Fabrication and Characterization of Highly Efficient As-Synthesized WO₃/Graphitic-C₃N₄ Nanocomposite for Photocatalytic Degradation of Organic Compounds

Abstract

The incorporation of tungsten trioxide (WO₃) by various concentrations of graphitic carbon nitride (g-C₃N₄) was successfully studied. X-ray diffraction (XRD), Scanning Electron Microscope (SEM), and Diffused Reflectance UV-Vis techniques were applied to investigate morphological and microstructure analysis, diffused reflectance optical properties, and photocatalysis measurements of WO₃/g-C₃N₄ photocatalyst composite organic compounds. The photocatalytic activity of incorporating WO3 into g-C₃N₄ composite organic compounds was evaluated by the photodegradation of both Methylene Blue (MB) dye and phenol under visible-light irradiation. Due to the high purity of the studied heterojunction composite series, no observed diffraction peaks appeared when incorporating WO₃ into g-C₃N₄ composite organic compounds. The particle size of the prepared composite organic compound photocatalysts revealed no evident influence through the increase in WO₃ atoms from the SEM characteristic. The direct and indirect bandgap were recorded for different mole ratios of WO₃/g-C₃N_4, and indicated no apparent impact on bandgap energy with increasing WO₃ content in the composite photocatalyst. The composite photocatalysts’ properties better understand their photocatalytic activity degradations. The pseudo-first-order reaction constants (K) can be calculated by examining the kinetic photocatalytic activity.

1. Introduction

The photocatalytic performance of semiconductor materials, based on oxide semiconductors in their crystalline phases, has recently become desirable for producing hydrogen and oxygen via visible-light water splitting. Designing semiconductor composites produced between two semiconductor materials necessitates crystalline phase engineering [1,2,3]. The use of heterogeneous photocatalysis to convert water into hydrogen gas is regarded as one of the most promising options for addressing global energy and pollution problems [1,4,5]. In the present century, the rapid decline in energy sources, and increased power, waste, and renewable H₂ energy sources, are attributable to worldwide demand for fossil fuels. A. Fujishima et al. studied for a long time how to establish a clean and renewable photocatalytic hydrogen-production method [6]. The absorption wavelength range of semiconductor materials such as oxides, sulfides, nitrides, and solid solutions was examined under visible light, to improve the absorption wavelength range [7,8].

Heterojunction photocatalysts were used extensively to improve the separation efficiency of photoexcited electron-hole pairs. A single-phase photocatalyst has significant limitations in a photocatalytic response, because photogenerated electrons and holes are quickly mixed [9] to develop optical properties. Jiang et al. doped phosphorus nanosheets with g-C3N4 and added carbon defects, which significantly increased the rate of hydrogen evolution through the photocatalytic method [9]. Furthermore, studies report an improved hydrogen production rate through using a mineral acid or phosphoric acid etching of g-C₃N₄ nanosheets to increase the number of active sites [10,11].

Graphite carbon nitride (g-C₃N₄) has recently gained new interest as a promising material in different applications such as photocatalysts, fuel cell electrodes, light-emitting devices, and chemical sensors. g-C₃N₄ stands out amongst many types of photocatalysts. A polymer photocatalyst bandgap is about 2.7 eV, allowing visible light to absorb up to 460 nm. Nevertheless, g-C₃N₄ can absorb visible light effectively since graphitic C₃N₄ has an adequate conduction band under illumination conditions that is able to be more harmful than protons formed by hydrogen [12,13]. Tungsten oxide (WO₃) is considered a promising material [13,14]. It has been reported that synthesis of WO₃/g-C₃N₄ composite organic compounds shapes a heterostructural composite photocatalyst [15,16],provided that the electron donor is an aqueous solution for triethanolamine. In the color-sensitization method, we looked at the composite catalyst’s hydrogen production behavior under visible-light irradiation. g-C₃N₄ composite, blended with WO₃ using a planetary mill, was prepared using hydrothermal treatment to improve its photocatalytic activity [15]. The photocatalytic activity incorporated WO₃ in g-C₃N₄ composite organic compounds [17]. J. Liang et al. [18] investigated the use of crystalline phase engineering in WO₃/g-C₃N₄ composite organic compounds to improve photocatalytic activity under visible light. They demonstrated that WO₃/g-C₃N₄ composites with h-WO₃ show better dispersion of WO₃, higher charge separation, and higher photocatalytic activity through visible-light photocatalytic degradation of Rhodamine B (RhB).

In this research, incorporating WO₃ into g-C₃N_4, composite organic compound photocatalysts were prepared, and exhibited improved photocatalytic activity under visible light. Their morphological, diffused reflectance UV-Vis optical, and (Methylene blue (MB) and phenol) photocatalytic activity properties were analyzed and discussed in detail, to understand the effects of WO₃ on g-C₃N₄ composite organic compounds.

2. Experimental Parts

2.1. Regents

In the experiment, an analytical grade of melamine, sodium tungstate dihydrate (Na₂WO₄·2H₂O) ACS reagent, ≥99%, and other chemicals purchased from Sigma-Aldrich were used. These chemicals did not need further purification as they were of analytical grade. De-ionized water was utilized to avoid contamination.

2.2. Synthesis of Pure g-C₃N₄ and WO₃/g-C₃N₄ Composite Organic Compounds

Simple g-C₃N₄ synthesis involves melamine pyrolysis prepared in an air atmosphere. Initially, 8 g of melamine was ground inside a crucible, then heated at a rate of 5 °C per min until it attained 550 °C, and then adjusted at this temperature for 2 h. Afterwards, the samples were left to cool down to ambient temperature. In the mortar, the resultant powder was then ground with a pestle. Following the first stage, Na₂WO₄·2H₂O was added to melamine to synthesize the doped WO₃/g-C₃N₄ composite organic compounds, after which the procedure for pure g-C₃N₄ was repeated.

2.3. Characterization Techniques

The effect of WO₃ on the physical structure of the g-C₃N₄ composite organic compounds for the 13 structures was characterized by X-ray diffraction. Shimadzu X-ray Diffractometer (XRD-6000 Series) was utilized with the standard copper X-ray tube at 30 kV and 30 mA with a wavelength equal to 1.5406 Å. The surface structure of all samples was evaluated by scanning electron microscopy (SEM-JSM6360 Series, with an acceleration voltage = 20 kV).

The effect of WO₃ on g-C₃N₄ nanocomposite organic compounds was tested employing a UV-vis-NIR spectrophotometer model (Shimadzu UV-3600) with diffused reflectance in a wide range between 200 and 800 nm. The device was designed with the BaSO₄ built-in sphere attachment as reference material. To counteract the WO₃/g-C₃N₄ composites within the holder, as a guide for the BaSO₄ and the other holder, a specifically constructed holder attached to the integrating sphere device was employed. The thickness of the holder corresponds to the thickness of the WO₃/g-C₃N₄ nanocomposites. To track the photo-removal phase, a UV-visible spectrophotometer was used.

The diffused reflectance was measured in ambient conditions using a JASCO UV-Vis-NIR-V-570 double beam spectrophotometer.

2.4. Photocatalytic Measurements

A simple wooden photoreactor tested the photocatalytic activity of WO3/g-C3N4 nanocomposites under visible-light spectrum radiation of samples, using both methylene blue (MB) and aqueous phenol as a pollutant example in wastewater. I.S. Yahia and his group designed the photoreactor in NLEBA, Ain Shams University (ASU), Egypt, consisting of two parts: the outer part was in the form of a box made from wood (height: 100 cm, width: 65 cm); the inner part had seven visible lamps (18 W, 60 cm length, 425 to 600 nm), and could be separately controlled. Each set of nanopowders was placed in a beaker containing 50 mL of MB (20 mg/L) (i.e., for the phenol photodegradation). The sample was magnetically stirred in the dark system to reach equilibrium between dye and photocatalyst. The sample was removed from the solution after a specific time (every 15 min), and the amount of sample remaining was then exposed to visible light. A UV-Vis spectrophotometer was used to analyze the samples.

3. Results and Discussions

3.1. Structural XRD Measurements

The crystalline structure phase was analyzed with the XRD technique to reveal the effect of doping WO₃ in the final products. XRD patterns are shown in Figure 1 for pure g-C₃N₄ and its doping with WO₃ on the g-C₃N₄ composite organic compounds, with various WO₃ effects. As visualized in Figure 1, the g-C₃N₄ powder sample highlights two peaks at 2θ = 13.1°, and 2θ = 27.4°, which can be indexed to a small peak, (100) plane, pertinent to the in-plane structural packing of graphitic material, and a sharp peak, (002) plane, assigned to the interlayer spacing of the conjugated aromatic system. The (002) peak indicates a higher-density packing of the g-C₃N₄ molecules. This corresponds to the characteristic interplanar staking peaks of aromatic systems and the inter-layer structural packing, respectively [19,20]. These results are compared with Mo et al. [21]. The peak at 2θ = 27.4° (JCPDS 87-1526) confirms the formation of hexagonal phase g-C₃N₄ powders [18,20,21]. The introduction of doping WO₃ on g-C₃N₄ composite organic compounds reduces the maximum intensity of peaks. In Figure 1, we notice that only two peaks appear in the entirety of the prepared samples related to pure g-C₃N₄ except for the last sample, noted as 0.5 g WO₃-doped g-C₃N_4, showing the other two peaks related to WO₃. The two peaks related to WO₃ appear at 2θ = 16.9° and 32.51°, related to (101) and (022), according to JCPDS 01-083-0950.

The average crystallite size for the prepared samples was measured using the Debye–Scherrer formula, as follows [22]:

$$D=0.94\lambda /\beta \cos \theta$$

(1)

The calculated crystallite size D depends on the broadening diffraction peak β, the diffraction angle $\theta$, and the X-ray wavelength λ. Both dislocation density $\delta$ and lattice strain $\epsilon$ were calculated using the following equations [23,24]:

$$\delta =n/D^2$$

(2)

$$\epsilon =\beta \cos \theta /4$$

(3)

The XRD structural parameters for the prepared samples summarized in

Table 1. N₄ and its WO₃/g-C₃N₄ with various amounts of tungsten oxide (0.001, 0.01, 0.05, 0.1, and 0.5% WO₃).

Samples	Phases	Mean Values
		Crystallite Size, (nm)	Dislocation Density, (1/(nm)2)	Lattice Strain
Pure g-C3N4	Pure g-C3N4	39.17	6.51 × 10−4	8.850 × 10−4
0.001 g WO3-doped g-C3N4	Pure g-C3N4	35.91	7.999 × 10−4	9.753 × 10−4
0.01 g WO3-doped g-C3N4	Pure g-C3N4	35.93	7.993 × 10−4	9.749 × 10−4
0.05 g WO3-doped g-C3N4	Pure g-C3N4	40.84	6.866 × 10−4	8.886 × 10−4
0.1 g WO3-doped g-C3N4	Pure g-C3N4	44.83	9.336 × 10−4	9.667 × 10−4
0.5 g WO3-doped g-C3N4	Phase 1: Pure g-C3N4	81.76	1.710 × 10−4	4.436 × 10−4
	Phase 2: WO3	52.93	3.641 × 10−4	6.593 × 10−4

show that the average crystallite size for the pure g-C₃N₄ is 39.17 nm, and this increases from 35.91, 35.93, 40.84, 44.83, to 81.67 nm for the 0.5, 0.1, 0.05, 0.01, and 0.001 WO₃-doped g-C₃N₄ composites, respectively.

The lower dislocation density values reflect the higher quality of the prepared samples.

3.2. Morphologies and Microstructure Analysis

To assess the effect of WO₃ in g-C₃N₄ composite organic photocatalyst compounds, the morphology and microstructure of the pure g-C₃N₄ and its WO₃-doped g-C₃N₄ composite organic compounds was studied by using SEM image analysis. Figure 2a–f presents the SEM images of the investigated composite photocatalysts. The morphology of pure g-C₃N₄ exhibited apparent granular aggregates and a typical sheet that consisted of small particles created from some irregular particles. Figure 2c–f shows that the WO₃ particles were attached to the surface of the sheet g-C₃N₄. The particle sizes obtained from SEM images of the investigated composite organic compounds were gathered in

Table 2. Particle size and corresponding rate constants of pure g-C₃N₄ and its WO₃/g-C₃N_4, with various amounts of tungsten oxide (0.001, 0.01, 0.05, 0.1, and 0.5% WO₃).

Samples	Particle Size, (µm)	K, (min−1)
		MB	Phenol
Pure g-C3N4	1.65	0.0028	0.0053
0.001 g WO3/g-C3N4	1.21	0.0054	0.0064
0.01 g WO3/g-C3N4	1.26	0.0049	0.0092
0.05 g WO3/g-C3N4	1.3	0.0108	0.0194
0.1 g WO3/g-C3N4	1.5	0.0052	0.0172
0.5 g WO3/g-C3N4	1.51	0.0045	0.0145

. The particle sizes varied between 1.65 and 1.51 µm. As the WO₃ doping particle contents increased, the g-C₃N₄ composite photocatalyst had no noticeable influence on the obtained particle size. A similar result is reported by X. Chu et al. [25].

3.3. Optical Properties of WO₃ Doped g-C₃N₄ Nanocomposites

Figure 3a shows the optical UV-Vis diffused reflectance spectroscopy study to identify the bandgap energy of pure g-C₃N₄ and WO₃-doped g-C₃N₄ composite organic photocatalyst compounds. The optical UV-Vis diffused reflectance spectra for other WO₃/g-C₃N₄ photocatalyst composite samples should be the superimposed signals of g-C₃N₄. All diffused reflection spectra increased with increasing wavelengths up to 400 nm. The absorption edge spectra start to redshift towards the visible region, allocated to the intrinsic g-C₃N₄ bandgap [25].

The redshift suggested that the photocatalyst composite compounds could use sunlight and produce more electron-hole pairs, which would help the photocatalytic response. The above findings may be caused by the interactions between the WO₃ and g-C₃N₄ incorporated into the heterojunction materials. Figure 3b,c displays the direct and indirect energy bandgap of pure g-C₃N₄ and its WO₃ atom-doped g-C₃N₄ composite organic photocatalyst compounds, which can be estimated from the plots of (ahυ)² and (ahυ)^1/2 vs. the incident photon energy (hυ) using Tauc’s formula [26]. The Kubelka–Munk function and its related absorption coefficient (α) are as follows [27,28,29]:

Ahυ = (F(R) hυ/d) = A(hυ−Eg)^r

(4)

where A is a pre-factor, E_g is the energy bandgap, and r limits the transition band type. When r = 2 is related to the indirect allowed band, r = 1/2 is associated with the direct allowed band. The bandgap of the photocatalyst was determined by Tauc’s plot, where (αhυ)^1/2 with hυ axis (i.e., the linear portion of the plots (αhυ)² with hυ axis). By drawing a tangent line of each curve, each photocatalyst composite material’s bandgap was obtained from the intercept hυ axis. Therefore, the estimated bandgaps of pure g-C₃N₄ and its WO₃ atom-doped g-C₃N₄ composite organic photocatalyst compounds, with various WO₃ content, confirmed that the direct and indirect bandgaps for g-C₃N₄ were 2.83 [30] and 2.56 eV, respectively. This result was also reported by J.Y. Tai et al. [31]. For the indirect bandgap, we noted a slight change from 2.56 eV for the pure g-C₃N₄ changing to 2.6, 2.65, 2.66, 2.86, to 2.71 eV for the 0.5, 0.1, 0.05, 0.01, and 0.001 WO₃-doped g-C₃N₄ composite, respectively. For the direct bandgap, a small change was also noted, from 2.83 eV for the pure g-C₃N₄ changing to 2.86. 2.88, 2.89, 2.90, 2.92 eV for the 0.5, 0.1, 0.05, 0.01, and 0.001 WO₃-doped g-C₃N₄ composite, respectively. This indicated that the increase in WO₃ content in the composite photocatalyst had no evident influence on bandgap energy. However, it was found that the movement and lifetime of the photogenerated charge carriers in g-C₃N₄ could be significantly affected by such a combination.

3.4. Kinetics of the Photodegradation Process of WO₃-Doped g-C₃N₄ Nanocomposites

The photocatalytic performances of pure g-C₃N₄ and its WO₃/g-C₃N₄ composites, with various concentrations, were assessed using MB and phenol contaminants under visible-light irradiation. The extent of the concentration of MB and phenol contaminants on the photocatalyst was measured in the dark system until equilibrium. The equilibrium between the prepared photocatalyst and the investigated organic pollutants, MB, as a colored dye, and phenol, as a colorless organic compound, was recognized after 30 min of stirring. The photocatalytic performances of pure g-C₃N₄ and its WO₃/g-C₃N₄ composites with different photocatalysts were investigated for the photodegradation of MB and phenol contaminants. The achieved results are shown in Figure 4a,b. Figure 4a shows a limited photocatalytic performance of the pure g-C₃N₄ toward MB. However, the improved photocatalytic performance of WO₃/g-C₃N₄ composites was recorded, indicating a major effect on the photocatalytic activity of g-C₃N₄ when modified with WO₃. In general, the composite’s photocatalytic activity increased after WO₃ was applied, and enhanced photocatalytic activity of 0.05% WO₃ was noted.

Nevertheless, the increase in WO₃ (0.1 and 0.5) has been found to reduce photocatalytic efficiency, which may be due to the decrease in visible-light absorption catalytic sites. Additionally, the same behavior was observed for the degradation of phenol. The composite material containing 0.5% WO₃ showed the maximum performance for phenol degradation, which can be attributed to the production of the WO₃-semiconductor heterojunction effectively, which leads to transfer of charge from g-C₃N₄ under the visible-light irradiation. Moreover, the kinetics of the degradation of MB and phenol was noticed to follow the pseudo-first-order model, and the reaction rates were calculated by Equation (2) [32]:

ln(C_o/C) = Kt,

(5)

Here, C_o and C are the concentrations at the initial step and regular time intervals, respectively. Additionally, K is the pseudo-first-order reaction-rate constant (min⁻¹) for investigated organic contaminants, and t is the time (min). Figure 5a,b shows the linear relationship between ln(C_o/C) and t. According to these results, the photodegradation of MB and phenol pollutants follows pseudo-first-order reaction kinetics. Additionally, the degradation rate relies on the concentration of the organic substrate. Figure 6 shows the constant rate values for the degradation of MB and phenol contaminants. Percentage of degradation can be calculated to determine the efficiency of the prepared catalysts in the photocatalytic reaction, as illustrated in Figure 5. The efficiency of degradation increases with WO₃ content to 0.5%, then the efficiency decreases. It can be noted that the photocatalytic degradation rates of MB had better rate-constant characteristics than the photocatalytic degradation rates of phenol. The photodegradation using MB and phenol contaminants in aqueous solution improvement onto pure g-C₃N₄ and its WO₃/g-C₃N₄ photocatalyst composites with various concentrations was evaluated under visible light. Dependant on variations in the WO₃ content, the plot of the irradiation time (t) against −ln(C/C_o) was nearly a straight line. This result has also been reported by G. Lui et al. [33]. The corresponding estimated degradation rate constants (K) were added to Table 2.

3.5. Photodegradation Conduction Mechanism of WO₃-Doped g-C₃N₄ Nanocomposites

The proposed mechanism for photocatalytic behavior of WO₃/g-CN composite under the visible spectrum is interpreted in Figure 6. It is a successfully designed WO₃/g-CN composite with various WO₃. The Kubelka–Munk method was used to calculate the valence band (VB) and conduction band (CB) edge potentials of the prepared pure g-CN and WO₃/g-CN with different concentrations of WO₃. The direct and indirect bandgaps for g-CN are 2.83 and 2.56 eV, respectively. The increase in WO₃ content in the WO₃/g-CN composite significantly influences bandgap energy [34]. The proposed system suggests that both g-CN and WO₃ will likely offer photogenerated charge carriers. The photogenerated electron in CB of WO₃ then flows to VB of g-C₃N₄ due to electrostatic forces of attraction between the electron in CB of WO₃ and the hole in VB g-CN, reducing the recombination of electrons and holes in the WO₃/g-C₃N₄ composite and assisting in enhancing the charge separation spatially.

Furthermore, the electrons in the CB of g-C₃N₄ can also be retained by reducing the molecular oxygen O₂ to form O^−2•, due to the negative nature of the CB edge potential compared to the standard redox capability of O₂/O^−2• [35]. Thus, the hole created in the VB of WO₃ leads interacts with H₂O to generate OH^• radicals, giving the more positive potential of the VB than the standard redox capability of OH/OH⁻. In this manner, the production of O^−2• and OH^• radicals plays a significant role in the photodegradation of MB and phenol degradation under visible light. With both OH^• and O⁻², the phenol and Methylene Blue dyes produced CO₂ and H₂O [36,37].

WO₃-g-C₃N₄ + hυ → e⁻ + h⁺

(6)

O₂ + e⁻ → O^−2•

(7)

H₂O + h⁺ → OH^•

(8)

OH^• + O^−2• + organic dye (MB or Ph) → CO₂+ H₂O

(9)

In comparison to the previous work, our 0.5% WO₃/g-C₃N₄ has the highest photocatalytic activity of both MB and phenol in the presence of visible light, as shown in

Table 3. Photocatalytic comparison between the prepared WO₃/g-C₃N₄ and various previous works.

Photocatalyst	Method of Preparation	Organic Solution	Irradiation Time	Source	% Degradation	Refs.
WO3/g-CN	Pyrolysis	MB	120 min	Visible Light lamp	80%	Present work
		Phenol			90%
C-doped g-C3N4/WO3	Hydrothermal impregnation	Tetracycline	60 min	UV-light lamp	75%	[33]
WO3/g-C3N4	Calcination method	Tetracycline	120 min	Xenon lamp	90.54%	[34]
Ag/WO3/g-C3N4	Calcination method	MB	300 min	UV-light lamp	-----	[35]
WO3/g-C3N4	Thermal polymerization	MO	120 min	Xenon lamp	93%	[36]
WO3/g-C3N4	Hydrothermal	RhB	90 min	UV-light lamp	91%	[37]
WO3/g-C3N4/ Photo-Fenton	Calcination method	P-nitrophenol	240 min	Xenon lamp	91%	[38]

. Using the hydrothermal impregnation and calcination method, Cong Zhao et al. [38] obtained simple and inexpensive C-doped g-C₃N₄/WO₃ as highly active photocatalysts. Tao Pan et al. [39] prepared WO₃/g-C₃N₄ that showed improved photocatalytic activity for TC degradation. Perhaps most importantly, the UV-C-induced–TC degradation activity outperformed all other degradations regardless of concentration or pH level. According to Zhao et al. [40], the Ag/WO_2.9/g-C₃N₄ composite outperforms pure g-C₃N₄ and pure WO₃ in terms of light-selective adsorption and photocatalysis. Hong Yan et al. [41] proved that WO₃/g-C3N4-modified nanocomposites had more significant photocatalytic activity than pure WO₃ and pure g-C₃N₄. Junling Zhao et al. [19] discovered that WO₃/g-C₃N₄ photocatalysts have increased degradation activities towards RhB dye when exposed to simulated sunlight. Minji Yoon et al. [42] demonstrated that WO₃/g-C₃N₄ does not exhibit any photocatalytic activity on the degradation of p-nitrophenol. On the other hand, the degradation rate of p-nitrophenol was remarkably increased by the addition of Fenton to WO₃/g-C₃N₄, which means that photocatalytic activity was further enhanced in the presence of hydrogen peroxide (H₂O₂) because of the generation of hydroxyl radicals.

3.6. Recycling of WO₃-Doped g-C₃N₄ Nanocomposites

A 0.05 WO₃/g-CN was subjected re-use in photodegradation of MB and phenol for 4 cycles. Figure 7 represents the recycling and reusability of WO₃/g-C₃N₄. The samples exhibited high performance without further decreased reaction rate, which means that WO₃/g-C₃N₄ is a promising material in MB and phenolic compound photodegradation.

4. Conclusions

Novel, efficient WO₃/g-C₃N₄ nanocomposite with various concentrations of graphitic carbon nitride (g-C₃N₄) was prepared. The prepared composites exhibited improved photocatalytic activity on the degradation of MB and phenol under visible-light irradiation. The composite material containing 0.5% WO₃ showed the maximum MB and phenol degradation performance. The kinetic study showed that the pseudo-first-order kinetic best fitted the photocatalytic process. A mechanism was proposed to degrade organic pollutants using WO₃/g-C₃N₄ composites under visible-light irradiation.