Reduced Graphene Oxide–Metal Oxide Nanocomposites (ZrO 2 and Y 2 O 3 ): Fabrication and Characterization for the Photocatalytic Degradation of Picric Acid

: Herein, reduced graphene-oxide-supported ZrO 2 and Y 2 O 3 (rGO-ZrO 2 and rGO-Y 2 O 3 ) nanocomposites were synthesized by hydrothermal method and used as the catalysts for photodegradation of picric acid. The structural and morphological properties of the synthesized samples were characterized by using an X-ray diffractometer (XRD), scanning electron microscope (SEM) with energy dispersive absorption X-ray spectroscopy (EDAX), UV-Vis spectrophotometer, Raman spectrophotometer and Fourier transformation infrared spectrophotometer (FT-IR) techniques. In this work, the wide band gap of the ZrO 2 and Y 2 O 3 was successfully reduced by addition of the reduced graphene oxide (rGO) to absorb visible light for photocatalytic application. The performance of as synthesized rGO-ZrO 2 and rGO-Y 2 O 3 nanocomposites in the photocatalytic degradation of picric acid were evaluated under UV light irradiation. The photodegradation study using picric acid was analyzed with different energy light sources UV (254, 365 and 395 nm), visible light and sunlight at different pH conditions (pH = 3, 7 and 10). The photocatalytic activity of rGO-ZrO 2 and rGO-Y 2 O 3 nanocomposites showed excellent photocatalytic activity under optimum identical conditions with mild variations in pH 3. Compared to rGO-Y 2 O 3 , the rGO-ZrO 2 nanocomposite showed a better action, with a degradation percentage rate of 100, 99.3, 99.9, 100 and 100% for light conditions of UV-252, 365, 395, visible and sunlight, respectively. The excellent degradation efﬁciency is attributed to factors such as oxygen-deﬁcient metal oxide phase, high surface area and creation of a greater number of hydroxyl groups.


Introduction
Various organic pollutants are increasing day by day because of rapidly growing industries such as textiles [1], plastic [2], food [3], gun powder [4], pesticides [5], tanning [6,7], etc. These industries regularly release their pollutants, which contain chemical substances, into the environment, causing health issues [8]. These pollutants have been highly toxic and non-degradable over many years. Many traditional methods, such as thermal destruction [9], precipitation techniques [10], membrane separations [11] and ultra-filtration [12], were used for purifying industrial pollutants. In addition, absorbents such as activated carbons [13] and zeolites [14,15] were also used to remove the organic chemical contaminants. However, these methods have several disadvantages, such as enormous energy requirements, extended treatment times, high power consumption, etc.

Results and Discussion
The synthesized rGO-ZrO 2 and rGO-Y 2 O 3 nanocomposite were subjected to analysis of the crystalline structure and crystal size by an X-ray diffractometer. Figure 1 shows the diffractogram for rGO-ZrO 2 , which displayed the diffraction peaks at 2θ = 30.06, 34.90, 50.90, 60.12, 3.66, 81.64 and 85.34 • , which are closely matched to the file number JCPDS 27-0997, which corresponds to its cubic structure. The lattice constant of a = 5.1448, the average crystal size is 22 nm and the calculated d-spacing for a plane (1 1 1) is 2.9704 nm. The diffractor image obtained for the rGO-Y 2 O 3 , which exhibits the diffraction peaks at 2θ = 14.98, 20.78, 29.29, 33.88, 35.38, 43.54, 48.74, 53.03 and 57.84 • , corresponds to the cubic structure of Y 2 O 3 (JCPDS card No. 71-0099) [35]. A small hump appears in the range of 2θ 25-27 degrees. This is due to the presence of graphene oxide. The calculated lattice constant is a = 5. 712. The average particle size of rGO-Y 2 O 3 is 45 nm. The calculated d-spacing for the plane (2 2 2) is 2.6437 nm. Both samples' XRD results confirm the synthesized ZrO 2 and Y 2 O 3 and show that the crystal sizes are in the nano range.
Catalysts 2022, 12, x FOR PEER REVIEW 3 of 13 method. The synthesized rGO-ZrO2 and rGO-Y2O3 nanocomposites were applied for picric acid photodegradation with different light sources and various pH (3, 7 and 10). To enhance the rGO-ZrO2 and rGO-Y2O3 photodegradation properties, Fenton's reagents were also added in a low quantity. Furthermore, the percentage of degradations and mechanisms of picric acid are investigated and discussed.

Results and Discussion
The synthesized rGO-ZrO2 and rGO-Y2O3 nanocomposite were subjected to analysis of the crystalline structure and crystal size by an X-ray diffractometer. Figure 1 shows the diffractogram for rGO-ZrO2, which displayed the diffraction peaks at 2θ = 30.06, 34.90, 50.90, 60.12, 3.66, 81.64 and 85.34°, which are closely matched to the file number JCPDS 27-0997, which corresponds to its cubic structure. The lattice constant of a = 5.1448, the average crystal size is 22 nm and the calculated d-spacing for a plane (1 1 1) [35]. A small hump appears in the range of 2θ 25-27 degrees. This is due to the presence of graphene oxide. The calculated lattice constant is a = 5. 712.The average particle size of rGO-Y2O3 is 45 nm. The calculated d-spacing for the plane (2 2 2) is 2.6437 nm. Both samples' XRD results confirm the synthesized ZrO2 and Y2O3 and show that the crystal sizes are in the nano range. SEM micrographs obtained for rGO-ZrO2 and rGO-Y2O3 nanocomposite are shown in Figure 2. Figure 2a,b show the rGO-ZrO2 nanocomposite SEM image, which reveals the formation of a multilayer of r-GO sheets. In the meantime, larger aggregated particles of ZrO2 (represented in yellow circles) were found on the surface of GO sheets. The synthesized ZrO2 shows uneven morphology with an average particle size of <100 nm. Figure  2d,e also show the r-GO sheets and aggregated Y2O3 nanoparticles supported on it. This aggregation is due to heterogeneous solid nucleation between GO and Y2O3. The synthesized Y2O3 comprises spherically shaped particles with varying sizes between 80 to 200 nm. Figure 2c shows the results of an analysis of the elements present in the nanocomposite using energy dispersive spectroscopy (EDAX). The investigation of EDAX data reveals that Zr, C and O were present in the percentages of 64, 25.2 and 10.0%, respectively. This distribution of elements (Zr, C, O) gives positive results for the formation of the rGO-ZrO2 nanocomposite. The microstructure of rGO-Y2O3 reveals that particles are heavily agglomerated on the GO sheets, as shown in Figure 2e. Figure 2f depicts the energy dispersive X-ray spectrum (EDAX) of the rGO-Y2O3 nanocomposite. The detected sharp SEM micrographs obtained for rGO-ZrO 2 and rGO-Y 2 O 3 nanocomposite are shown in Figure 2. Figure 2a,b show the rGO-ZrO 2 nanocomposite SEM image, which reveals the formation of a multilayer of r-GO sheets. In the meantime, larger aggregated particles of ZrO 2 (represented in yellow circles) were found on the surface of GO sheets. The synthesized ZrO 2 shows uneven morphology with an average particle size of <100 nm. Figure 2d,e also show the r-GO sheets and aggregated Y 2 O 3 nanoparticles supported on it. This aggregation is due to heterogeneous solid nucleation between GO and Y 2 O 3 . The synthesized Y 2 O 3 comprises spherically shaped particles with varying sizes between 80 to 200 nm. Figure 2c shows the results of an analysis of the elements present in the nanocomposite using energy dispersive spectroscopy (EDAX). The investigation of EDAX data reveals that Zr, C and O were present in the percentages of 64, 25.2 and 10.0%, respectively. This distribution of elements (Zr, C, O) gives positive results for the formation of the rGO-ZrO 2 nanocomposite. The microstructure of rGO-Y 2 O 3 reveals that particles are heavily agglomerated on the GO sheets, as shown in Figure 2e. Figure 2f depicts the energy dispersive X-ray spectrum (EDAX) of the rGO-Y 2 O 3 nanocomposite. The detected sharp related elements in the EDAX spectrum for rGO-Y 2 O 3 (Y, C, O) are 83.8, 7.2 and 9.0%, respectively. This result confirms the formation of the rGO-Y 2 O 3 nanocomposite. Since the rGO single layer appears along with metal oxides as a nanocomposite form, it appears as multi-layers. Moreover, because of the limited resolution of the SEM, it also appears as multilayered.
Catalysts 2022, 12, x FOR PEER REVIEW 4 of 13 related elements in the EDAX spectrum for rGO-Y2O3 (Y, C, O) are 83.8, 7.2 and 9.0%, respectively. This result confirms the formation of the rGO-Y2O3 nanocomposite. Since the rGO single layer appears along with metal oxides as a nanocomposite form, it appears as multi-layers. Moreover, because of the limited resolution of the SEM, it also appears as multilayered. FT-IR analysis was carried out for the synthesis of nanocomposites rGO-ZrO2 and rGO-Y2O3, as shown in Figure 3a. For Y2O3, the sharp and intense peak of stretching vibration bands appears at 614.88 cm −1 . This result corresponds to the formation of the Y-O metal-oxygen bond. In the case of rGO-Y2O3 nanocomposite, the metal peak shifted to 620.2 cm −1 . The above shift may be due to heterojunction formation between Y2O3 and rGO. The band at 1351.8 cm −1 is responsible for C-OH. A peak appearing in the range of 1581.97 cm −1 is accountable for the vibration mode from the water molecules absorbed and C-C skeleton vibration of rGO-ZrO2 nanocomposites, which is shown in Figure 3a. The peak 566.61 and 650.0 cm −1 represent the metal oxides' stretching vibration [36].
The new peak appears at the range of 1449.66 cm −1 , corresponding to the O-H stretching and bending vibrations of graphene oxide [37]. This result indicates that GO has successfully been composited with ZrO2, which is shown in Figure 3a. The Raman spectra were recorded to investigate the surface-related defects of rGO-Y2O3 and ZrO2 nanocomposites and are presented in Figure 3b. The Eg, A1g + B1g, B1g and Eg mode is responsible for the vibration peak for the Y2O3 and ZrO2 metal oxide nanoparticles. The metal oxide peaks of ZrO2 at 159, 273, 568 and 640 cm −1 and rGO peaks of D band and G band appear at 1268 cm −1 and 1608 cm −1 , respectively [38]. Also, other nanocomposites of the rGO-Y2O3 reveal the metal oxide peaks at 300 to 400 cm −1 , and the rGO is confirmed by the presence of the D and G band at 1200 cm −1 and 1623 cm −1 , respectively. It represents the carbon atom's lattice defect and the in-plane stretching vibration of the carbon atom SP 2 hybridization [39]. The results confirmed the successful formation of reduced graphene oxide-Y2O3 and ZrO2 composite [40]. FT-IR analysis was carried out for the synthesis of nanocomposites rGO-ZrO 2 and rGO-Y 2 O 3 , as shown in Figure 3a. For Y 2 O 3 , the sharp and intense peak of stretching vibration bands appears at 614.88 cm −1 . This result corresponds to the formation of the Y-O metal-oxygen bond. In the case of rGO-Y 2 O 3 nanocomposite, the metal peak shifted to 620.2 cm −1 . The above shift may be due to heterojunction formation between Y 2 O 3 and rGO. The band at 1351.8 cm −1 is responsible for C-OH. A peak appearing in the range of 1581.97 cm −1 is accountable for the vibration mode from the water molecules absorbed and C-C skeleton vibration of rGO-ZrO 2 nanocomposites, which is shown in Figure 3a. The peak 566.61 and 650.0 cm −1 represent the metal oxides' stretching vibration [36]. UV-visible absorption spectroscopy is a non-destructive tool used to determine the optical properties of synthesized nanocomposites, as shown in Figure 4a. The rGO-ZrO2 and rGO-Y2O3 samples were characterized by solid-state absorption in the wavelength range of 800 nm to 200 nm. Both rGO-ZrO2 and rGO-Y2O3 nanocomposites showed strong and broad absorption, whereas the bare Y2O3 and ZrO2 showed weak absorption, as  The new peak appears at the range of 1449.66 cm −1 , corresponding to the O-H stretching and bending vibrations of graphene oxide [37]. This result indicates that GO has successfully been composited with ZrO 2 , which is shown in Figure 3a. The Raman spectra were recorded to investigate the surface-related defects of rGO-Y 2 O 3 and ZrO 2 nanocomposites and are presented in Figure 3b. The E g , A 1g + B 1g , B 1g and E g mode is responsible for the vibration peak for the Y 2 O 3 and ZrO 2 metal oxide nanoparticles. The metal oxide peaks of ZrO 2 at 159, 273, 568 and 640 cm −1 and rGO peaks of D band and G band appear at 1268 cm −1 and 1608 cm −1 , respectively [38]. Also, other nanocomposites of the rGO-Y 2 O 3 reveal the metal oxide peaks at 300 to 400 cm −1 , and the rGO is confirmed by the presence of the D and G band at 1200 cm −1 and 1623 cm −1 , respectively. It represents the carbon atom's lattice defect and the in-plane stretching vibration of the carbon atom SP 2 hybridization [39]. The results confirmed the successful formation of reduced graphene oxide-Y 2 O 3 and ZrO 2 composite [40].
UV-visible absorption spectroscopy is a non-destructive tool used to determine the optical properties of synthesized nanocomposites, as shown in Figure 4a. The rGO-ZrO 2 and rGO-Y 2 O 3 samples were characterized by solid-state absorption in the wavelength range of 800 nm to 200 nm. Both rGO-ZrO 2 and rGO-Y 2 O 3 nanocomposites showed strong and broad absorption, whereas the bare Y 2 O 3 and ZrO 2 showed weak absorption, as shown in Figure 4. The broad absorption of nanocomposite is due to the π→π* transitions of the C = C bond present in the sample; this criterion is also applicable for the samples. The band gap of the rGO-ZrO 2 and rGO-Y 2 O 3 nanocomposites is determined by the absorption value of the corresponding samples. It shows the band gap value of 2.56 and 2.78 eV for rGO-ZrO 2 and rGO-Y 2 O 3 nanocomposites, respectively (inset of Figure 4b). The narrow band gap of the nanocomposites is due to shifting the valance band/conduction band position by using reduced graphene oxide. Thus, the obtained nanocomposites were expected to show an improved photoactivity under simulated light irradiations. UV-visible absorption spectroscopy is a non-destructive tool used to determine the optical properties of synthesized nanocomposites, as shown in Figure 4a. The rGO-ZrO2 and rGO-Y2O3 samples were characterized by solid-state absorption in the wavelength range of 800 nm to 200 nm. Both rGO-ZrO2 and rGO-Y2O3 nanocomposites showed strong and broad absorption, whereas the bare Y2O3 and ZrO2 showed weak absorption, as shown in Figure 4. The broad absorption of nanocomposite is due to the π→π* transitions of the C = C bond present in the sample; this criterion is also applicable for the samples. The band gap of the rGO-ZrO2 and rGO-Y2O3 nanocomposites is determined by the absorption value of the corresponding samples. It shows the band gap value of 2.56 and 2.78 eV for rGO-ZrO2 and rGO-Y2O3 nanocomposites, respectively (inset of Figure 4b). The narrow band gap of the nanocomposites is due to shifting the valance band/conduction band position by using reduced graphene oxide. Thus, the obtained nanocomposites were expected to show an improved photoactivity under simulated light irradiations.  Using organic contaminants such as picric acid, the photocatalytic degradation activity of the produced nanocomposites (rGO-ZrO 2 and rGO-Y 2 O 3 ) were determined. posites at pH 7 and 10. But the degradation was increased to 99% at the period of 15 min to 30 min at pH 3 at the light conditions of UV 395, visible light and sunlight. This is due to the electron charge carrier capacity of rGO. 5 and 6 depict a comparative graph of picric acid solution photodegradation under various light circumstances, including UV (254, 365 and 395 nm), visible light and sunlight, at three distinct pH values (3, 7 and 10). The photocatalytic effectiveness of produced nanocomposites was determined by monitoring the changes in a picric acid solution under various light conditions using UV-Visible spectra. The photocatalytic degradation was carried out with and without the catalysts. To increase degrading efficiency, Fenton's regents were combined with nanocomposites such as rGO-ZrO2 and rGO-Y2O3. The photo degradation of picric acid shows less degradation for rGO-ZrO2 and rGO-Y2O3 nanocomposites at pH 7 and 10. But the degradation was increased to 99% at the period of 15 min to 30 min at pH 3 at the light conditions of UV 395, visible light and sunlight. This is due to the electron charge carrier capacity of rGO. In the presence of a UV-light source with a wavelength range of UV-254, 365 and 395 nm, the solution was stirred continuously in dark conditions. In the same situation, the picric acid with rGO-ZrO2 and rGO-Y2O3 nanocomposites and rGO-ZrO2 and rGO-Y2O3, along with Fenton's reagent, was analyzed in visible light and sunlight to check the degradation of picric acid. Every 5 min, 3 mL of solution was taken and recorded for UV analysis. Under all the light irradiation, the sample rGO-ZrO2 degraded the picric acid, and the absorption peak was also decreased. The rGO-ZrO2 and rGO-Y2O3 with Fenton's reagent samples exhibit remarkable photocatalytic activity compared to the rGO-ZrO2 and rGO-Y2O3 nanocomposites. The absorption of picric acid degradation has been noted at 0 min. After the time interval, the absorption peaks start to reduce. There are no absorption peaks after 30 min of irradiation in UV (254, 365 and 395 nm), visible light or sunlight. These photodegradation results explain why the degradation of picric acid decreases when the pH increases. At pH 3, picric acid is completely degraded in acidic conditions due to free radical production, including the hydroxyl radical (• OH) and superoxide radical (O2 •−) generated during the Fenton reaction [41]. The overall graph is shown in Figure 5f for the photodegradation activity of rGO-ZrO2 at pH 3.   Photodegradation of ZrO2/Y2O3 nanoparticles was improved by making a composite with graphene oxide (GO), which creates recombination of photogenerated electron-hole pairs of ZrO2/Y2O3 and increases the amount of surface-absorbed reactant species. When exposed to light, the electrons in ZrO2/Y2O3 are excited to the conduction band. Charge carriers get diffused on the surface of the particles, interact with the water molecules present in the solution and produce reactive oxygen species of peroxide (O 2− ) and hydroxide In the presence of a UV-light source with a wavelength range of UV-254, 365 and 395 nm, the solution was stirred continuously in dark conditions. In the same situation, the picric acid with rGO-ZrO 2 and rGO-Y 2 O 3 nanocomposites and rGO-ZrO 2 and rGO-Y 2 O 3 , along with Fenton's reagent, was analyzed in visible light and sunlight to check the degradation of picric acid. Every 5 min, 3 mL of solution was taken and recorded for UV analysis. Under all the light irradiation, the sample rGO-ZrO 2 degraded the picric acid, and the absorption peak was also decreased. The rGO-ZrO 2 and rGO-Y 2 O 3 with Fenton's reagent samples exhibit remarkable photocatalytic activity compared to the rGO-ZrO 2 and rGO-Y 2 O 3 nanocomposites. The absorption of picric acid degradation has been noted at 0 min. After the time interval, the absorption peaks start to reduce. There are no absorption peaks after 30 min of irradiation in UV (254, 365 and 395 nm), visible light or sunlight. These photodegradation results explain why the degradation of picric acid decreases when the pH increases. At pH 3, picric acid is completely degraded in acidic conditions due to free radical production, including the hydroxyl radical (• OH) and superoxide radical (O2 •−) generated during the Fenton reaction [41]. The overall graph is shown in Figure 5f for the photodegradation activity of rGO-ZrO 2 at pH 3.
As shown in Figure 6a Photodegradation of ZrO 2 /Y 2 O 3 nanoparticles was improved by making a composite with graphene oxide (GO), which creates recombination of photogenerated electron-hole pairs of ZrO 2 /Y 2 O 3 and increases the amount of surface-absorbed reactant species. When exposed to light, the electrons in ZrO 2 /Y 2 O 3 are excited to the conduction band. Charge carriers get diffused on the surface of the particles, interact with the water molecules present in the solution and produce reactive oxygen species of peroxide (O 2− ) and hydroxide radicals (OH) responsible for picric acid degradation [42,43]. Figure 7a depicts the efficiency with and without scavenger studies. The improved photocatalytic activity of ZrO 2 /Y 2 O 3 is the small bandgap energy that allows the photons to be absorbed in the 395, sunlight and visible light range. The step-reactions in the photocatalytic process leading to the degradation of picric acid appeared in the following sequence: •OH + picric acid →gaseous product (9) shows a comparison of rGO-Y2O3 and rGO-ZrO2 of the present work with previously reported photocatalysts. Figure 7b shows the results: the catalyst is stable and there is no significant loss in degrading efficiency, and it achieves 90% even after five cycles of the photocatalytic process; but the efficiency may decrease after five cycles due to catalyst loss during washing. It is apparent that the rGO-Y2O3, rGO-ZrO2 catalyst might brilliantly enable realistic applications (Figure 8).   . These results indicate that rGO-ZrO 2 and rGO-Y 2 O 3catalysts with Fenton's reagent exhibit high activity, degrading picric acid to 100% and 99%, respectively, whereas hydrogen peroxide degrades to less than 22% in both nanocomposites. Fenton's reagent demonstrates nearly 25% degradation. Thus, rGO-ZrO 2 -Fenton's reagent will be a low-cost, highly effective material for picric acid degradation. Table 1 shows a comparison of rGO-Y 2 O 3 and rGO-ZrO 2 of the present work with previously reported photocatalysts. Figure 7b shows the results: the catalyst is stable and there is no significant loss in degrading efficiency, and it achieves 90% even after five cycles of the photocatalytic process; but the efficiency may decrease after five cycles due to catalyst loss during washing. It is apparent that the rGO-Y 2 O 3, rGO-ZrO 2 catalyst might brilliantly enable realistic applications ( Figure 8).

Materials and Methods
Analytical grade chemicals were used for the whole synthesis without any further purification. Chemicals such as graphite powder were purchased from Merck. Also used

Materials and Methods
Analytical grade chemicals were used for the whole synthesis without any further purification. Chemicals such as graphite powder were purchased from Merck. Also used were concentrated sulphuric acid (H 2 SO 4 ; 98% were purchased from Merck, India), potassium permanganate (KMnO 4 ; Merck, India), sodium nitrate (NaNO 3 ; Merck, India) and hydrogen peroxide solution (H 2 O 2 ; Merck). To synthesize ZrO 2 nanoparticles, zirconium oxychloride was used as the precursor and was purchased from Aldrich. Potassium hydroxide (KOH) GR grades were purchased from Aldrich. For Y 2 O 3 synthesis, the reagents of Y(NO 3 ) 3 ·6H 2 O (99.9%) (Sigma Aldrich, India) were used as the starting precursor. Thiourea 99.0% was purchased from Merck with ACS grade. Picric acid (2,4,6-Trinitrophenol) 98% was from Merck, India. For solution preparation, deionized water was used.

Synthesis of Graphene Oxide (GO)
Hummer's method was used to make graphene oxide from graphite powder. H 2 SO 4 , graphite powder and NaNO 3 were taken as precursors for this method. To the graphite powder, KMnO 4 (30.0 g) was gradually added by maintaining the temperature at 100 • C. A total of 50 mL of 10% H 2 O 2 was added to the aforesaid mixture, which was then placed in an oil bath at 100 • C and heated for 1 h. The residue was centrifuged three times with HCl solution. The resultant solid was again redispersed in dilute hydrochloric acid to eliminate any remaining salts or acids. Graphene oxide powder (GO) was then obtained.

Synthesis of rGO-ZrO 2 Nanocomposite
A simple technique prepared the rGO-ZrO 2 nanocomposite by dispersing 0.5 g of GO in DD water and stirring it for 30 min. Zirconium oxychloride (ZrOCl 2 ·8H 2 O) was added to the GO solution. KOH was dissolved in deionized water and added dropwise to the above mixture to obtain a homogenous mixture by stirring. The whole reaction was conducted at a pH of 10.5. The suspension was placed in a Teflon-lined autoclave. Finally, the autoclave was sealed, maintained at a temperature of 100 • C in a furnace for 8 h and allowed to return to ambient temperature. The precipitate was filtered and washed with deionized water to eliminate any excess chloride ions. The finished product was calcinated at 500 • C for three hours in a muffle furnace.

Synthesis of rGO-Y 2 O 3 Nanocomposite
The nanocomposite rGO-Y 2 O 3 was synthesized using yttrium nitrate Y(NO 3 ) 3 ·6H 2 O as a precursor. A total of 0.5 g of GO was dispersed in 80 mL deionized water and agitated for 30 min, and then the suspension was then supplemented with thiourea (0.5 mM). A total of 0.2 g of Y(NO 3 ) 3 ·6H 2 O was added to the graphene oxide solution and thoroughly mixed for 1 h using a magnetic stirrer. The mixture was transferred to a Teflon-lined autoclave and maintained at a temperature of 120 • C for 4 h. At room temperature, the combined solution was allowed to settle. To remove extra contaminants, the separated particles were washed many times in deionized water. Calcination of the final product at 500 • C resulted in the formation of rGO-Y 2 O 3 nanocomposites.

Photodegradation Activity
The degradation of picric acid was used to assess the photocatalytic activity of rGO-ZrO 2 and rGO-Y 2 O 3 . A total of 100 mL (200 mg L −1 ) picric acid is taken in a beaker. Each catalyst was added at a concentration of 5 mg to the picric acid solution. Fenton's reagent was a solution of 1% hydrogen peroxide and 1mM ferrous sulphate. UV radiation with a wavelength of 254, 365, 395 nm, visible light and sunlight were employed in the experiment to measure photocatalytic activity at various pH values (3, 7 and 10). The distance between the beaker and the bulb was determined (10 cm). In a dark environment, the above-mentioned solution was agitated to achieve absorption equilibrium between picric acid and catalyst. A total of 3 mL of the first sample was taken after dark adsorption and its initial concentration C 0 was determined using a UV-Vis spectroscopy. The solution was then treated with light to generate active photocatalytic activity. The suspension was taken at regular intervals of 5 min, and the maximum absorption C was determined. C/C 0 determined the picric acid degradation ratio. This experiment used picric acid to achieve the lowest possible absorption with rGO-ZrO 2 , rGO-Y 2 O 3 , Fenton's reagent, rGO-ZrO 2 and rGO-Y 2 O 3 -Fenton's reagent as a catalyst.

Physical Characterization
The XRD pattern of rGO-ZrO 2 and rGO-Y 2 O 3 nanocomposites was obtained using a Philips instrument and Cu K radiation (λ = 1.541) at 36 kV. SEM and EDAX analysis were carried out using the ZEISS instrument with VPSE G3 software. Ultraviolet-Visible Spectroscopy (UV-Vis) of nanocomposites was carried out at room temperature using a Perkin Elmer Lamda-900 spectrophotometer in the range of 200-800 nm. For the photocatalytic degradation experiment, a special UV light with a wavelength range of 254, 365, 395 nm was used and investigations were also carried out in visible light and sunlight. The photocatalyst, 100 mg L −1 , was suspended in distilled water with 20 mg L −1 picric acid. Dissolution was stirred under different light conditions at room temperature.

Conclusions
In summary, rGO-ZrO 2 and rGO-Y 2 O 3 nanocomposites were successfully synthesized by hydrothermal method and rGO prepared by Hummer's method. The XRD pattern confirms the cubic phase structure of both the nanocomposites. The morphological analyses of nanocomposites were discovered using SEM, and elemental analyses were reviewed. The wide band gap of the ZrO 2 and Y 2 O 3 was significantly reduced by addition of the rGO to capture visible light for photocatalytic application. The photocatalytic performance of rGO-ZrO 2 and rGO-Y 2 O 3 samples for picric acid degradation was evaluated at different pH levels under different light irradiation conditions. In all light conditions, the rGO-ZrO 2 outperformed the rGO-Y 2 O 3 nanocomposite at pH 3. The presence of a small amount of oxygen-deficient zirconium oxide phase and a high density of surface hydroxyl groups contributed to the pronounced catalytic activity of rGO-ZrO 2 . As a result, the two nanocomposites proved to be innovative picric acid degradation systems.