Hydrothermal Synthesis of Bimetallic (Zn, Co) Co-Doped Tungstate Nanocomposite with Direct Z-Scheme for Enhanced Photodegradation of Xylenol Orange

Abstract

In the present study, pristine ZnWO₄, CoWO_4, and mixed metal Zn_0.5Co_0.5WO₄ were synthesized through the hydrothermal process using a Teflon-lined autoclave at 180 ℃. The synthesized nanomaterials were characterized by various spectroscopic techniques, such as TEM, FTIR, UV–vis, XRD, and SEM-EDX-mapping to confirm the formation of nanocomposite material. The synthesized materials were explored as photocatalysts for the degradation of xylenol orange (XO) under a visible light source and a comparative study was explored to check the efficiency of the bimetallic co-doped nanocomposite to the pristine metal tungstate NPs. XRD analysis proved that reinforcement of Co²⁺ in ZnWO₄ lattice results in a reduction in interplanar distance from 0.203 nm to 0.185 nm, which is reflected in its crystallite size, which reduced from 32 nm to 24 nm. Contraction in crystallite size reflects on the optical properties as the energy bandgap of ZnWO₄ reduced from 3.49 eV to 3.33 eV in Zn_0.5Co_0.5WO₄, which is due to the formation of a Z-scheme for charge transfer and enhancement in photocatalytic efficiency. The experimental results suggested that ZnWO₄, CoWO_4, and Zn_0.5Co_0.5WO₄ NPs achieved a photocatalytic efficiency of 97.89%, 98.10%, and 98.77% towards XO in 120 min of visible solar light irradiation. The kinetics of photodegradation was best explained by pseudo-first-order kinetics and the values of apparent rate const (k_app) also supported the enhanced photocatalytic efficiency of mixed metal Zn_0.5Co_0.5WO₄ NPs towards XO degradation.

1. Introduction

The development of societies and industries has led to the deterioration of freshwater qualities through the release of untreated effluents containing hazardous chemical contaminants, such as dyes and pharmaceutical products, etc. [1,2]. The toxic organic compounds, especially dyes and pharmaceuticals, contaminated wastewater in the environment, which can cause a variety of health issues to humans [3,4]. Among various organic pollutants, xylenol orange (XO; C₃₁H₃₂N₂ O₁₃S) belongs to the acidic dye family commonly used in the textile, paper, and printing industries, but beyond the permissible limit, it causes nervous system breakdown, liver–kidney damage, blood disorders, and skin irritation [5,6]. So, it has become a matter of serious concern to develop new technology-based methods and resolutions which can treat these hazardous chemicals in the water and regulate their limits in the aquatic environment. Among various chemical, physical, and biological wastewater treatment methods, photocatalysis enables the adsorbed organic molecules to be degraded at a low cost into H₂O and CO₂ using solar irradiation, which is abundant, renewable, and environmentally friendly [7,8]. Moreover, advanced oxidation processes (AOPs) were created to promote the removal or degradation of contaminant species through redox processes, focused on the remediation of contaminants in wastewater; they are a group of techniques that use reactive oxidant species (ROS) to mineralize a wide range of resistant organics [9,10].

Metal tungstate nanoparticles (NPs) have received a lot of interest among the different types of semiconductor photocatalysts [11]. For this reason, the study and synthesis of single photocatalysts that successfully use visible light to stimulate photocatalysis have recently become a promising idea. Until now, various semiconductors with sufficient energy band gaps to absorb visible light, such as Fe₂O₃, Ag₂O, Bi₂WO₆, g-C₃N₄, MnWO₄, CdWO₄, CdS, NiMoO₄, Cu₂O, ln₂S_3, and Si₃N₄, have been effectively synthesized and utilized for photocatalysis, because of its high average refractive index, high chemical stability, excellent light absorbing property, and high catalytic activity [12,13,14]. In the tungstate family, zinc tungstate (ZnWO₄) has been a widely known semiconductor photocatalyst owing to its high physical and chemical stability and difficulty to dissolve [15]. However, due to homogeneity, it is only UV light active and possesses a high rate of electron–hole pair recombination, which poses an obstacle in the practical applications of ZnWO₄ [16]. Various methods have been applied previously to increase the photocatalytic efficiency of ZnWO₄, such as semiconductor coupling [17], noble metal loading [18], and metal deposition [19]. The metal deposition of ZnWO₄ with other metals having a narrow band gap could effectively inhibit the electron–hole pair recombination by expanding the light absorption range, and thus a robust enhancement in photocatalytic efficiency will appear [20,21]. So, the Co deposition method was applied in this study to synthesize the Co-doped nanocomposite Zn_0.5Co_0.5WO₄ with improved energy bandgap, optical properties (UV–vis light active), and photocatalytic efficiency. Since the ionic radii of both Co²⁺ and Zn²⁺ are approximately the same, it can be easily deposited in the solid matrix of ZnWO₄ and result in the reduction of the energy bandgap from 3.87 eV (ZnWO₄) to 1.95 eV (Zn_0.5Co_0.5WO₄). ZnWO₄ was identified as a large band gap semiconductor (band gap = 3.87 eV) with the appropriate conduction band and valance band locations based on its electronic characteristics, while, on the other hand, cobalt tungstate (CoWO₄) has been recognized as having excellent phase composition, strong chemical stability, and a low energy bandgap, which makes it adaptable to degrade the dyestuff in the visible light range [22].

In the present work, a hydrothermal method was used for the synthesis of pristine metal tungstate nanomaterials (ZnWO₄, CoWO₄ NPs) and mixed metal tungstate nanocomposite (Zn_0.5Co_0.5WO₄ NPs) using a Teflon line autoclave. The synthesized NPs were explored for the photocatalytic degradation of XO dye and a comparative study was designed to explore the photocatalytic mechanism and efficiency of the photocatalyst.

2. Results and Discussion

2.1. Material Characterization

Figure 1 represents the FTIR of ZnWO₄, CoWO_4, and Zn_0.5Co_0.5WO₄, in which the characteristic peaks of ZnWO₄ are 3448 and 1644 cm⁻¹ (stretching and bending vibrations of –OH group) due to surface adsorbed water, 473 to 881 cm⁻¹ (W₂O₈ bonds (WO₆) groups shared at edges and WO₂ groups), and 473 cm⁻¹ (stretching vibrational of the Zn–O) [23]. Three peaks at 541, 685, and 716 cm⁻¹ correspond to symmetric and asymmetric stretching vibrations of the bridging atom of the WO₂ groups of distorted octahedral (WO₆) clusters [24]. Broad absorption bands at 816 cm⁻¹ and 881 cm⁻¹ are caused by symmetrical vibrations of bridge oxygen atoms of Zn–O–W groups [24]. Similar types of stretching and vibrational bands are also shown by CoWO₄ with different peak values and 469 cm⁻¹ belongs to the Co–O bond. As compared to ZnWO₄, the FTIR of mixed metal Zn_0.5Co_0.5WO₄ showed the difference in peak shapes and intensities due to the partial replacement of Zn²⁺ ions by Co²⁺ ions [25].

The formation of ZnWO₄, CoWO₄, and Zn_0.5Co_0.5WO₄ was confirmed by the XRD analysis. Figure 2 shows the XRD pattern of the ZnWO₄, CoWO_4, and Zn_0.5Co_0.5WO₄ prepared by the hydrothermal method at 180℃ for 24 h. The XRD spectra of ZnWO₄ shows characteristic peaks at 2θ value of 15.58°, 19.04°, 23.94°, 24.66°, 30.59°, 31.38°, 36.54°, 38.47°, 41.41°, 44.40°, 46.55°, 50.39 51.84°, 53.76°, 54.11°, 61.85°, 64.90°, and 68.34°, which belong to miller indices (010), (100), (011), (110), (111), (020), (021), (200), (121), (112), (211), (220) (130), (221), (202), (113), (132), and (041), respectively (JCPDs card no. 96-210-1675). Then, the XRD spectra of CoWO₄ shows characteristic peaks at 2θ value of 19.02°, 23.88°, 27.72°, 31.50°, 36.44°, 38.69°, 48.52°, 52.17°, 61.89°, and 65.14°, which belong to miller indices (100), (011), (110), (020), (002), (200), (022), (031), (310), and (040), respectively (JCPDs card no. 96-591-0317). Finally, the XRD pattern of Zn_0.5Co_0.5WO₄ NPs shows peaks ascribed to the ZnWO₄ at 15.54° (010), 36.41° (021), 41.35° (211), 51.95° (220), 53.94° (130), and 68.56° (041), and peaks ascribed to the CoWO₄ at 48.81° (022), 61.81° (310), and 64.96° (040), respectively, which suggests that Co has been successfully doped in the solid matrix of ZnWO₄. Moreover, the peak intensities of Zn_0.5Co_0.5WO₄ NPs are greater than pristine ZnWO₄ suggesting the increase in crystallinity of the complex upon being mixed with Co²⁺, which has been proved by data given in

Table 1. XRD parameter of ZnWO₄, CoWO_4, and Zn0.5Co0.5WO₄.

Crystallinity (%)	Dislocation Density (δ) Lines (1014 × m2)	Crystallite Size (nm) at 2θ value	Interlayer Spacing (nm) at 2θ	FWHM (βhkl)	2θ	Component
45.24	9.78	31.98	0.203	0.45	30.72	ZnWO4
48.46	9.65	32.19	0.195	0.81	30.05	CoWO4
42.18	17.02	24.24	0.198	0.76	30.33	Zn0.5Co0. 5WO4

. There are several extra peaks observed in the XRD spectra of Zn_0.5Co_0.5WO₄ which do not belong to either ZnWO₄ or CoWO₄. These peaks are due to the formation of Na₂W₂O₇ and Na₂W₄O₁₃ phases with the Zn_0.5Co_0.5WO₄ phase [26,27].

Further information about crystallite size and dislocation density; Scherrer’s equation was taken into consideration [28].

$$\mathrm{D}=\frac{0.9\mathsf{\lambda }}{\mathsf{\beta }\cos \mathsf{\theta }}$$

(1)

$$\mathrm{Dislocation} \mathrm{Density}(\mathsf{\delta })=\frac{1}{{\mathrm{D}}^2}$$

(2)

$$\mathrm{Interlayer} \mathrm{Spacing}({\mathrm{d}}_{111})=\frac{\mathrm{n}\lambda }{2\mathrm{Sin}\mathsf{\theta }}$$

(3)

$$\%\mathrm{Crystallinity}=\frac{\mathrm{Area} \mathrm{under} \mathrm{the} \mathrm{crystalline} \mathrm{peaks}}{\mathrm{Total} \mathrm{area}}\times 100$$

(4)

where D is the crystallite size, $\lambda$ is the characteristic wavelength of the X-ray, β represents the angular width in radian at an intensity equal to half of its maximum (HWMI) of the peak, and $\theta$ is the diffraction angle. Using Equation (1), the average particle size of ZnWO₄, CoWO_4, and Zn_0.5Co_0.5WO₄ was 31.98, 32.19, and 24.24 nm suggesting a contraction in particle size upon Co inclusion in the ZnWO₄ solid lattice.

The surface morphology of the ZnWO₄, CoWO_4, and Zn_0.5Co_0.5WO₄ were studied using SEM analysis given in (Figure 3). ZnWO₄ reveals an aggregation of irregularly shaped crystals with plenty of nanorods (Figure 3a). CoWO₄ exhibit uniform nanoparticles of various size (Figure 3b), while Zn_0.5Co_0.5WO₄ revealed an assembly of fine nanoparticles with a porous surface (Figure 3c). The elemental composition of the synthesized Zn_0.5Co_0.5WO₄ NPs was observed by EDX analysis and the results showed the presence of the element (weight%) as O (28.52%), Zn (4.55%), Co (4.22%), and W (62.81%), which confirms the successful deposition of Co²⁺ in the ZnWO₄ lattice (Figure 3d). The high atomic percentage of W in the EDX spectra is due to the presence of impurity in the form of Na₂W₂O₇ and Na₂W₄O₁₃ phases with the Zn_0.5Co_0.5WO₄, which resulted in an extra amount of W in the EDX spectra [27].

Furthermore, to figure out the size of the ZnWO₄, CoWO_4, and Zn_0.5Co_0.5WO₄, TEM analysis was performed (Figure 4a–c). ZnWO₄ (Figure 4a) is composed of particles with a rod-like shape; CoWO₄ (Figure 4b) presented a morphologically spherical shaped particle. The synthesized material Zn_0.5Co_0.5WO₄ (Figure 4c) exhibited particles mixed of nanorods with some spherical particles with size in a range from 5 to 60 nm, and the Gaussian distribution of the average size was found to be 31 nm given in Figure 4d. Figure 4e represents the TEM image of Zn_0.5Co_0.5WO₄, which suggested the shape of particles to be distorted monoclinic due to the doping of Co into the ZnWO₄ lattice. Selected area electron diffraction (SAED) analysis (Figure 4f) was further considered to simulate the TEM data with XRD to assess the purity and shape of the nanoparticles. It contains patterns originating from diffraction points in the TEM screen belonging to randomly oriented nanoparticles. The occurrence of diffraction patterns in Figure 4f suggested that the material is crystalline and pure. The hkl values belonging to yellow fringes simulate the XRD hkl planes of Zn_0.5Co_0.5WO₄ suggesting the shape of the nanoparticles to be monoclinic.

The optical properties of the synthesized NPs and their energy bandgap were estimated by using UV–vis spectroscopy in the range of 200–700 nm and the results are given in Figure 5. The absorption spectra of all the synthesized NPs ZnWO₄, CoWO_4, and Zn_0.5Co_0.5WO₄ exhibited two peaks, one around 260–312 nm and another broad peak around 530–650 nm, suggesting the UV–vis activeness of the NPs. The inset in Figure 5 shows Tauc’s plot, which is used to calculate band gap energy (Eg) of the prepared samples by using the Equation (5) [29];

$$\left(\alpha h\nu \right)=A{(h\nu -E_g)}^n$$

(5)

where α is the absorption coefficient, A is constant, h is plank constant, ν is the frequency of radiations, and n is a constant of transition variations, i.e., n = 1/2 for direct transitions and n = 2 for the indirect transitions. Tauc’s plot indicated that the synthesized ZnWO₄, CoWO_4, and Zn_0.5Co_0.5WO₄ were found to have an energy bandgap value of 3.49 eV, 3.42 eV, and 3.33 eV, respectively. Results showed that the deposition of Co²⁺ in the ZnWO₄ solid lattice results in the increase in absorption edge by decreasing the energy bandgap, which inhibits the rate of electron–hole pair recombination, and thus improves the photocatalytic efficiency of the synthesized material [30]. The doping of Co²⁺ into ZnWO₄ leads to the induction of deformations in the pure monoclinic lattice of ZnWO₄, which creates mobile oxygen vacancies in the cluster and makes an improvement in the catalytic [31,32].

Figure 6 consists of the photoluminescence emission spectra of ZnWO₄, CoWO_4, and Zn_0.5Co_0.5WO₄ recorded at a 594 nm excitation wavelength. It can be seen that the PL spectra spans in the range of 545 to 575 nm with a prominent emission peak at 558 nm. The information obtained from the PL spectra suggested that the PL intensity of synthesized Zn_0.5Co_0.5WO₄ was found to be lower than that of ZnWO₄ and CoWO₄. The inclusion of Co in the wolframite monoclinic structure of ZnWO₄ results in emissions due to the radiative transition between W and O in the WO₆⁶⁻ molecular complex, which effectively slows down the electron–hole [33,34,35].

2.2. Photocatalytic Experiments and Optimization of Reaction Parameters

2.2.1. Effect of Variable XO Concentration

The concentration of the dye solution is an important parameter for elucidating the photocatalytic efficiency of the material. Photocatalytic experiments were conducted by taking 10 mg of NPs as a catalyst with variable (20–100 ppm) XO concentration for 120 min of visible light irradiation. The results obtained are given in Figure 7a–d, which suggest that Zn_0.5Co_0.5WO₄ possesses better photocatalytic efficiency (97.17%) as compared to CoWO₄ (91.81%), followed by ZnWO₄ (89.51%) for 60 ppm of XO, but as the concentration of XO increases beyond 60 ppm, the percentage degradation decreases (Figure 7d). The causes could include: (1) XO may cover more ZnWO₄, CoWO₄, and Zn_0.5Co_0.5WO₄ active sites, which inhibits the production of oxidants (^•OH or ^•O₂⁻ radicals) and lowers the efficiency of degradation: and (2) higher XO concentration absorbs a greater number of photons, and as a result, there are not enough photons to activate the surfaces of ZnWO₄, CoWO₄, and Zn_0.5Co_0.5WO₄ nanoparticles, which slows down XO’s degradation at higher concentrations [36]. So, 60 ppm XO concentration was chosen as the optimum concentration for further photocatalytic experiments.

2.2.2. Effect of Variable Catalyst Dose

The physicochemical characteristics of nanomaterials, such as surface area, phase structure, particle size, and interface charge, make them able to complete adsorption action [27]. To efficiently degrade XO through catalysis using the catalyst, the photocatalytic experiment was performed by varying catalyst doses, such as 5, 10, 15, and 20 mg with 10 mL of 10 ppm XO. The results given in Figure 8a–d suggested that the photocatalytic efficiency increased from 5 mg catalyst dose to 10 mg and starts declining with further increase. All the synthesized NPs exhibit maximum photocatalytic efficiency at 10 mg catalyst dose as 97.89% for ZnWO₄, 98.10% for CoWO_4, and 98.77% for Zn_0.5Co_0.5WO_4. More active sites are produced on the catalyst’s surface at lower concentrations, which promotes the creation of ^•OH or ^•O₂⁻ radicals. After that, increasing the catalytic dose causes the particles to agglomerate and sediment, which increases the turbidity and opacity of the slurry. Hence, the generation of ^•OH or ^•O₂⁻ radicals automatically decrease [24]. Therefore, the catalyst dose of 10 mg was chosen to be the optimum dose for the degradation.

2.2.3. Effect of pH

Experiments were performed to evaluate the impact of pH on the photocatalytic efficiencies of ZnWO₄, CoWO₄, and Zn_0.5Co_0.5WO₄. A total of 10 mg of the catalyst was dispersed in 20 mL of 60 ppm XO solution with a variable pH range from 1–7, and irradiated for 120 min under the visible light source. The obtained results after the experiments are given in Figure 9, which suggests that the photocatalytic efficiency of all the catalysts increased until pH 3, giving a maximum value of 93.35% for ZnWO₄, 96.21% for CoWO₄, and 97.97% for Zn_0.5Co_0.5WO₄. Further, an increase in pH value beyond 3 results in a decrease in photocatalytic efficiency with a continuous fall until pH 7. So, pH 3 was taken as the optimum pH value for the photocatalytic experiments. The trend can be explained by the fact that as XO is an anionic dye, it can easily bind with the positive surface of the catalyst at low pH through electrostatic interactions, and thereby, in the presence of light, it gets degraded by ^•OH or ^•O₂⁻ radicals [7]. As the pH values increase, the surface becomes negative, which will exert a repulsive force on the anionic XO molecule, and hence, the photocatalytic efficiency decreases.

2.3. Kinetics of Photodegradation

A variety of experiments were carried out by taking 10 mL of 20 ppm XO with 10 mg of catalyst at varying irradiation time from 0 to 120 min under visible solar radiation, and the data obtained was adjusted to the Langmuir–Hinshelwood (L–H) pseudo-first-order kinetic model for the quantitative analysis of XO degradation [37,38].

$$-\ln \left(\frac{{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{C}}_{\mathrm{o}}}\right)={\mathrm{k}}_{\mathrm{app}}\times \mathrm{t}$$

(6)

where C_o represents the initial concentration and C_t represents the final concentration of XO at time t. The results are given in Figure 10a–d, in which it was observed that with an increase in irradiation time, the photocatalytic efficiency increases. This may be due to an increase in irradiation time, the surface of the catalyst becomes more activated by absorbing the solar radiation and generates a greater number of ROS ^•OH or ^•O₂⁻ radicals which interact with dye molecules and degrade them [39]. The slope of the pseudo-first-order reaction (Figure 10d) can be utilized to determine the value of k_app from the plot of –ln (C_e/C₀) vs. time (min), which shows a linear response. The obtained values of k_app given in

Table 2. The kinetic parameters for the degradation of XO dye (20ppm) using 10 mg ZnWO₄, CoWO_4, and Zn_0.5Co_0.5WO₄ with variable irradiation time (5–120 min).

Material	k1 (min−1)	R2	t1/2 (min)
ZnWO4	0.018	0.99	38.50
CoWO4	0.021	0.99	33.00
Zn0.5Co0.5WO4	0.025	0.99	27.72

as 0.018 min⁻¹ for ZnWO₄, 0.021 min⁻¹ for CoWO_4, and 0.025 min⁻¹ for Zn_0.5Co_0.5WO₄ proved the high efficiency of Zn_0.5Co_0.5WO₄ as compared to pristine ZnWO₄ and CoWO₄. The obtained value of R² = 0.99 for all the nanoparticles suggested that the photocatalytic data can be realistically simulated by the L–H model. The outcomes demonstrate that the deposition of Co²⁺ in ZnWO₄ of both catalysts considerably improves the degrading ability compared to each catalyst individually. Finally, Zn_0.5Co_0.5WO₄was the novel composite having distinctive characteristics when compared to other composites.

2.4. Scavenging Study and Mechanism of Photodegradation

It was suggested that a variety of reactive species, including the hydroxyl radical (^•OH), superoxide (^•O₂⁻), valence band hole (h⁺), and electron (e⁻) play important roles in the photodegradation of organic pollutants, and their effectiveness depends upon the band structure and chemical nature of the photocatalysts [40]. So, scavenger studies were performed using a 1.3 mM solution of Benzoic acid (BA), Potassium dichromate (K₂Cr₂O₇), (AA), ethylenediaminetetraacetic acid (EDTA), and benzoquinone (BQ) by taking 10 mL of 20ppm XO with 10 mg at pH 3 for an irradiation time of 120 min. We know that BA is a scavenger for ^•OH, EDTA for h⁺, K₂Cr₂O₇ for e⁻, and BQ for ^•O₂⁻ [41,42]. Figure 11 displays the results, which show that without the use of a scavenger, the degrading efficiency of Zn_0.5Co_0.5WO₄ towards XO achieved 98.27%. After adding BA and EDTA, there appears no appreciable change in photocatalytic efficiency, suggesting no primary role of ^•OH and h⁺ on the degradation of XO at the optimized conditions. On the contrary, when BQ and K₂Cr₂O₇ are added, an appreciable decrease in photocatalytic efficiency happens, demonstrating that photogenerated electrons (e⁻) and ^•O₂⁻ radicals are predominant species in this photodegradation reaction scheme. The results showed that the degradation of XO was primarily controlled by e⁻ and ^•O₂⁻ radicals (majorly).

The plausible mechanism of degradation of XO by ^•O₂⁻ radicals is given in Figure 12 [43,44].

2.5. Comparison with the Literature

The outcomes of this study were compared with the information available in the literature, and a comparison in

Table 3. Comparison of the literature information with the present study.

Catalysts	Irradiation Time (min)	Light Source	Dye Used	% Degradation	References
BiOBr/ZnWO4	170	UV-A light	RhB	99.40	[45]
La: ZnWO4	90	UV light	MB	97.00	[46]
CoWO4/ZnWO4 p-n heterojunction	40	Xe lamp	RhB	93%	[47]
ZnWO4/WO3 heterojunction	120	Visible light	MB	83.60	[48]
ZnWO4-(ZnO)	120	UV-illumination	MO	99.00	[49]
CDs-ZnWO4	150	UV-illumination	NGB	93.00	[50]
Zn0.5Co0.5WO4	120	Visible Solar Light	XO	98.77	Present Study

was made to investigate the upgraded scientific information added by the present study. It was found that there is no research where tungstate base nanomaterial is used for the photocatalytic degradation of xylenol orange.

2.6. Reusability Test

The reusability test was performed for the synthesized Zn_0.5Co_0.5WO₄ up to five cycles to assess the stability of the material towards XO degradation, and the results are given in Figure 13a. It can be seen that there is no appreciable change in photocatalytic efficiency up to the first two cycles with greater than 98% of XO degradation. Furthermore, from cycle 3 to cycle 5, there appears some appreciable decrease in photocatalytic efficiency attaining 85% of XO degradation in cycle 5. The overall results suggest that the synthesized Zn_0.5Co_0.5WO₄ have good stability toward the XO degradation under optimized reaction conditions. To support the stability of the material, XRD analysis of the synthesized material before and after the photocatalytic reaction was taken into consideration, which is given in Figure 13b. The XRD spectra of Zn_0.5Co_0.5WO₄ after five consecutive cycles of reuse represented almost similar hkl planes, but with reduced intensity as compared to pristine Zn_0.5Co_0.5WO₄ before the photocatalytic reaction. Only one change was observed, that after the photocatalytic reaction the (-111) plane diminished, which may be due to the absorption of water.

3. Materials and Methods

3.1. Chemicals and Reagents

Sodium tungstate dihydrate (Na₂WO₄·2H₂O, ACS grade, 99%) was purchased from Merck, Rahway, NJ, USA. Cobalt (II) nitrate hexahydrate (Co (NO₃)₂.6H₂O ACS grade 98%), Zinc nitrate hexahydrate (Zn (NO₃)₂.6H₂O, reagent grade 98%), were purchased from Sigma Aldrich, St. Louis, Missouri USA. Xylenol orange (C₃₁H₂₈N₂Na₄O₁₃S, GR) and Ammonia solution (25%) were acquired from Otto Chemie, Mumbai, India. All the solutions were prepared in deionized (DI) water and used throughout all the photocatalytic experiments. Figure 14 represents the molecular structure of xylenol orange.

3.2. Synthesis of ZnWO₄, CoWO₄, and Zn_0.5Co_0.5WO₄ Nanoparticles

The synthesis of pristine metal tungstate nanomaterials (ZnWO₄, CoWO₄ NPs) and mixed metal tungstate nanocomposite (Zn_0.5Co_0.5WO₄) was done according to the hydrothermal method reported elsewhere [51]. Three Erlenmeyer flasks of 50 mL capacity were equipped with 25 mL of 3mmol of sodium tungstate dihydrate (Na₂WO₄·2H₂O), Zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O; Flask 1), Cobalt(II)nitrate hexahydrate (Co (NO₃)₂·6H₂O; Flask 2), and 1:1 ratio, i.e., 1.5 mmol each, of Zn(NO₃)₂·6H₂O and Co (NO₃)₂·6H₂O (Flask 3). All the flasks were placed on a magnetic stirring plate for 30 min to completely dissolve the salts and attain homogeneity. Then, the mixtures (F1, F2, and F3) were transferred into a Teflon-lined steel autoclave and heated in a convection oven at 180 ℃ for 24 h. The resulting precipitates of ZnWO₄, CoWO_4, and Zn_0.5Co_0.5WO₄ were centrifuged at 8000 rpm for 5 min and purified several times with DI water and finally with ethanol (2 times). After that, the materials were dried at 100 °C for 5 h and calcined at 600 °C for 4 h, then stored in a desiccator using a polystyrene Petri dish for further characterization and photocatalytic experiments.

3.3. Characterization Techniques

The ZnWO₄, CoWO_4, and Zn_0.5Co_0.5WO₄ were characterized using various techniques. The change in crystallite size and interplanar distance on Co deposition in the crystal lattice of ZnWO₄ was analyzed by an X-ray powder diffractometer (XRD, Rigaku Ultima IV, Tokyo, Japan) at Cu Kα wavelength of 1.5418 Å as a source of radiation. The change of WO₆⁶⁻ octahedron vibrations, Zn–O, and formation of Co–O bonding was assessed through Fourier transform infrared (FTIR) spectroscopy using a Perkin Elmer Spectrum 2 ATR spectrometer (Perkin Elmer, Waltham, MA, USA). The optical properties, energy bandgap of the synthesized NPs, and remaining concentration of XO in the effluent after the photocatalytic experiment was calculated by using a Shimadzu UV–1900 double-beam spectrophotometer (Kyoto, Japan). The surface morphology of the synthesized NPs was observed by scanning electron microscopy (SEM; JEOL GSM 6510LV, Tokyo, Japan) associated with an X-ray energy dispersive detector (EDX) to investigate the elemental composition and mapping. The morphological change in shape and size of the nanocrystalline solid upon Co deposition in ZnWO₄ was investigated using a transmission electron microscope (TEM; JEM 2100, Tokyo, Japan).

3.4. Photocatalysis Experiment

The photocatalytic efficiency of ZnWO₄, CoWO₄, and Zn_0.5Co_0.5WO₄ was analyzed towards the degradation of xylenol orange under visible light irradiation in a photocatalytic reactor. An aliquot of 10 mL of 50 ppm XO was taken in a 20 mL 1.8 cm glass tube with a magnetic bar of 10 mg of synthesized NPs in a photocatalytic reactor using Xe lamp (420 W cm ⁻¹) for 5–120 min of irradiation. The aliquots are taken out of the photoreactor at a regular time interval, centrifuged to separate the nanocatalyst, and then placed in a UV–vis spectrophotometer at λ_max = 580 nm to check the photocatalytic efficiency of the synthesized NPs, which is given by Equation (7);

$$\mathrm{XO} \mathrm{Degradation}(\%)=\left(\frac{{\mathrm{C}}_{\mathrm{0}}-{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{C}}_{\mathrm{0}}}\right)\times 100$$

(7)

where C₀ and Ce represents the initial concentration of an organic pollutant at time t = 0 and at any time t, respectively. The experiments were repeated by changing the parameters, for instance, organic pollutant concentration (ppm), irradiation time (min), catalytic load (mg), and pH of the media to optimize the reaction condition and predict the mechanism of the degradation.

4. Conclusions

The present study reveals the hydrothermal synthesis of pristine ZnWO₄, CoWO_4, and mixed metal Zn_0.5Co_0.5WO₄ nanoparticles (NPs) in a Teflon-lined autoclave at 180 °C for 24 h. The structural, morphological, elemental, and optical analyses supported the formation of Zn_0.5Co_0.5WO₄. The XRD analysis revealed a contraction in crystallite size from 32 nm to 24 nm with a decrease in crystallinity from 45% to 42% in ZnWO₄ to Zn_0.5Co_0.5WO₄ suggesting the successful deposition of Co²⁺ in the ZnWO₄ lattice. The TEM analysis suggested the nanorod-like shape of the Zn_0.5Co_0.5WO₄ NPs with an average particle size of 31.74 nm. The optical studies revealed the UV–vis light activeness of materials with the energy band gap of ZnWO₄ (3.49 eV), CoWO₄ (3.42 eV), and mixed metal Zn_0.5Co_0.5WO₄ (3.33 eV). The material was explored as a catalyst for the photodegradation of XO dye under visible solar radiation and the results suggested the order of photocatalytic efficiency as Zn_0.5Co_0.5WO₄ (98.77%) > CoWO₄ (98.10%) > ZnWO₄ (97.89%) utilizing 10 mg of catalyst for 60 ppm XO for 120 min of solar irradiation. The photocatalytic data were well explained by L–H pseudo–first–order kinetics with R² = 0.99, and apparent rate constant (k_app) as 0.018 min⁻¹ for ZnWO₄, 0.021 min⁻¹ for CoWO₄, and 0.025 min⁻¹ for Zn_0.5Co_0.5WO₄. The scavenger experiments disclosed that superoxide anion radicals (•O₂⁻) and photogenerated electrons (e⁻) played important roles in the degradation of XO. The outcomes of this study revealed that the eco-friendly and economical hydrothermal synthesized material (Zn0.5Co0.5WO4) can be used more efficiently for the treatment of XO-contaminated water in industries without the generation of any secondary pollution.