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Article

Reduced Graphene Oxide–Metal Oxide Nanocomposites (ZrO2 and Y2O3): Fabrication and Characterization for the Photocatalytic Degradation of Picric Acid

by
Balasubramanian Usharani
1,
Govindhasamy Murugadoss
2,*,
Manavalan Rajesh Kumar
3,
Shaik Gouse Peera
4,* and
Varadharajan Manivannan
1,*
1
PG and Research Department of Chemistry, Thiruvalluvar Govt Arts College, Rasipuram 637401, Tamil Nadu, India
2
Centre for Nanoscience and Nanotechnology, Sathyabama Institute of Science and Technology, Chennai 600119, Tamilnadu, India
3
Institute of Natural Science and Mathematics, Ural Federal University, Yekaterinburg 620002, Russia
4
Department of Environmental Science, Keimyung University, 1095 Dalgubeol-daero, Dalseo-gu, Daegu 42601, Korea
*
Authors to whom correspondence should be addressed.
Catalysts 2022, 12(10), 1249; https://doi.org/10.3390/catal12101249
Submission received: 13 September 2022 / Revised: 11 October 2022 / Accepted: 13 October 2022 / Published: 16 October 2022
(This article belongs to the Special Issue New Trends in Photocatalytic Materials for Efficient Performance)
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Abstract

:
Herein, reduced graphene-oxide-supported ZrO2 and Y2O3 (rGO-ZrO2 and rGO-Y2O3) 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 ZrO2 and Y2O3 was successfully reduced by addition of the reduced graphene oxide (rGO) to absorb visible light for photocatalytic application. The performance of as synthesized rGO-ZrO2 and rGO-Y2O3 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-ZrO2 and rGO-Y2O3 nanocomposites showed excellent photocatalytic activity under optimum identical conditions with mild variations in pH 3. Compared to rGO-Y2O3, the rGO-ZrO2 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.

1. 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.
Photocatalytic degradation of pollutants has emerged as an alternative approach to the traditional techniques to remove pollutants from wastewater, one that is low-cost and eco-friendly [16,17]. Several metal-oxides-based photocatalytic materials have been investigated as possible photocatalysts due to their abundance, affordability, biocompatibility, and stability under various environmental conditions. Some metal oxides, such as TiO2, ZnO, WO3, ZrO2, SnO2, Fe2O3, CuO, PbS, CdS and Y2O3 [18,19,20], have been successfully applied for photocatalytic degradations and have been studied in previous research. Under UV/light irradiation, metal oxides such as TiO2, ZnO, CuO, etc., quickly get photoactivated and generate reactive oxygen species (ROS), i.e., (O2) radicals and hydroxyl (•OH) radicals on their surfaces. Among the various metal oxides, ZrO2 and Y2O3 are the most attractive material because of their excellent semiconducting properties. Zirconium oxide (ZrO2) is a typical n-type semiconductor material with a band gap of ~5 eV [21]. ZrO2 or doped Zr ions have also gained attention recently in the field of photocatalysis because of their high surface-to-volume ratio. ZrO2 under UV irradiation shows excellent photocatalytic performance due to its wideband properties [22]. ZrO2 exists in three different crystal structures, viz., m-monoclinic (m-ZrO2), t-tetragonal (t-ZrO2) and c-cubic phase (c-ZrO2). Obtaining these crystal phases strongly depends on the chosen synthesis methods and temperature [23]. The applications of ZrO2 nanoparticles depend on crystal structures and phase transformation. Taguchi et al. synthesized m-ZrO2 nanoparticles by a hydrothermal method [24]. T-ZrO2 nanoparticles are synthesized by a microwave-irradiated sol-gel method, as reported by Dwivedi et al. [25]. Despite their several advantages, the high recombination rate of photogenerated electron-hole pairs of ZrO2 gives a low quantum efficiency and strong affinity for negative charge moiety. To mitigate the recombination, a strategy of combining ZrO2 with carbon materials is proposed by some researchers. For example, ZrO2 loaded with carbon-based materials such as graphene oxide (GO) increases the beneficial effects of photocatalytic activity due to enhanced electron transfer process [26].
On the other hand, a rare earth metal oxide, Yttrium oxide (Y2O3) also attracted special attention as a photocatalyst due to its narrow band gap of (~5 eV), excellent thermal stability, good adsorptions and electronic stability [27]. Like ZrO2, Yttrium oxide (Y2O3) also exists in three structural phases: cubic, hexagonal and monoclinic. A cubic phase of yttrium oxide is extremely stable and withstands temperatures of up to 2400 °C. Yttrium oxide (Y2O3) is a dopant that is employed in a variety of metal oxides. Y3+-TiO2, Y3+/Bi5Nb3O15 [28] and PbYO [29] are a few examples. Liu et al. [30] developed Y2O3-doped ZrO2-CeO2 nanocomposites with a tetragonal phase in Y2O3. Numerous studies indicate that Y-doped metal oxides such as TiO2 and ZnO exhibit enhanced optical characteristics [31].
Reduced graphene oxide (r-GO) is an excellent and promising material due to its good electrical conductivity, large surface area, absorption of pollutants and high transparency [32]. Only a few papers are available for rGO-ZrO2 and rGO-Y2O3 nanocomposites for applications such as electrochemical sensors, supercapacitors and dye degradation. Some researchers have reported the removal of inorganic pollutants such as arsenic (III and V) and chromium ions. rGO-ZrO2 and Y2O3 nanocomposites have not yet been reported for the degradation of organic pollutants such as picric acid (2,4,6 trinitrophenol). Picric acid is a very dangerous environmental pollutant generated from chemical and dye industries due to its high toxicity. Picric acid is toxic if inhaled or absorbed through the skin [33,34], inhalation may cause lung damage and chronic exposure may cause liver or kidney damage. When in contact with metals, picric acid readily forms salts (including copper, lead, mercury, zinc, nickel and iron). When subjected to heat and friction, picric acid forms a salt with ammonia and amines that is highly prone to explosions.
In this work, rGO-ZrO2 and rGO-Y2O3 nanocomposites have been successfully synthesized by the hydrothermal method. Graphene oxide (GO) was prepared by Hummer’s 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.

2. 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) is 2.9704 nm. The diffractor image obtained for the rGO-Y2O3, 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 Y2O3 (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-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 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 SP2 hybridization [39]. The results confirmed the successful formation of reduced graphene oxide-Y2O3 and ZrO2 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-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-ZrO2 and rGO-Y2O3) were determined. Figure 5 and Figure 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.
As shown in Figure 6a–e, the degradation percentage rate of the rGO-Y2O3 nanocomposite is 70, 100, 84, 100 and 100% under UV (254, 365, and 390 nm), visible and sunlight conditions, respectively. Compared to rGO-Y2O3, the rGO-ZrO2 nanocomposite shows even better photocatalytic activity with a degradation percentage rate of 100, 99.3 and 99.9; 100 and 100% for light conditions of UV (254, 365 and 395 nm), visible and sunlight, as shown in Figure 5a–e. Photocatalytic activity mainly depends on crystallinity, surface area and morphology. Based on the XRD and SEM results, rGO-ZrO2 shows better morphology and a crystal size of 22.44 nm, which may be the reason for its good photodegradation activity. By 15 min, 99% of picric acid has completely degraded in all light conditions with rGO-ZrO2. But only 97% of picric acid was degraded in light conditions (UV 395 nm, visible light, and sunlight) with rGO-Y2O3, even up to 32 min.
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 (O2−) 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 ZrO2/Y2O3 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:
rGO-ZrO2/Y2O + hʋ → e− + h+
h+ + OH−→ •OH (Oxidation)
h+ + H2O → •OH + H+
O2 + e− → O2•− (Reduction)
H2O2 + e−→ •OH + OH−
O2•− + H+ → HO2•−
Fe(III) + HO2• → Fe(II) + H+ + O2•−
Fe(II) + H2O2 → Fe (III) + •OH + OH−
•OH + picric acid →gaseous product
The comparative bar diagram in Figure 5f and Figure 6f illustrates the degradation of picric acid in the presence of various catalysts, including rGO- ZrO2-Fenton’s reagent (PFC), rGO- Y2O3-Fenton’s reagent (PFC), rGO-ZrO2, rGO-Y2O3 (PC) Fenton’s reagent (F) and hydrogen peroxide (H2O2). These results indicate that rGO-ZrO2 and rGO-Y2O3-catalysts 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-ZrO2-Fenton’s reagent will be a low-cost, highly effective material for picric acid degradation. Table 1 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).

3. 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 (H2SO4; 98% were purchased from Merck, India), potassium permanganate (KMnO4; Merck, India), sodium nitrate (NaNO3; Merck, India) and hydrogen peroxide solution (H2O2; Merck). To synthesize ZrO2 nanoparticles, zirconium oxychloride was used as the precursor and was purchased from Aldrich. Potassium hydroxide (KOH) GR grades were purchased from Aldrich. For Y2O3 synthesis, the reagents of Y(NO3)3·6H2O (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.

3.1. Synthesis of Graphene Oxide (GO)

Hummer’s method was used to make graphene oxide from graphite powder. H2SO4, graphite powder and NaNO3 were taken as precursors for this method. To the graphite powder, KMnO4 (30.0 g) was gradually added by maintaining the temperature at 100 °C. A total of 50 mL of 10% H2O2 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.

3.2. Synthesis of rGO-ZrO2 Nanocomposite

A simple technique prepared the rGO-ZrO2 nanocomposite by dispersing 0.5 g of GO in DD water and stirring it for 30 min. Zirconium oxychloride (ZrOCl2·8H2O) 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.

3.3. Synthesis of rGO-Y2O3 Nanocomposite

The nanocomposite rGO-Y2O3 was synthesized using yttrium nitrate Y(NO3)3·6H2O 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(NO3)3·6H2O 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-Y2O3 nanocomposites.

3.4. Photodegradation Activity

The degradation of picric acid was used to assess the photocatalytic activity of rGO-ZrO2 and rGO-Y2O3. 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 C0 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/C0 determined the picric acid degradation ratio. This experiment used picric acid to achieve the lowest possible absorption with rGO-ZrO2, rGO-Y2O3, Fenton’s reagent, rGO-ZrO2 and rGO-Y2O3-Fenton’s reagent as a catalyst.

3.5. Physical Characterization

The XRD pattern of rGO-ZrO2 and rGO-Y2O3 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.

4. Conclusions

In summary, rGO-ZrO2 and rGO-Y2O3 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 ZrO2 and Y2O3 was significantly reduced by addition of the rGO to capture visible light for photocatalytic application. The photocatalytic performance of rGO-ZrO2 and rGO-Y2O3 samples for picric acid degradation was evaluated at different pH levels under different light irradiation conditions. In all light conditions, the rGO-ZrO2 outperformed the rGO-Y2O3 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-ZrO2. As a result, the two nanocomposites proved to be innovative picric acid degradation systems.

Author Contributions

Conceptualization, methodology, B.U.; validation, formal analysis, G.M. and M.R.K.; writing—original draft preparation, B.U.; writing—review and editing, S.G.P. and V.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Not Applicable.

Acknowledgments

The author, G.M. thanks the Chancellor, President and Vice Chancellor, Sathyabama Institute of Science and Technology, Chennai, Tamilnadu, India for their support and encouragement. One of the authors, M.R.K. thanks contract No. 40/is2.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. XRD pattern of rGO-ZrO2 and rGO-Y2O3 nanocomposites.
Figure 1. XRD pattern of rGO-ZrO2 and rGO-Y2O3 nanocomposites.
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Catalysts 12 01249 g002 550
Figure 2. SEM of r-GO-ZrO2 (a,b) and rGO-Y2O3 (d,e). EDAX images of (c) of r-GO-ZrO2 and (f) r-GO-Y2O3.
Figure 2. SEM of r-GO-ZrO2 (a,b) and rGO-Y2O3 (d,e). EDAX images of (c) of r-GO-ZrO2 and (f) r-GO-Y2O3.
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Catalysts 12 01249 g003 550
Figure 3. (a) FT-IR (b) Raman spectra of rGO-ZrO2 and rGO-Y2O3 nanocomposites.
Figure 3. (a) FT-IR (b) Raman spectra of rGO-ZrO2 and rGO-Y2O3 nanocomposites.
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Catalysts 12 01249 g004 550
Figure 4. (a)UV-Visible spectra of rGO-Y2O3, rGO-ZrO2, Y2O3 and ZrO2 (b) UV-DRS spectra of rGO-Y2O3, rGO-ZrO2 Figure 4. Inset Tauc plot of rGO-Y2O3, rGO-ZrO2.
Figure 4. (a)UV-Visible spectra of rGO-Y2O3, rGO-ZrO2, Y2O3 and ZrO2 (b) UV-DRS spectra of rGO-Y2O3, rGO-ZrO2 Figure 4. Inset Tauc plot of rGO-Y2O3, rGO-ZrO2.
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Figure 5. Photodegradation activity of rGO-ZrO2 (a) UV 254 nm, (b) UV 365 nm, (c) UV 395 nm, (d) Visible light and (e) sunlight (f) Comparison of various methods with present studies, PFC: Photo Fenton’s with rGO-ZrO2 catalyst, PC: Photocatalyst (FO), F: Fenton’s process, P-H2O2: Photo process with hydrogen peroxide at pH 3.
Figure 5. Photodegradation activity of rGO-ZrO2 (a) UV 254 nm, (b) UV 365 nm, (c) UV 395 nm, (d) Visible light and (e) sunlight (f) Comparison of various methods with present studies, PFC: Photo Fenton’s with rGO-ZrO2 catalyst, PC: Photocatalyst (FO), F: Fenton’s process, P-H2O2: Photo process with hydrogen peroxide at pH 3.
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Figure 6. Photodegradation activity of rGO-Y2O3 (a) UV 254 nm, (b) UV 365 nm, (c) UV 395 nm, (d) Visible light and (e) sunlight (f) Comparison of various methods with present studies, PFC: Photo Fenton’s with rGO-Y2O3 catalyst, PC: Photocatalyst (FO), F: Fenton’s process, P-H2O2: Photo process with hydrogen peroxide at pH 3.
Figure 6. Photodegradation activity of rGO-Y2O3 (a) UV 254 nm, (b) UV 365 nm, (c) UV 395 nm, (d) Visible light and (e) sunlight (f) Comparison of various methods with present studies, PFC: Photo Fenton’s with rGO-Y2O3 catalyst, PC: Photocatalyst (FO), F: Fenton’s process, P-H2O2: Photo process with hydrogen peroxide at pH 3.
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Figure 7. (a) Scavenger test (b) reusability study of rGO-ZrO2 and rGO-Y2O3 nanocomposites for Picric acid degradation.
Figure 7. (a) Scavenger test (b) reusability study of rGO-ZrO2 and rGO-Y2O3 nanocomposites for Picric acid degradation.
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Figure 8. Schematic diagram of catalyst preparation and photocatalytic picric acid degradation.
Figure 8. Schematic diagram of catalyst preparation and photocatalytic picric acid degradation.
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Table
Table 1. Comparison studies of recently reported literature on photocatalytic parameters.
Table 1. Comparison studies of recently reported literature on photocatalytic parameters.
S. No.CatalystsPollutant NameSourceDurationEfficiency(%)Ref.
1rGO-TiO2Picric acidUv-light24 min100%[44]
2rGO-TiO2Picric acidVisible light18 min100%[44]
3rGO-TiO2Picric acidSun light18 min100%[44]
4CeO2
Nanocompisite		Picric acidUv-light45 min100%[45]
5CeO2
Nanocompisite		Picric acidVisible light40 min100%[45]
6CeO2
Nanocompisite		Picric acidSun light35 min100%[45]
7rGOPicric acidSun light60 min99%[46]
8MnO2Picric acidSun light40 min99%[46]
9rGO-MnO2Picric acidSunlight30 min100%[46]
10rGO-TiO2(5%)Picric acidSunlight18 min100%[47]
11rGO-TiO2(10%)Picric acidSunlight15 min100%[47]
12TiO2Picric acidVisible light27 min100%[48]
13rGO-Y2O3Picric acidUV light30 min100%Present study
14rGO-Y2O3Picric acidSunlight35 min100%Present study
15rGO-ZrO2Picric acidUV light16 min100%Present study
16rGO-ZrO2Picric acidSunlight15 min100%Present study
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Usharani, B.; Murugadoss, G.; Rajesh Kumar, M.; Gouse Peera, S.; Manivannan, V. Reduced Graphene Oxide–Metal Oxide Nanocomposites (ZrO2 and Y2O3): Fabrication and Characterization for the Photocatalytic Degradation of Picric Acid. Catalysts 2022, 12, 1249. https://doi.org/10.3390/catal12101249

AMA Style

Usharani B, Murugadoss G, Rajesh Kumar M, Gouse Peera S, Manivannan V. Reduced Graphene Oxide–Metal Oxide Nanocomposites (ZrO2 and Y2O3): Fabrication and Characterization for the Photocatalytic Degradation of Picric Acid. Catalysts. 2022; 12(10):1249. https://doi.org/10.3390/catal12101249

Chicago/Turabian Style

Usharani, Balasubramanian, Govindhasamy Murugadoss, Manavalan Rajesh Kumar, Shaik Gouse Peera, and Varadharajan Manivannan. 2022. "Reduced Graphene Oxide–Metal Oxide Nanocomposites (ZrO2 and Y2O3): Fabrication and Characterization for the Photocatalytic Degradation of Picric Acid" Catalysts 12, no. 10: 1249. https://doi.org/10.3390/catal12101249

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