Synthesis and Photocatalytic Performance of g-C₃N₄/ZnO Nanocomposites for the Efficient Degradation of Dyes Under Sunlight ^†

Abstract

In recent years, carbon-derived nanomaterials have been efficiently utilized for wastewater treatment. Herein, we report a facile route to synthesize g-C₃N₄/ZnO nanocomposites (NCs) by utilizing thermal condensation methods and demonstrateits photocatalytic performance against Rhodamine-B (RhB) and Reactive blue-171 (Rb171) under open air sunlight. The crystalline nature of synthesized NCs was examined by utilizing X-ray diffraction; however, their purity was evaluated through Fourier transform spectroscopy (FT-IR), which conveyed that the synthesized NCs exhibited excellent crystallinity and purity. Moreover, the g-C₃N₄/ZnO NCs displayed efficient photocatalytic activity of 82.60 and 74.46% for RhB and Rb171, respectively, in 125 min of sunlight exposure. The degradation of the dyes followed the pseudo-first-order kinetics with apparent rate constants of 0.01531 and 0.01012 min⁻¹ for RhB and Rb171, respectively, and their half-life was found to be 45.27 and 68.49 min.

1. Introduction

In recent years, water contamination caused by industrial effluent discharges from textile, paper making, paints, cosmetics, and food processing industries, etc., has become a significant environmental concern [1,2]. It is estimated that over 100,000 tons of dyes have been exclusively used in the textile industry, and 15% of them are improperly disposed of into aquatic resources and running water bodies [3,4,5]. However, these effluents produce toxic products during hydrolysis and oxidation; indeed, they persist in the water bodies for a longer period of time, which causes serious damage to both flora and fauna [6,7]. Since the discovery of carbon nanotubes by Sumio Iijima in 1991, carbon-basednanomaterials have gained importance over the traditional method for water purification, and photocatalytic degradation has gained significant attention for its ability to efficiently degrade organic pollutants [8]. Therefore, we decided to synthesize g-C₃N₄/ZnO NCs with a new strategy for a sustainable solution of pollutants. Owing to this, it was previously observed that ZnO nanoparticles conveyed a broad intellectual significance for their degradation effects. Moreover, Qazi et al. reported that various sizes, including attractive morphology, like nanoparticles, were synthesized using chemical and green techniques [9,10]. On behalf of this conduction work, a facile route to synthesize g-C₃N₄/ZnO nanocomposites was utilized using thermal condensation methods. After successful synthesis, targeted composites were characterized using analytical tools named XRD for the property of crystals, FT-IR for identification of specific functional active groups, etc.; the photocatalytic activity of the synthesized nanocomposites was also tested under direct sunlight radiation using different organic dyes named RhB and Rb171 at ambient temperature.

2. Methods and Materials

2.1. Chemicals

All the chemicals used were of analytical grade: Zinc acetate dihydrate (Zn (CH₃COO)₂·2H₂O Mol.wt.= 219.49 g/mol), urea (N₂H₄CO Mol.wt.= 60.06 g/mol), RhB and Rb171 dyes, and ethanol as a drying agent were purchased from Gyan Scientific, Lucknow.

Moreover, the physical properties of Rb171 and RhB dyes are shown in

Table 1. Physical properties of Azo dyes.

Properties	Reactive Blue (Rb171)	Rhodamine B
Chemical Formula	C40H23O19 N15Na6S6Cl2	C28H31ClN2O3
Molecular Weight (g/mol)	1419.83 approx.	479.02 approx
Dye Type	Anionic (Acidic)	Cationic (Basic)
Solubility in water at 25 °C	Soluble	Soluble

[11].

2.2. Synthesis of ZnO and g-C₃N₄

For the preparation of ZnO, an adequate amount of zinc salt (8.77 g 200 mL D.I. water) was dissolved to obtain a 0.2 M concentration solution under continuous stirring with pH= 6.14 (37 °C); afterwards, an additional 100 mL NaOH solution was gradually added under continuous stirring to regulate the solution pH of 10.41 (43 °C). Subsequently, 100 mL of absolute ethanol was added to the above solution to prevent the agglomeration of precipitate, and the resultant solution was transferred into a three-neck flask reactor for reflex-heating for 6 h at 80 °C. After a stipulated time interval, the reaction mixture was allowed to cool at ambient temperature and then filtered, washed three times with D.I. water, then with ethanol, and oven-dried, followed by calcination at 400 °C for 2 h in a muffle furnace to obtain ZnO NPs. However, g-C₃N₄ was prepared by thermal condensation of urea; an adequate amount of urea (15 g) was heated in an alumina crucible at 525 °C for two hours in a Muffle furnace. The obtained pale-yellow color product was washed with ethanol, followed by drying in an oven at 100 °C for 1 h, as reported in [12]. The schematic representation of the synthesis of ZnO and g-C₃N₄ is shown in Figure 1a.

2.3. Synthesis of g-C₃N₄/ZnO Nanocomposite

The g-C₃N₄/ZnO NCs were prepared by mixing equal weights of 0.2507 g of g-C₃N₄ and ZnO in 40 mL ethanol using a mortar and pestle, followed by sonication and drying to remove the solvent, and then the homogeneous mixture was poured into a alumina crucible, covered with aluminum foil, and placed in a muffle furnace at 525 °C for 2 h. After stipulated time interval; the resulting sample was allowed to cool to normal temperature and again finely ground to obtain the nanocomposites, denoted as g-C₃N₄/ZnO NCs, then collected for characterization and photocatalytic testing. Figure 1b shows the schematic layout for the preparation of the NCs.

2.4. Photocatalytic Test Evaluation

The photocatalytic potential of fabricated g-C₃N₄/ZnO NCs was evaluated against RhB and Rb171. Initially, a 10 ppm dye solution was prepared in 500 mL of D.I. water. Additionally, 2 mL of NaOH solution was poured into the RhB solution to make the solution pH 8. Then, 100 mg of NCs photocatalyst was added into 100 mL of each dye solution, and the mixture was stirred in the dark for 20 min to allow for adsorption and desorption equilibrium. Afterward, the stable dye–NCs suspension was exposed to sunlight between 12 pm and 2 pm. After a stipulated time interval of 25 min, the sample was collected to monitor the absorption of the decomposed dye solution. Prior to this, it was centrifuged to separate the particles of NCs, and the degradation process was monitored using UV-Vis spectroscopy.

2.5. Characterizations

Crystal structures and the desired functional group of the synthesized samples were studied using X-ray Diffraction (XRD), (XRD Model: miniFlex600/600-C) and FTIR (Perkin Elmer spectrum IR Version), respectively, and the photodegradation of RhB and Rb171 was monitored using UV-Vis absorption spectra on a UV-1800 instrument (Perkin Elmer-Lambda25).

3. Result and Discussion

3.1. Fourier Transform Infrared (FTIR) Analysis

Figure 2a displays the FTIR spectrum of the NCs; the three distinct bands appearing at 529.84, 682.68, and 825.17 cm⁻¹ convey the Zn-O characteristic stretching; interestingly, it was noticed that there was a shifting of the characteristic peak of Zn-O, which could be due to the formation of new bonds between the ZnO and g-C₃N₄ [13]. The supporting Figures S1 and S2 reveal the FTIR spectrums of the as-prepared ZnO and g-C₃N_4, which show their typical nature. However, a strong peak between 1350 and 1550 cm⁻¹ arose due to C-N stretching modes of g-C₃N₄ [14,15]; moreover, a new peak at 2935 cm⁻¹ may havearisen due to the development of a new C–N bond alongside the existing sp² bonds of the triazine rings, potentially resulting from the breaking of these sp² bonds by ZnO [16].

3.2. Phase Analysis and Characterization of Crystalline Structures

XRD is a non-destructive analytical tool that is utilized to analyzethe crystalline nature of NCs, phase structure, crystallite size, and matrix impurities. Figure 2b displays the prominent reflection at 31.8°, 34.43°, 36.03°, 47.23°, 56.6°, 62.9°, 66.30°, 67.90°, 69.90°, and 76.90° and a 66.30° Bragg’s angle (2θ), which conveys that the synthesized NCs exhibited excellent crystallinity. Additionally, Figure S2 reveals the typical XRD pattern of as-prepared bare ZnO and g-C₃N₄, which conveys that the synthesized material exhibits good crystallinity. The intensity of XRD diffraction of the g-C₃N₄ peak diminishes in g-C₃N₄/ZnO NCs, suggesting that it is primarily deposited on the ZnO surface rather than integrated into its crystalline lattice, and it also indicates that ZnO fusion may impede the growth of pure g-C₃N₄ crystals [17,18].

3.3. Evaluation of Photocatalytic Potential of g-C₃N₄/ZnO Nanocomposite

The photocatalytic potential of as-prepared g-C₃N₄/ZnO NCs was demonstrated against basic dyes under open air sunlight, and their results are shown in Figure 3. Interestingly, it was noticed that negligible degradation was observed in the dark. On the contrary, as the dye sample was exposed to sunlight, it exhibited appraisable degradation, which conveyed the photocatalytic potential of the as-synthesized g-C₃N₄/ZnO NCs. The extent of degradation was monitored by observing the change in the optical density of characteristic UV-vis absorption peaks of RhB at 553 nm and Rb171 at 602 nm, in which the intensity was diminished with respect to time that conveyed the mineralization of the dyes. The photocatalytic degradation efficiency was enumerated by the following empirical relation [19]:

$$\mathrm{E}\mathrm{x}\mathrm{t}\mathrm{e}\mathrm{n}\mathrm{t} \mathrm{o}\mathrm{f} \mathrm{d}\mathrm{e}\mathrm{g}\mathrm{r}\mathrm{a}\mathrm{d}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}(\%):{\displaystyle \frac{{\mathbf{C}}_0-{\mathbf{C}}_{\mathbf{t}}}{{\mathbf{C}}_0}}\times 100$$

where C₀ and C_t represent the absorption of dye at instant time t = 0 min and at variable time intervals, respectively. It was found that the extent of degradation of the dyes progressively increases with time, as shown in Figure 3a,b, and 82.5 and 74.46% degradation of RhB and Rb171, respectively, was monitored in a 125 min time interval, as shown in Figure 3c. The degradation of the dyes followed the pseudo-first-order kinetics; as revealed in Figure 3d, the slope of the graph between ln C₀/C and time represents the apparent rate constant for the degradation of the dyes.

Table 2. Summary of photocatalytic performance of g-C₃N₄/ZnO NCs.

Dye	Degradation Efficiency (%)	Sunlight Exposure (min)	Half-Life Time (min)	Rate Constant (min−1)	R2 Value
RhB	82.50	125	45.27	0.01531	0.89207
Rb171	74.46		68.49	0.01012	0.96733

represents the highlight of photocatalytic performance of g-C₃N₄/ZnO NCs, and the results were consistent with previously reported work [20,21,22].

4. Conclusions

Herein, we report a facile route to synthesize g-C₃N₄/ZnO NCs by using equal amounts of ZnO and g-C₃N₄ via the thermal condensation method. The XRD results displayed prominent diffractions, which confirmed that the prepared NCs had excellent crystallinity. However, FTIR results exhibited strong peaks between 1350 and 1550 cm⁻¹, and 529.84, 682.68, and 825.17 cm⁻¹ represent the nature of the as-prepared g-C₃N₄/ZnO NCs. Moreover, the photocatalytic demonstration of the as-prepared NCs showed excellent results; 82.50 and 74.46% of degradation was monitored for RhB and Rb171, respectively, under open air sunlight in 125 min of time that confirmed its photocatalytic potential.