°C (Liang et al., 2018;Shandilya et al., 2019;Song et al., 2017).

Photoluminescence excitation
The luminescence properties of GdVO 4 :Eu 3+ and PVA-GdVO 4 :Eu 3+ were measured by their fluorescence spectrum.The peak shapes of both samples are almost similar.The emission wavelength of 616 nm (Yi et al., 2006), at which excitation occurs is shown in Figure 6.The excitation consists of a narrow band with a strong and sharp peak center at about 312 nm and another peak at 280 nm.This indicates that there is a complete transfer of energy from the GdVO 4 host to the Eu 3+ ions (Tang et al., 2012).The strong and sharp peak at 312 nm in both GdVO 4 :Eu 3+ and PVA-GdVO 4 :Eu 3+ can be ascribed to the general f-f transitions of Eu 3+ .These sharp peaks observed between 200 and 350 nm can be attributed to the charge transfer band of Eu-O and V-O (Jovanović et al., 2013).The peaks centered at 280 nm can be assigned to the GdVO 4 host (VO 4 3-) excitation band and the 8 S-6 D transitions, respectively (Thakur et al., 2021).spectra show a strong and sharp red emission peak at 520 nm and another peak at 571 nm.These narrow and sharp peaks observed in the spectrum can be attributed to the shielding of the 4f electrons by the 5s and 5p electrons in the outer shells in the Eu 3+ ions, and the emission peaks observed at about 521 nm are due to Eu 3+ 5 D 1 -7 F 1 transitions (Chung et al., 2009;Shim et al., 2007).
When the as-synthesized materials were exposed to 254 (365) nm UV light, the materials emit bright red light.The relevant UV-vis absorption spectra for GdVO 4 :Eu 3+ and PVA-GdVO 4 :Eu 3+ are depicted in Figure 8.There are both clearly strong UV and weak visible light absorption peaks in the entire absorption spectrum of the samples.The major peak in the UV region for the GdVO 4 :Eu 3+ may be as a result of charge transfer band between V 5+ and O 2-.These strong UV absorption bands may also originate from the vanadate (VO 4 ) 3-group, which acts as a self-activated host (Chung et al., 2009;Shim et al., 2007).

Effect of pH on the degradation of Eosin Y using PVA-GdVO 4 :Eu 3+ catalyst
From Figure 10, it was obvious that the rate of degradation increased significantly in Eosin Y with a decrease in pH.This is because Eosin Y (anionic dye) degrades faster at lower pH.This may be due to the creation of positive charges on the surface of the catalyst which leads to the attraction of the anionic dye for enhanced degradation.

Effect of catalyst concentration on the degradation of Eosin Y using PVA-GdVO 4 :Eu 3+ at pH = 2
The effect of Eosin Y dye concentration on PVA-GdVO 4 : Eu 3+ loading on the rate of degradation was performed by varying the amount of PVA-GdVO 4 :Eu 3+ catalyst loading from 50 mg to 200 mg at pH = 2 with fixed dye concentration (30 ppm) (Agorku et al., 2014;Maksimchuk et al., 2020;Oppong et al., 2018).Figure 9 shows the effect of the loading of the catalyst on the rate of photodegradation of the dye.From Figure 11, it can be observed that the amount of the PVA-GdVO 4 :Eu 3+ influences the degradation rate of Eosin Y.The rate increased with an increasing amount of the catalyst and the concentration of the Eosin Y dye inclined from 85% (50 mg) to 92% (200 mg) after 150 min of illumination.The rise in the rate of photodegradation can be attributed to OH. radicals formed which has a great impact on the process of degradation (Oppong et al., 2021).A rise in catalyst mass corresponds to an   increase in the formation of OH. radicals, leading to a rise in the rate of degradation of the dye molecule.Active charge carrier transfer, a narrower band gap, and delayed charge carrier recombination as a result of Gd and Eu trapping electrons in the conduction band could all account for the higher photocatalytic activity (Figure 12).In general, it can be noted that the rate of degradation of the dye did not change much with catalyst loading at 150 mg (91%) and 200 mg (92%) and this may be due to an increase in light scattering (Agorku et al., 2014).As a result, using more catalysts may not be beneficial due to the possibility of clumping as well as the lower irradiation field resulting from a rise in dispersion (Sakthivel et al., 2003).

Conclusion
Luminescent GdVO 4 :Eu 3+ was prepared by coprecipitation method and PVA-GdVO 4 :Eu 3+ nanocomposite by the simple wet-processing method.The TEM and SEM results of the GdVO 4 :Eu 3+ powder showed a rice-like structure and are uniform in shape with an average size of 37 nm in width and 96 nm in length with good dispersity.XRD, FTIR, and Raman spectroscopy results confirmed the formation of both GdVO4:Eu 3+ and PVA-GdVO4:Eu 3 + nanocomposite.Both materials displayed visible red-light emission under UV excitation.TGA results showed insignificant weight loss for GdVO 4 :Eu 3+ powder over the entire temperature range between 50°C and 800°C, whereas the PVA-GdVO 4 : Eu 3+ nanocomposite exhibited significant weight loss (67%) between 120°C and 240°C, and this may be due to pyrolysis of PVA in the nanocomposite.The photocatalytic activities of the PVA-GdVO4:Eu 3+ decreased markedly with a decrease in pH.The PVA-GdVO4:Eu 3+ showed a high rate of Eosin Y degradation at low catalyst loading, which suggests that low catalyst loading may be more useful than high loading.The optimum catalyst dosage required for the degradation of 30 mg/L of Eosin Y in 150 min at a pH of 2 was 200 mg.

Data accessibility
The data used in this research article came from our experiments.The data used to support the study's findings are included in the article and can also be obtained upon request from the corresponding author.

Figure 10 .
Figure 10.(a) Effect of pH on the degradation of Eosin Y and (b) percentage removal of Eosin Y at different pH using PVAGdVO 4 :Eu 3+ .Catalyst amount = 200 mg/100 mL of 30 ppm Eosin Y.

Figure 11 .
Figure 11.(a) Effect of catalyst concentration on the degradation of Eosin Y (30 ppm) using PVA-GdVO 4 :Eu 3+ at pH=2 and (b) percentage removal vs irradiation time as a function of catalyst loading.

Table 1 .
d, values measured by XRD diffraction

Table 2 .
Comparison of photocatalytic activity of material with other works