Photodegradation under Ultraviolet Light Irradiation of RhB by ZnO–ZnCr₂O₄/g-C₃N₄ Nanocomposites Fabricated by Urea Combustion Method

Abstract

Due to the rapid national development of various countries, organic dyes have been applied in manufacturing products such as leather, textiles, paper, and cosmetics. However, the wastewater produced by these industries is harmful to the environment and organisms. Moreover, organic dyes contain toxic carcinogens and cause the reduction of the oxygen content in water, which is harmful to nature and the water resources people use. Photodegradation is a low-cost, highly efficient, and low-energy way to remove these substances. Zinc-based materials were applied as a degradation catalyst in this study. ZnO–ZnCr₂O₄/g-C₃N₄ nanocomposites were fabricated by the urea combustion method and used as photocatalysts for rhodamine B (RhB) degradation under ultraviolet A (UVA) light irradiation. ZnO–ZnCr₂O₄/g-C₃N₄ was investigated by XRD, FESEM, BET, UV-Vis, and TEM to confirm the crystalline microstructure. Based on the various annealing temperatures of ZnO–ZnCr₂O₄/g-C₃N₄ nanocomposite, the specific surface area varied from 36.33 m²/g to 107.55 m²/g. In addition, the photocatalytic activities of ZnO–ZnCr₂O₄/g-C₃N₄ nanocrystals were investigated through the degradation of RhB under UV light for 12 hours. After 12 hours, 95.45% of the RhB was degraded under UV light irradiation. ZnO–ZnCr₂O₄/g-C₃N₄ nanocomposites annealed at 500°C exhibited the highest rate constant, up to 6.11 × 10⁻³ min⁻¹, and ZnO–ZnCr₂O₄/g-C₃N₄ revealed excellent stability based on the results of the cyclic test.

1. Introduction

Organic dyes are used in various industrial fields, such as manufacturing leather, textiles, paper, and cosmetics. Because many organic dyes have been produced, wastewater containing them has become a major source of pollution in recent years. According to a previous study, there were about 10,000 different textile dyes in the world, and their annual output is about 700,000 tons.¹⁾ Advanced oxidation technology is a potential process for application to wastewater treatment.^2–5) Rhodamine B (RhB) is one of the common organic dyes used in industrial products. However, RhB can cause environmental damage, cancer, and irritation of the eyes and skin. Therefore, removing this dye from water is an important goal. Several methods have been reported to be effective in dye removal, some examples being laser cavitation,⁶⁾ ozonation,⁷⁾ and photodegradation.⁸⁾

In recent years, graphitic-type carbon nitride (g-C3N4) has been used as a potential photocatalyst, as it is activated when irradiated at a wavelength over 420 nm. In addition, g-C₃N₄ is metal-free, non-toxic, and highly stable, so it was often used as a functional photocatalyst and applied to water splitting,^9,10) organic pollutant degradation,¹¹⁾ carbon dioxide reduction^12,13) and nitrogen fixation.¹⁴⁾ In previous work, g-C₃N₄ was synthesized by the thermal condensation reaction of compounds of carbon and nitrogen (such as melamine, urea, thiourea, and dicyandiamide), which revealed the lower specific surface area and faster recombination of electron-hole pairs. Based on this, the catalyst’s performance was greatly limited by its shortcomings.^15–20) Due to spinel’s excellent physicochemical characteristics, spinel structure oxide (AB₂O₄) has attracted attention in the fields of superhard materials,²¹⁾ magnetic materials²²⁾ and high temperature ceramics,²³⁾ and spinel has practical applications as low-cost sensors for the detection of toxic and hazardous substances. Among these, ZnCr₂O₄ is one of the most important spinel compounds due to its high potential for non-homogeneous applications in chemical reactions such as carbon monoxide oxidation,²⁴⁾ sensing properties,²⁵⁾ catalytic combustion of hydrocarbons,²⁶⁾ and degradation reduction of various organic dyes.^8,27) To improve the defects of g-C₃N₄, different precursors have been prepared with various dopant elements to allow microscopic morphology control and the formation of nanosized grain structures. Among these approaches, the development of nanocomposite materials with synergistic effects was highly efficient in improving the catalytic performance of g-C₃N₄. The energy band of the photocatalyst is extended and the light absorption efficiency is enhanced by different metal oxide combinations (such as ZnO/TiO₂, NiO/TiO₂, NiO/V₂O₅ and TiO₂/Bi₂O₃), which suppress the photogenerated electron-hole pair recombination efficiency.^28–31) In addition, water-soluble polymers have been widely used in inorganic synthesis and can be classified as dispersants or templates depending on their role in the preparation process.^32,33) Moreover, when water-soluble polymers are used as the dispersants, the agglomeration of nanoparticles can be effectively controlled.³⁴⁾ Therefore, the biomacromolecules starch,³⁵⁾ carrageenan,³⁶⁾ and gelatin^37,38) have made important contributions in the field of inorganic synthesis.

Mazeyar Parvinzadeh Gashti et al. reported that zinc phosphate-based nanosheets could be prepared by diffusing zinc ions in gelatin solution.³⁹⁾ Moreover, the gelatin exhibited the role of an organic precursor (biotemplate) in the preparation of the porous catalysts with functional groups, as it was an excellent miscible agent for metal cations to fabricate metal oxide catalysts with excellent metal dispersion and porosity.⁴⁰⁾ In addition, the urea combustion method was one of the material fabrication methods proposed by A.G. Merzhanov et al.⁴¹⁾ The gas released during the reaction promotes a porous structure of the materials, which can be used in flat panels, lithium batteries, etc.⁴²⁾ Based on the above, ZnO–ZnCr₂O₄/g-C₃N₄ ternary nanocomposite porous powder fabricated by urea combustion method was applied to organic dye photodegradation. Furthermore, the porous structure of the ZnO–ZnCr₂O₄/g-C₃N₄ powder effectively improved the absorption and efficiency of the catalyst. The physicochemical characteristics of the catalysts were investigated. The catalytic reaction of ZnO–ZnCr₂O₄/g-C₃N₄ nanocomposite photodegradation of RhB under ultraviolet A (UVA) light irradiation was also evaluated.

2. Experimental Procedure

2.1 Materials and methods

The starting reagents, namely, zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O, 99.0% purity), chromium nitrate nonahydrate (Cr(NO₃)₃·9H₂O, 99.0% purity), cold water fish skin gelatin (300 bloom, EP) and urea ((NH₂)₂CO, 99.0% purity) were purchased from SHOWA and Sigma-Aldrich.

This study used an X-ray diffractometer (XRD, D2 Phaser, Bruker) with a Cu target (Cu Kα = 1.5418 Å) as the X-ray source to identify the crystal phase of the catalyst, with an operating voltage of 30 kV, current of 10 mA, and scanning range of 10°–60°. The catalyst morphology and structure of the prepared materials were observed with a field emission scanning electron microscope (FESEM, SU8100, HITACHI), and the particle size and elemental distribution were studied with a transmission electron microscope (TEM, JEM2100F, JEOL) at an operating voltage of 200 kV and energy dispersive X-ray spectroscopy (EDX). The specific surface area, adsorption–desorption isotherms, and pore size distribution of the ZnO–ZnCr₂O₄/g-C₃N₄ catalyst were determined by Brunauer-Emmett-Teller analyzer (BET, Micromeritics, TriStar II plus 3030). Before BET measurement, the appropriate amount of catalyst was degassed at 200°C for 24 hours with high-purity nitrogen (N₂). During catalyst adsorption of the N₂, the N₂ adsorption isotherms and pore size distribution were measured and studied at different relative pressures (P/P₀) of 0–1.0. The absorbance, energy band gap, and the absorbance of the dye during the photocatalytic degradation process of the nanocomposite were investigated using a UV-visible spectrophotometer (UV-vis, UV-2600, Shimadzu) equipped with an integrating sphere over a wavelength range of 300–1100 nm.

2.2 Preparation of ZnO–ZnCr₂O₄/g-C₃N₄ and ZnCr₂O₄/g-C₃N₄ nanocomposites

ZnO–ZnCr₂O₄/g-C₃N₄ and ZnCr₂O₄/g-C₃N₄ nanocomposites were fabricated by urea combustion method. Zinc nitrate and chromium nitrate were used as the precursor metal sources. First, molar ratios of 1:1 and 1:2 of zinc nitrate to chromium nitrate were used to prepare ZnO–ZnCr₂O₄/g-C₃N₄ and ZnCr₂O₄/g-C₃N₄, after which the metal nitrate was mixed with 0.1 mole urea and 5 ml 10 mass% gelatin solution dissolved in DI water. The mixed starting reagents were stirred and dissolved in 5 ml DI water at 80°C and uniformly mixed. After that, the precursor was dehydrated at 80°C to form a gel-like state, which was dried in an oven to form a brownish gel-like substance.

The precursor gel material was transferred to an alumina crucible and annealed for 3 hours at 450°C, 500°C, and 550°C in air with a heating rate of 2.3°C/min, respectively. After the reaction, the gelatin was burned off, resulting in spontaneous combustion. The resulting product was collected and subjected to characteristic analysis.⁴³⁾

2.3 Catalyst test

The the catalytic performance of the as-combusted Zn-based nanocomposites was investigated by organic dye photodegradation in this study. Figure 1 shows a schematic diagram of the photocatalyst test for RhB photodegradation. 10⁻⁵ M RhB solution was used as the target dye for photocatalytic degradation. 30 mg of the nano-composite catalyst was dispersed in 40 ml of the 10⁻⁵ M RhB solution and pre-stirred in the dark for 30 min to ensure adsorption–desorption equilibrium of the catalyst at 25°C. The photocatalytic degradation of 10⁻⁵ M RhB was carried out under UVA light irradiation of 8 W and a 254 nm wavelength for 12 hours to evaluate the activity of the nanocomposites. Each hour, 5 ml of the supernatant solution was retrieved from the mixture solution of the catalyst and RhB, which were separated by centrifugation and measured with a UV-visible spectrophotometer. The degradation degree of RhB (D) was determined by eq. (1) where C₀ is the initial concentration of RhB and C_t is the concentration of RhB at time t of the photocatalysis.⁴⁴⁾   

\begin{equation} \mathrm{D} = \frac{C_{0} - C_{t}}{C_{0}} \times 100\% \end{equation} (1)

Fig. 1

Flowchart diagram of the photodegradation test.

3. Results and Discussions

3.1 Characteristics of ZnO–ZnCr₂O₄/g-C₃N₄ and ZnCr₂O₄/g-C₃N₄ nanocomposites prepared by urea combustion method

Figure 2 exhibits the diffraction patterns of ZnO–ZnCr₂O₄/g-C₃N₄ and ZnCr₂O₄/g-C₃N₄ nanocomposites prepared by the urea combustion method at different annealing temperatures. The patterns in Fig. 2 show the diffraction peaks of the spinel phase ZnCr₂O₄ (PDF#87-0028) and the hexagonal phase ZnO (PDF#79-0206). The diffraction peaks of spinel phase ZnCr₂O₄ powder were observed at 18.4°, 30.3°, 35.7°, 43.4°, 53.8°, 57.4°, and 63.1°, which correspond to the (111), (220), (311), (400), (422), and (511) crystal planes, respectively. The strongest diffraction peaks of the hexagonal phase ZnO were observed at 31.7°, 34.4°, and 36.2°, which correspond to the (100), (002), and (101) crystal planes, respectively. In addition, the presence of g-C₃N₄ in the nanocomposite powder was observed in the finely scanned X-ray diffraction pattern in Fig. 3, and the (100) crystal plane of g-C₃N₄ was observed at 12.5°–13.1° at different annealing temperatures. Among the XRD patterns of the ZnO–ZnCr₂O₄/g-C₃N₄ and ZnCr₂O₄/g-C₃N₄ nanocomposites, incompatibility between ZnO and ZnCr₂O₄ was judged by the non-shifted XRD diffraction peak, and no other secondary phases such as Cr₂O₃ and ZnCrO₄ were observed in the pattern.

Fig. 2

XRD patterns of ZnO–ZnCr₂O₄/g-C₃N₄ and ZnCr₂O₄/g-C₃N₄ nanocomposites fabricated with the urea combustion method and annealed at various temperatures (450°C–550°C).

Fig. 3

Fine XRD patterns (10°–20°) of ZnO–ZnCr₂O₄/g-C₃N₄ and ZnCr₂O₄/g-C₃N₄ nanocomposites fabricated with the urea combustion method and annealed at various temperatures (450°C–550°C).

Figure 4(a)–(f) shows FESEM images of ZnO–ZnCr₂O₄/g-C₃N₄ and ZnCr₂O₄/g-C₃N₄ nanocomposites prepared at different annealing temperatures. In the SEM images, it can be observed that the ZnO–ZnCr₂O₄/g-C₃N₄ and ZnCr₂O₄/g-C₃N₄ nanocomposites powder produced by the urea combustion method crystallized due to the heat generated during the process. Meanwhile, large amounts of gas were released, causing the formation of a porous structure. In Fig. 4(b), Fig. 4(d), and Fig. 4(f), hexagonal flakes can be observed, and the corresponding XRD results suggested that these hexagonal flakes were ZnO.

Fig. 4

FESEM images of ZnO–ZnCr₂O₄/g-C₃N₄ and ZnCr₂O₄/g-C₃N₄ nanocomposites fabricated with the urea combustion method and annealed at various temperatures (450°C–550°C). (a) 450°C ZnCr₂O₄/g-C₃N₄, (b) 450°C ZnO–ZnCr₂O₄/g-C₃N₄, (c) 500°C ZnCr₂O₄/g-C₃N₄, (d) 500°C ZnO–ZnCr₂O₄/g-C₃N₄, (e) 550°C ZnCr₂O₄/g-C₃N₄ and (f) 550°C ZnO–ZnCr₂O₄/g-C₃N₄.

The microstructure of the ZnO–ZnCr₂O₄/g-C₃N₄ nanocomposite was analyzed by TEM and HRTEM, as shown in Fig. 5(a)–(b). In Fig. 5(a), it can be observed that the ZnO–ZnCr₂O₄/g-C₃N₄ nanocomposite annealed at 500°C had a homogeneous mixture microstructure and a porous structure generated by the release of a large amount of gas during the urea combustion method. From Fig. 5(b), the particle size of the ZnO–ZnCr₂O₄/g-C₃N₄ nanocomposite was approximately determined to be 5–15 nm, confirming that the ZnO–ZnCr₂O₄/g-C₃N₄ prepared by the urea combustion method had a porous nanostructure.

Fig. 5

(a) TEM image and (b) HR-TEM image of ZnO–ZnCr₂O₄/g-C₃N₄ nanocomposite fabricated with the urea combustion method and 500°C annealing.

Figure 6(a) shows a STEM image of ZnO–ZnCr₂O₄/g-C₃N₄ after annealing at 500°C. Figure 6(b)–(g) present the STEM-EDX mapping of the ZnO–ZnCr₂O₄/g-C₃N₄ nanocomposite, revealing that all the elements were evenly distributed. Moreover, according to the results of the STEM-EDX mapping, C and N were uniformly coated on the surface of the ZnO–ZnCr₂O₄ nanocomposite. Based on this, it was deduced that g-C₃N₄ was formed by the thermal decomposition reaction of urea during the annealing process after the formation of ZnO–ZnCr₂O₄.

Fig. 6

ZnO–ZnCr₂O₄/g-C₃N₄ nanocomposite fabricated with the urea combustion method and 500°C annealing (a) STEM image, (b) Zn, Cr and N overlap, (c) Zn, (d) Cr, (e) N, (f) C and (g) O.

The BET specific surface areas of ZnO–ZnCr₂O₄/g-C₃N₄ and ZnCr₂O₄/g-C₃N₄ nanocomposites prepared by urea combustion method are listed in Table 1. After 24 hours of dehydration with high-purity N₂ gas at 200°C, the nitrogen adsorption amounts were calculated at various relative pressures using the equation P/P₀ = 0–1.0. Catalysts with high surface areas are desirable for achieving good catalytic performance. Based on the BET results listed in Table 1, the surface area of the ZnO–ZnCr₂O₄/g-C₃N₄ and ZnCr₂O₄/g-C₃N₄ nanocomposites ranged from 36.33 m²/g to 107.55 m²/g. Furthermore, the specific surface areas of the ZnO–ZnCr₂O₄/g-C₃N₄ and ZnCr₂O₄/g-C₃N₄ nanocomposites decreased with increases in annealing temperature, with or without the ZnO coating on the powders.

Table 1 The specific surface areas of ZnO–ZnCr₂O₄/g-C₃N₄ and ZnCr₂O₄/g-C₃N₄ nanocomposite fabricated with the urea combustion method and annealed at various temperatures (450°C–550°C).

Figure 7 and Fig. 8 show the N₂ adsorption–desorption curves and pore size distribution plots of the ZnO–ZnCr₂O₄/g-C₃N₄ and ZnCr₂O₄/g-C₃N₄ nanocomposites, respectively. Table 2 indicates that the pore sizes of ZnO–ZnCr₂O₄/g-C₃N₄ and ZnCr₂O₄/g-C₃N₄ nanocomposites were distributed in the range of 6.02–10.61 nm. As seen in Fig. 7, the N₂ adsorption–desorption curves of ZnO–ZnCr₂O₄/g-C₃N₄ and ZnCr₂O₄/g-C₃N₄ nanocomposites were similar to the IUPAC Type IV and exhibited obvious hysteresis due to the shape of the pores, which was consistent with the H3-type. During the desorption process, a lower relative pressure was required to vaporize a large amount of adsorbed substances due to the hysteresis. Therefore, the pore size distribution of the powders was mostly mesoporous. Moreover, the specific surface area of the nanocomposite decreased due to the attachment of ZnO to the catalyst surface, which could cover the micropores on the catalyst surface, as shown in Fig. 8.

Fig. 7

N₂ adsorption/desorption curves of ZnO–ZnCr₂O₄/g-C₃N₄ and ZnCr₂O₄/g-C₃N₄ nanocomposites fabricated with the urea combustion method and annealed at various temperatures (450°C–550°C).

Fig. 8

Pore size distribution of ZnO–ZnCr₂O₄/g-C₃N₄ and ZnCr₂O₄/g-C₃N₄ nanocomposites fabricated by the urea combustion method and annealed at various temperatures (450°C–550°C).

Table 2 Pore volumes and average pore diameters of ZnO–ZnCr₂O₄/g-C₃N₄ and ZnCr₂O₄/g-C₃N₄ nanocomposites fabricated with the urea combustion method and annealed at various temperatures (450°C–550°C).

The optical absorption spectra of the ZnO–ZnCr₂O₄/g-C₃N₄ and ZnCr₂O₄/g-C₃N₄ nanocomposites were measured at wavelengths of 220–1100 nm. Figure 9(a) shows the UV-Vis absorption spectrum of ZnO–ZnCr₂O₄/g-C₃N₄ and ZnCr₂O₄/g-C₃N₄ nanocomposites prepared by urea combustion method. Figure 9(b)–(f) present the Tauc plots of ZnO–ZnCr₂O₄/g-C₃N₄ and ZnCr₂O₄/g-C₃N₄ at different reaction temperatures. ZnCr₂O₄ had several absorption edges in the spectrum, and the edges of the ZnCr₂O₄, estimated to be 420–440 nm and 580–600 nm, could be attributed to the octahedral Cr³⁺(d³) transition.⁴⁵⁾ In addition, to evaluate the optical band gap and absorption, the band gaps (E_g) of the samples were calculated according with eq. (2), where α, h, υ, A and E_g represent absorption coefficient, Planck’s constant, optical frequency, constant and band gap energy, respectively,⁴⁶⁾ and the n value is a constant of 1/2 for direct allowed transitions, 3/2 for direct forbidden transitions and 2 for indirect allowed transitions.   

\begin{equation} \alpha h\upsilon = A(h\upsilon - E_{g})^{\frac{1}{\text{n}}} \end{equation} (2)

Fig. 9

The UV–Vis DRS absorption spectra of (a) ZnO–ZnCr₂O₄/g-C₃N₄ and ZnCr₂O₄/g-C₃N₄ nanocomposites, and Tauc plots of the estimated bandgaps of (b) 450°C ZnCr₂O₄/g-C₃N₄, (c) 450°C ZnO–ZnCr₂O₄/g-C₃N₄, (d) 500°C ZnCr₂O₄/g-C₃N₄, (e) 500°C ZnO–ZnCr₂O₄/g-C₃N₄, (f) 550°C ZnCr₂O₄/g-C₃N₄, and (g) 550°C ZnO–ZnCr₂O₄/g-C₃N₄.

The Tauc plots revealed that the energy band gaps of the nanocomposite oxides were approximately 2.09–2.22 eV for ZnCr₂O₄, 2.53–2.80 eV for g-C₃N₄, and 2.91–3.01 eV for ZnO under different annealing conditions. The energy band gaps of ZnCr₂O₄, g-C₃N₄ and ZnO were investigated with eq. (3) and are listed in Table 3, respectively.   

\begin{equation} E(\textit{eV}) = \frac{\mathrm{hc}}{\lambda} = \frac{1240}{\lambda}(\text{nm}) \end{equation} (3)

Table 3 The energy bandgap of ZnO–ZnCr₂O₄/g-C₃N₄ and ZnCr₂O₄/g-C₃N₄ nanocomposites fabricated with the urea combustion method and annealed at various temperatures (450°C–550°C).

3.2 ZnO–ZnCr₂O₄/g-C₃N₄ and ZnCr₂O₄/g-C₃N₄ nanocomposites prepared by GNP and applied to photodegradation of RhB organic dyes

The photocatalytic activities of the ZnO–ZnCr₂O₄/g-C₃N₄ and ZnCr₂O₄/g-C₃N₄ nanocomposites were evaluated according to their degradation efficiency of 10⁻⁵ M RhB solution under irradiation with UVA light (8 W, 254 nm) for 12 hours. When the catalyst absorbs light and the frequency of the incident wavelength is higher than its energy band gap, electron-hole pairs will be generated, and the recombination rate of electron-hole pairs can used as an indicator of the catalytic performance. Therefore, a faster the recombination rate causes a worse catalytic performance. In addition, electron holes will react with H₂O and hydroxyl groups (–OH) to form highly efficient non-selective oxidants (·OH) for RhB degradation. Figure 10(a) and Fig. 10(b) respectively present the absorbance spectra after the degradation of RhB solution with and without dispersed catalyst. RhB solution was hardly degraded under UVA irradiation for 12 hours without the assistance of a catalyst. After the RhB solution was mixed with the ZnO–ZnCr₂O₄/g-C₃N₄ nanocomposite annealed at 500°C and then irradiated with UVA light for 12 hours, the photocatalytic degradation efficiency of the RhB reached 95.45%. As shown in the schematic diagram of Fig. 1, the solution became almost transparent and colorless after degradation. In Fig. 10(c) and Table 4, the rate constant (k) of photocatalytic degradation can be calculated by its slope, and the k value and linear correlation coefficient (R²) of each sample are listed in Table 4.

Fig. 10

The test of RhB photodegradation by ZnO–ZnCr₂O₄/g-C₃N₄ and ZnCr₂O₄/g-C₃N₄ nanocomposites. The absorbance of RhB photodegradation with (a) 500°C ZnO–ZnCr₂O₄/g-C₃N₄ nanocomposite and (b) only UV light. (c) The plot of the rate constant of ln(C/C₀) vs. irradiation time. (d) Cycling-life and (e) photocatalytic degradation efficiency and stability of 500°C ZnO–ZnCr₂O₄/g-C₃N₄ nanocomposite in the photodegradation of RhB.

Table 4 The efficiency and rate constant of ZnO–ZnCr₂O₄/g-C₃N₄ and ZnCr₂O₄/g-C₃N₄ nanocomposites fabricated with the urea combustion method and annealed at various temperatures (450°C–550°C).

Among the results of the RhB photodegradation test, 500°C ZnO–ZnCr₂O₄/g-C₃N₄ nanocomposite presented the best degradation rate constant of 6.11 × 10⁻³ min⁻¹, and R² was about 0.771. The reason for its lower R² could have been its porous structure, which did not allow the light to be fully absorbed and trigger the catalyst.

Zhang et al. found that incorporating ZnO into a catalyst had an impact on the particle size, and an appropriate amount of ZnO content could modify the active site on the surface and prevent particle agglomeration.⁴⁷⁾ The incorporation of ZnO into ZnCr₂O₄/g-C₃N₄ led to enhancement of the photocatalytic activity, and the resulting ternary crystal structure (ZnO–ZnCr₂O₄/g-C₃N₄) had a close connection between the hexagonal ZnO, spinel ZnCr₂O₄, and g-C₃N₄ phases, which improved their synergistic effect. Gao et al. reported that the morphology and surface structure of ZnO played a crucial role in its catalytic activity. The (0001) facets of ZnO, which are terminal polar surfaces, were more active for catalysis than were the nonpolar surfaces perpendicular to them.⁴⁸⁾

In addition, in this study, the stability of the catalyst was tested, and even after 4 cycles of photodegradation reaction, 500°C ZnO–ZnCr₂O₄/g-C₃N₄ could still achieve a high photocatalytic degradation efficiency of up to 92.40%. In summary, ZnO–ZnCr₂O₄/g-C₃N₄ nanocomposite catalyst prepared by urea combustion method revealed high degradation efficiency and high catalytic activity for removal of organic dyes by photodegradation under UVA light irradiation, and it can be applied to environmental clean-up efforts.

4. Conclusions

In this study, ZnO–ZnCr₂O₄/g-C₃N₄ and ZnCr₂O₄/g-C₃N₄ nanocomposites were successfully prepared by urea combustion method, and they exhibited high efficiency, low cost, and excellent performance in removing organic dyes. The as-prepared ZnO–ZnCr₂O₄/g-C₃N₄ and ZnCr₂O₄/g-C₃N₄ nanocomposites were characterized by XRD, FE-SEM, STEM-EDX, BET, and UV-Vis, and their catalytic properties were investigated. The study results of UVA photodegradation of Rhodamine B corresponding to FE-SEM, TEM and BET studies demonstrated that the porous structure obtained by the urea combustion method effectively increased the specific surface area of the catalyst, and the nanoscale size of the catalyst particles allowed better dispersion, thus reducing the efficiency of electron-hole pair recombination. In the UVA photodegradation test, 500°C ZnO–ZnCr₂O₄/g-C₃N₄ nanocomposite showed the best photodegradation efficiency and rate constant (k) of 95.45% and 6.11 × 10⁻³ min⁻¹, respectively. The catalyst also revealed excellent stability in cyclic tests and performed well as a highly active catalyst in the field of environmental purification.

Acknowledgments

This work was supported by the Ministry of Science and Technology of Taiwan (MOST 108-2221-E-027-056, MOST 109-2221-E-027-068, MOST 109-2221-E-027-059, and MOST 109-2113-M-027-001-MY3, MOST 110-2221-E-027-041). This work was supported by the National Science and Technology Council of Taiwan (NSTC 111-2221-E-027-104). The authors are grateful to the Precision Research and Analysis Centre of the National Taipei University of Technology (NTUT) for providing the measurement facilities.

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