In Situ Hydrothermal Synthesis of Ni_1−xMn_xWO₄ Nanoheterostructure for Enhanced Photodegradation of Methyl Orange

Abstract

The monoclinic nanocrystalline Ni_1−xMn_xWO₄ heterostructure has been successfully synthesized by the hydrothermal technique for achieving better sensitive and photocatalytic performances. Different characterization techniques such as X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), ultraviolet-visible (UV–Vis), and photoluminescence (PL) spectroscopy have been employed to investigate their structural, microstructural, and optical properties. Mn-ion incorporation in the NiWO₄ lattice reduces the particle size of the sample compared with the pure undoped NiWO₄ sample, which has been confirmed from the transmission electron microscope image. The Tauc plot of the Ni_1−xMn_xWO₄ sample exhibits a significant decrease in bandgap energy compared with the pure undoped NiWO₄ sample due to the quantum confinement effect. Finally, the material was explored as a photocatalyst for the degradation of methyl orange (MO) dye from wastewater under visible light irradiation. Various reaction parameters such as pH, catalyst dose, reaction time, and kinetics of the photodegradation were studied using the batch method. The results showed that the Ni_1−xMn_xWO₄ is highly efficient (94.51%) compared with undoped NiWO₄ (65.45%). The rate of photodegradation by Ni_1–xMn_xWO₄ (0.067) was found to be 1.06 times higher than the undoped NiWO₄ (0.062).

1. Introduction

The advancement in industrial productions, such as paper, dyeing, and textiles, to boost up the economy has resulted in extreme use of heavy metals, azo dyes, and other organic compounds which are discarded directly into surface and ground water [1,2]. The toxic organic and inorganic compounds that have a non-biodegradable and persistent nature pose a threat to human health and other creatures [3]. These azo dyes are very harmful in their own context, even at ng/L concentrations, and may turn into more harmful products by reacting with other chemical species in the water streams or interacting with UV light [4]. Among various azo dyes, methyl orange (MO) has been categorized as an omnipresent water pollutant which can cause diarrhea, vomiting, and skin allergies; moreover, exposure to higher concentrations can cause death [5,6]. MO and its transformation products in water can reflect certain wavelengths of sunlight and thus hinder the photosynthesis reactions of aquatic flora [7]. This process leads to a decrease in the amount of dissolved oxygen which is mandatory for the sustainability of aquatic life [8]. Thus, it is necessary for researchers to develop efficient methods to remove MO from wastewater completely without producing any secondary pollutants. Various methods currently exist, such as chlorination, ozonation, adsorption, sedimentation, and coagulation [9,10,11]. However, these methods transfer MO from one medium to another medium and produce secondary pollution. So, a green procedure, i.e., photocatalytic degradation, was taken into consideration, which constitutes a more effective and energy-efficient method as compared with its counterparts in the scope of advanced oxidation processes (AOPs). The process forms highly reactive radical species generated by the irradiation of light on a catalyst surface [12]. When interacting with pollutant molecules, these highly active radicals decompose them into small non-toxic molecules or even mineralize them into CO₂ and H₂O [13]. However, the conversion efficiency of this method depends upon the chemical and optical behavior of the developed catalyst material.

In recent years, vast research has been carried out to explore the novel properties of the metal oxide semiconductor nanomaterials, including TiO₂ [13], Cu₂O [14], ZnO [15], Fe₂O₃ [16], etc., and mixed-metal oxide nanomaterials, such as BiFeO₃ [17], BiVO₄ [18], Bi₂MoO₆ [19], CuWO₄ [20], etc., as a photocatalyst for the degradation of organic pollutants. Among these, nanocrystalline metal tungstates of the general formula MWO₄ (M = Zn, Co, Ni, Mn, etc.) have attracted the attention of scientists as an efficient semiconductor photocatalyst because of their excellent optical properties and energy bandgap [21,22]. Among various metal tungstates, NiWO₄ has been recognized as one of the important semiconductor photocatalysts, with an energy bandgap ≂3.5 eV [23]. NiWO₄ can be easily synthesized by the hydrothermal method, decomposition approach, and electrochemical method, but the hydrothermal route has attracted more interest to synthesize a vast number of nanomaterials due to the formation of a large crystal phase [24,25]. NiWO₄ exhibits a monoclinic wolframite-type structure with oxygen deposited around the tungsten-associating [WO₆] type octahedron [26]. Photocatalytic degradation involves the formation of electron-hole pairs in the catalyst under radiation, which react with surrounding water and surface-adsorbed oxygen to generate ^•OH and ^•O₂⁻ radicals to degrade MO [1,5]. However, the fast recombination of these electron-hole pairs hinders the photocatalytic degradation of pollutants and poses a drawback on the efficiency of the material [27]. To overcome this drawback, various steps have been applied previously in the literature, such as composite forming [28], morphological improvement [29], doping with metal elements [30], and heterojunctions [31]. Therefore, one of the effective strategies, namely heterostructure formation, was applied in this study, which creates a methodical partition of photogenerated electron-hole pairs through band alignment to improve the photocatalytic efficiency [32]. In the present study, manganese (Mn) was doped in the crystal lattice of NiWO₄ to improve its photocatalytic activity through changes in morphology, crystal lattice, and optical properties. One of the most important aspects of doping is to substitute the metal ion of the host material through another metal ion of smaller radius, thus creating lattice defects and oxygen vacancy, which hinders the rate of electron-hole pair recombination, reduces the energy bandgap, and thus improves the photocatalytic activity of the material towards the pollutant [26,33,34]. From the literature, the ionic radii of Ni²⁺ and Mn²⁺ are 0.069 nm and 0.065 nm; this suggests that Mn²⁺ can easily substitute some of the Ni²⁺ in the NiWO₄ lattice and thus form a Ni_1−xMn_xWO₄ nanocomposite-type heterostructure, where x is the moles of Mn added [35]. The synthesized material was explored as a photocatalyst for the degradation of MO under a visible light source. The efficiency of the synthesized material, Ni_1−xMn_xWO₄ NC, was optimized by varying different reaction parameters, such as irradiation time, pH of the reaction medium, and catalyst dose. Finally, the kinetics of degradation was outlined depending on the photocatalytic results.

2. Results and Discussion

2.1. Material Characterization

2.1.1. Crystal Structure Studies

The XRD patterns of synthesized NiWO₄ and Ni_1−xMn_xWO₄ are given in Figure 1. The pristine NiWO₄ exhibited the characteristic peaks at Miller indices (100), (011), (110), (111), (002), (200), (102), (112), (211), (022), (130), (202), (113), (132), and (041), which are indexed for the monoclinic wolframite structure of NiWO₄ with standard JCPDS no. 072–0480. The XRD spectra of synthesized Ni_1−xMn_xWO₄ also exhibited maximum peaks from NiWO₄ except for the appearance of some new Miller indices values, namely (121), (030), (220), and (023), which were found to have good agreement with the lattice structure of MnWO₄ with standard JCPDS no. 72-0478. The XRD results suggested that Mn²⁺ has been successfully incorporated in the NiWO₄ lattice; moreover, there is an increase in the intensity of peaks after Mn incorporation, indicating an increase in the crystallinity of material. The crystallite size was determined using the Debye–Scherrer equation given as [36]:

$${\mathrm{D}}_{\mathrm{c}}=\frac{\mathrm{K}\mathsf{\lambda }}{\mathsf{\beta }\times \mathrm{Cos}\mathsf{\theta }}$$

(1)

where λ is the wavelength of X-ray source, K is a shape factor and is usually ~0.9, θ is the corresponding angle, and β is the breadth of the observed diffraction line at its half intensity maximum. The results obtained using Equation (1) disclosed that the particle size of NiWO₄ was 32 ± 1.05 nm, while for Ni_1−xMn_xWO₄, it was 26 ± 0.65 nm. The outcomes indicated a contraction in the crystallite size in NiWO₄ upon Mn²⁺ incorporation, suggesting an improvement in the optical activity of the material as well.

2.1.2. Functional Group Studies

The functional group study of the materials was done using FTIR spectroscopy and the results are given in Figure 2. From the FTIR spectra, NiWO₄ shows characteristic bands at 3414, 1630 cm⁻¹ belonging to stretching and bending vibration bands of –OH groups (surface adsorbed water), 879 and 830 cm⁻¹ antisymmetric stretching vibration bands of W–O bonds, 706, 622 cm⁻¹ stretching and bending vibrations of W–O bonds in [WO₆] octahedron, and 528, 434 cm⁻¹ stretching vibrations of Ni–O bonds from the NiO₆ octahedron [23,27]. The FTIR spectra of the as-synthesized Ni_1−xMn_xWO₄ NC exhibited the same bands with shifted values, which is due to the orbital hybridization and change of chemical environment of NiWO₄ by Mn. The band at 706 cm^–1 vanished completely, suggesting the acquirement of the portion associated with the [WO₆] octahedron by Mn²⁺ [37].

2.1.3. Optical Studies

The optical properties of the synthesized material were investigated by observing the UV-Vis spectra given in Figure 3. The UV-Vis spectra of NiWO₄ exhibited one peak of very low intensity at 278 nm, suggesting that the material is only UV-light active and the corresponding transition is due to a charge transfer process between Ni²⁺ (d–d) and octahedron [WO₆] clusters [38]. The UV spectra of Ni_1−xMn_xWO₄ NC exhibited three peaks at 286 nm, 355 nm, and 685 nm, suggesting the UV and visible light activity of the synthesized material. So, the incorporation of Mn in the NiWO₄ lattice enlarged its light absorbing capacity; moreover, the peaks that appeared are due to Ni²⁺ (d–d) to octahedron [WO₆] clusters CT and secondly Ni²⁺ (d–d) to octahedron [WO₆] clusters to Mn²⁺ CT, which increased its absorption intensity to visible light from the UV region [39]. The energy bandgap (E_g) value of the synthesized material can be calculated using Tauc’s equation given as [40]:

$$\mathsf{\alpha }\mathrm{h}\mathsf{\upsilon }={\mathrm{B}(\mathrm{h}\mathsf{\upsilon }-{\mathrm{E}}_{\mathrm{g}})}^{\mathrm{n}}$$

(2)

where B is a constant (dependent on the nature of the material), α is the absorption coefficient of the material (calculated from the absorption spectra), ν is the frequency of the radiation, h is the Plank’s constant, and E_g is the energy of the band gap in eV. The value of n (coefficient of transition) decides the type of energy bandgap, i.e., n = 2 value corresponds to allowed indirect energy bandgap, while n = 1/2 for allowed direct energy bandgap. Using Equation (2), the value of energy band gap (E_g) was calculated as 3.35 eV for NiWO₄ and 2.13 eV for Ni_1−xMn_xWO₄ NC. The contraction in Eg value upon incorporation of Mn in the NiWO₄ lattice resulted in improved optical and photocatalytic activities.

2.1.4. SEM-EDX-Mapping Analysis

The surface morphology, elemental composition, and their distribution in the crystal lattice were assessed by SEM-EDX analysis. Figure 4a represents the SEM image of pristine NiWO₄, which shows an aggregation of tiny interconnected particles, whereas the SEM image in Figure 4b represents a porous morphology with interconnected tiny particles. The EDX analysis given in Figure 4c,d confirms the composition of the synthesized material by weight % as O K (20.51), Ni K (19.74), and W M (59.75) for NiWO₄, and O K (22.97), Ni K (9.36), Mn K (6.85), and W M (60.82) for Ni_1−xMn_xWO₄ NC. The EDX results suggested a ratio of 1.4:1 of Ni:Mn in the synthesized material. The elemental mapping given in Figure 4e taken from the selected marked area of Ni_1–xMn_xWO₄ NC shows a uniform distribution of O, Ni, Mn, and W in the material.

For further insight into the morphology, including crystallite shape and size, TEM-SAED analysis was used. Figure 5a,b represents the TEM image of Ni_1−xMn_xWO₄ NC, which represents the monodispersed hexagonal crystals with mitigated morphology. The Gaussian particle size distribution given in Figure 5c revealed an average crystallite size of 26.84 nm, which is found to be in good agreement with the crystallite size calculated using the Debye–Scherer equation (26 ± 0.65 nm). Figure 5d consists of the selected area diffraction pattern (SAED) of Ni_1−xMn_xWO₄ NC, with marked yellow rings belonging to the Miller indices (110), (112), and (104) indexed in the XRD pattern of Ni_1−xMn_xWO₄ NC.

2.2. Photocatalytic Applications

2.2.1. Effect of pH

The pH of the reaction medium plays a vital role in controlling the rate of photodegradation of the organic pollutant by varying the charge density on the surface of the catalyst. Photocatalytic experiments were conducted by taking 20 mL of 50 ppm MO dye with 10 mg of the catalyst with varying pH values from 1–7 in cylindrical vessels inside the photocatalytic chamber associated with a tungsten lamp (150 mWcm⁻²) as the visible light source. The results obtained after completion of irradiation time are given in Figure 6a–c in which it was observed that with an increase in pH value from 1–5, the rate of degradation increases, achieving an efficiency of 97.16% for NiWO₄ and 98.32% for Ni_1−xMn_xWO₄ NC; however, further increases in the pH value results in a decrease in the rate of degradation. This trend can be explained based on point of zero charge (pH_pzc) value, which is found to be 5.8 for NiWO₄ and 6.2 for Ni_1−xMn_xWO₄ NC (Figure S2). So, at pH < pH_pzc, the surface of the catalyst is positive and is suitable to form adsorption–desorption equilibrium with a maximum number of anionic MO molecules and thus mineralize it in the presence of light utilizing photogenerated ^•OH or ^•O₂⁻ radicals. However, at pH > pH_pzc, the surface of the catalyst is negative, which will reflect the anionic MO molecules and thus a lesser number of dye molecules will be available for degradation; this is why a decrease in photocatalytic efficiency appeared at higher pH values [41]. The outcomes suggested that the Ni_1−xMn_xWO₄ NC possessed better photocatalytic efficiency as compared with pristine NiWO₄. Thus, Mn decoration in the NiWO₄ lattice, leading to the formation of the heterostructure, resulted in an enhancement of photocatalytic efficiency towards MO degradation.

2.2.2. Effect of Catalyst Dose

Experiments were conducted by taking 20 mL of 50 ppm MO with variable catalyst dose (5, 10, 15, 20, and 25 mg) for 70 min of irradiation and results obtained are given in Figure 7a–d. From Figure 7a,b, it was observed that from 5–10 mg of catalyst dose, the absorbance value decreases, indicating the increase in rate of degradation of MO; beyond 10 mg, the rate of degradation increases as supported by Figure 7c. From Figure 7d, the maximum degradation efficiency for NiWO₄ and Ni_1−xMn_xWO₄ NC was achieved as 93.58% and 97.46% at 10 mg of catalyst dose, respectively. Initially, with low concentration of catalyst, a greater number of surface-active sites are available to accommodate the MO molecules and thus there is a high rate of photodegradation. The decrease in the rate of photodegradation beyond a certain value of catalyst dose is due to an increase in the degree of aggregation of nanoparticles, which produces turbidity in the solution. The turbidity increases the opacity of the solution and impedes the light penetration intensity towards the catalyst surface. This phenomena decrease the rate of generation of ROS (^•OH or ^•O₂⁻ radicals) and thus cause a decrease in photocatalytic efficiency [42].

2.3. Kinetics of Photodegradation and Effect of Irradiation Time

Figure 8a,b represents the degradation of MO in the presence of pristine NiWO₄ and the as-synthesized Ni_1−xMn_xWO₄ NC with respect to variation in irradiation time. It can be seen from the results that with an increase in irradiation time from 5 min to 70 min, the intensity of the absorption maxima peak decreases continuously with any shift in wavelength. Thus, an increase in the rate of photodegradation of MO is observed with an increase in irradiation time until 70 min and the maximum photocatalytic efficiency was calculated as 98.79% for NiWO₄ and 99.06% for Ni_1−xMn_xWO₄ NC, respectively. The obtained photocatalytic data was adjusted to the Langmuir–Hinshelwood (L-H) pseudo first-order kinetic model to observe the rate of degradation quantitatively. The model is mathematically given as [12,43]:

$$-\ln \left(\frac{{\mathrm{C}}_{\mathrm{t}}}{{\mathrm{C}}_0}\right)={\mathrm{k}}_{\mathrm{app}}\times \mathrm{t}$$

(3)

where C₀ and C_t are the concentration of MO at the initial state (t = 0) and specific time intervals (t) and k_app is the apparent rate constant which can be obtained from the slope of linear graph of -ln (Ct/C0) vs. irradiation time (t). The obtained experimental data was applied to equation (3) and the results obtained are given in

Table 1. L-H first-order kinetic parameters for photocatalytic degradation of MO by NiWO₄ and Ni_1−xMn_xWO₄ NC.

Catalyst	kapp (min−1)	Error	t1/2 (min)	R2
NiWO4	0.063	6.16 × 10−4	11.00	0.99
Ni1−xMnxWO4 NC	0.067	1.31 × 10−3	10.34	0.99

and Figure 8c. It was found that the rate of degradation of MO was higher for Ni_1−xMn_xWO₄ (0.067 min⁻¹) as compared with pristine NiWO₄ (0.062 min⁻¹), which again quantitatively supports the idea of the enhancement of photocatalytic efficiency of material towards MO through Mn incorporation. The value of the correlation coefficient R² = 0.99 reveals that the simulation of the L-H model to the photocatalytic data has minimum error. The half lifetime of the reaction was calculated using t_1/2 = ln 2/k_app, and the values obtained are 11 min for NiWO₄ and 10.34 min for Ni_1−xMn_xWO₄ NC.

2.4. Detection of ROS

The scavenger’s test was performed to find out the primary ROS involved in the photodegradation of MO by Ni_1−xMn_xWO₄ NC under visible light source. In the present study, some of the scavengers used include EDTA for h⁺, benzoic acid (BA for ^•OH), acrylamide (AA for e⁻), and benzoquinone (BQ for ^•O₂⁻) and the results are given in Figure 9 [44]. The appearance of high absorption intensity for MO in the presence of BA suggests that the photocatalytic rate of MO is maximally inhibited by benzoic acid, which is responsible for the trapping of ^•OH radicals. So, ^•OH radicals are the primary ROS to degrade MO using Ni_1−xMn_xWO₄ NC.

Based on the trapping experiments, the plausible general mechanism of MO degradation is given by Equations (4–10) [1,4,8]:

$${\mathrm{Ni}}_{1-x}{\mathrm{Mn}}_x{\mathrm{WO}}_4+\mathrm{h}\nu \to {\mathrm{Ni}}_{1-x}{\mathrm{Mn}}_x{\mathrm{WO}}_4({\mathrm{h}}_{\mathrm{VB}}^{+}+{\mathrm{e}}_{\mathrm{CB}}^{-})$$

(4)

$${\mathrm{Ni}}_{1-x}{\mathrm{Mn}}_x{\mathrm{WO}}_4({\mathrm{h}}_{\mathrm{VB}}^{+})+{\mathrm{OH}}_{\mathrm{ad}}^{-}\to {}^{•}\mathrm{O}\mathrm{H}$$

(5)

$${\mathrm{Ni}}_{1-x}{\mathrm{Mn}}_x{\mathrm{WO}}_4({\mathrm{e}}_{\mathrm{CB}}^{-})+{\mathrm{O}}_2\to {}^{•}{\mathrm{O}}_2^{-}$$

(6)

$${}^{•}{\mathrm{O}}_2^{-}+{\mathrm{H}}^{+}\to {}^{•}\mathrm{H}{\mathrm{O}}_2$$

(7)

$$2{}^{•}\mathrm{H}{\mathrm{O}}_2\to {\mathrm{O}}_2+{\mathrm{H}}_2{\mathrm{O}}_2$$

(8)

$${\mathrm{H}}_2{\mathrm{O}}_2+{\mathrm{e}}_{\mathrm{CB}}^{-}\to {}^{•}\mathrm{O}\mathrm{H}+{\mathrm{OH}}^{-}$$

(9)

$${}^{•}\mathrm{O}\mathrm{H}+\mathrm{MO}\left(\mathrm{dye}\right)\to {\mathrm{CO}}_2+{\mathrm{H}}_2\mathrm{O}$$

(10)

Figure 10 represents the mechanism of photodegradation of MO by Ni_1−xMn_xWO₄ NC under visible light source. The irradiation of the photocatalyst initiates the promotion of electrons (e⁻) from the valence band (VB) to conduction band (CB) through the photoabsorption process, leaving lots of vacancies in VB, referred to as holes (h⁺), and photogenerated electrons (e^–) in CB Equation (4). These photogenerated electrons (e^–) in CB interact with surface-adsorbed O₂ molecules and oxidize to superoxide radicals (^•O₂⁻) Equation (6). The holes interact with hydroxyl anions (OH⁻) to produce hydroxy radicals (^•OH) Equation (5). The O₂ molecule reacts with H⁺ ions and produces H₂O₂, which further reacts with e^– (CB) generating super reactive ^•OH radical reaction (7–10); these are responsible for the degradation of MO under visible light source.

2.5. Reusability and TOC Test

To check the stability of the synthesized material towards treatment of MO in wastewater, a photocatalytic test was performed in repeated mode. In cycle 1, 10 mg of the Ni_1−xMn_xWO₄ NC photocatalyst was dispersed in 20 mL of 50 ppm MO solution under optimized reaction conditions. Then, after the completion of reaction, the photocatalyst was separated out by centrifugation and the supernatant was checked using a UV-Vis spectrophotometer. The photocatalyst was washed and dried and then again dispersed in the MO solution for cycle 2. In this way, the same material underwent 6 cycles of the photocatalytic test to check its stability and reusable efficiency under the given optimized conditions towards MO. The results obtained after the photocatalytic experiment was given in Figure 11a. The outcomes suggested that the synthesized Ni_1−xMn_xWO₄ NC material is highly stable to treat MO dye as only a very small change in photocatalytic efficiency from cycle 1 (98.79%) to cycle 6 (95.23%) was observed.

Similarly, to observe the mineralization process of MO by the photocatalyst, the total organic carbon (TOC) test was taken into consideration and the results are given in Figure 11b. It was observed from the graph that as the irradiation time increases, TOC value decreases continuously, and after 70 min of irradiation, the synthesized material Ni_1−xMn_xWO₄ NC exhibits 67.45% of TOC removal, which suggests that the synthesized material is a very effective catalyst to treat MO-contaminated water under visible light source.

2.6. Photocurrent Measurement

Furthermore, to investigate the photophysical behaviors of NiWO₄ and Ni_1−xMn_xWO₄ NC under visible light irradiation, photocurrent measurements were taken into consideration. The obtained results are given in Figure 12. It can be seen that the photocurrent response of NiWO₄ is found to be lower than Ni_1−xMn_xWO₄ NC under visible light irradiation, which suggests that the separation efficiency of charger carriers in Ni_1−xMn_xWO₄ NC is higher than NiWO₄. The photocatalytic experiments and optical studies are also found to be in close agreement with the photocurrent data which concludes that Ni_1−xMn_xWO₄ NC has higher photocatalytic efficiency than the NiWO₄. The photocurrent for NiWO₄ and Ni_1−xMn_xWO₄ NC was found to be 0.032 μA and 0.035 μA, respectively. The doping of Mn in the NiWO₄ solid matrix leads to the creation of mobile oxygen vacancies, which prompted the separation efficiency of charge carriers; consequently, higher photocurrent was observed for Ni_1−xMn_xWO₄ NC compared with pristine NiWO₄ [45,46].

2.7. Comparison with Literature

The present study was compared with the reported studies in the literature and the results are given in

Table 2. Comparison of the literature information with the present study.

Catalysts	Irradiation Time (min)	Light Source	Organic Pollutant	% Degradation	References
Cu-NiWO4	180	Visible light	Benzene	96.50	[47]
Bi-doped NiWO4	90	UV Irradiation	Rhodamine	86.71	[33]
rGO-NiWO4/Bi2S3	40	Visible light	Methyl Orange	72.00	[26]
WO3/NiWO4	80	UV Irradiation	Methylene blue	90.63	[48]
NiWO4-RGO	240	Visible light	o-Nitrophenol	82.00	[49]
Fe3O4/ZnO/NiWO4	300	Visible light	Rhodmaine B	97.90	[50]
Ni1−xMnxWO4	70	Visible light	Methyl Orange	99.06%	Present study

. It was found that the synthesized material and performed studies are purely novel as it is not reported in the literature. The material was found to be highly effective and energy efficient towards the degradation of methyl orange.

3. Materials and Methods

3.1. Chemical and Reagents

All precursor chemicals used for the synthesis, such as sodium tungstate [Na₂WO₄·2H₂O] (Sigma Aldrich, St. Louis, MO, USA), nickel nitrate [Ni (NO₃)₂.6H₂O] (Loba chemicals, Mumbai, India), manganese chloride [MnCl₂.4H₂O], and urea [CH₄N₂O, 99.5 %] (Merck chemicals, Rahway, NJ, USA) are of analytical grade (purity > 99%).

3.2. Synthesis of NiWO₄ and Ni_1−xMn_xWO₄ Nanocomposite

The nanomaterial was synthesized by the hydrothermal process by taking the precursor mixture in a Teflon-lined autoclave at 150 °C for 24 h [51]. In a beaker, 2 moles of Ni (NO₃)₂.6H₂O and 1 mole of MnCl₂.4H₂O were dissolved in 40 mL of deionized water and placed on a magnetic stirrer for 30 min to achieve homogeneity. After 30 min, 2 moles of Na₂WO₄·2H₂O in 30 mL deionized water was added dropwise followed by the addition of 1.5 g of urea. The mixture was left on a magnetic stirrer for 3 h at room temperature (22 °C) on continuous stirring. After 3 h, the mixture was transferred to a 100 mL Teflon-lined autoclave and placed under an oven at 120 °C for 24 h to achieve complete nucleation of Mn²⁺ in the NiWO₄ lattice. After the reaction, the mixture was taken out of the oven, cooled down and then the precipitate was collected via centrifugation (9000 rpm). The obtained material was washed with deionized water several times and ethanol (2 times) to remove unreacted entities. Finally, the material was dried in the oven at 100 °C for 4 h and then calcined at 600 °C for 3 h. Similarly, in the same way, pristine NiWO₄ was prepared without adding MnCl₂.4H₂O.

3.3. Characterization Techniques

The synthesized NiWO₄ and Ni_1−xMn_xWO₄ nanocomposites were characterized by various instruments to verify the synthesis of the material. The crystal structure of the material and lattice deformation upon Mn²⁺ doping was observed using powder X-ray diffraction (XRD, Bruker D8 Advance with Cu-Kα radiation, λ = 0.15418 nm, Billerica, MA, USA). The surface composition was assessed by scanning electron spectroscopy (SEM; Hitachi S-4800 Field Emission Scanning Electron Microscope, Ibaraki, Japan) and morphological information, shape and size, and their distribution in the lattice were investigated by transmission electron microscopy (TEM; JEM-2100F, Tokyo, Japan). Chemical structural information of the nanomaterial was obtained through Fourier transform infrared (FTIR; Perkin Elmer spectrum 2 ATR, Waltham, MA, USA). The optical properties of the nanoparticles were measured using ultraviolet visible spectroscopy (UV–1900 Shimadzu, Kyoto, Japan). To investigate the difference between the organic carbon (OC) and inorganic carbon (IC) during the photocatalytic reaction, a TOC analyzer (Shimadzu-00077) was used for the analysis of total organic carbon (TOC).

3.4. Photocatalysis Process

The photocatalytic efficiency of the as-synthesized NiWO₄ and Ni_1−xMn_xWO₄ nanocomposites was monitored by the degradation of methyl orange (MO) under visible light irradiation using a tungsten lamp (150 mW/cm⁻²) in a photoreactor chamber. We dispersed 10 mg each of NiWO₄ and Ni_1−xMn_xWO₄ in 10 mL of 50 ppm of MO dye solution; this was then mixed under magnetic stirring for 30 min in dark to establish the adsorption–desorption equilibrium. Finally, the mixture was taken in 20 mL cylindrical vessels (test tube, 20 mL) with a magnetic bar and irradiated in a photoreactor chamber; at a time interval, the vessels were taken out of the chamber, centrifuged to separate the catalyst from the MO solution, and then tested using a UV-Vis spectrophotometer at λmax = 492 nm to evaluate the photocatalytic efficiency as given by Equation (11):

$$\mathrm{Degradation}\mathrm{Efficieny}(\%)=\left(1-\frac{{\mathrm{C}}_{\mathrm{t}}}{{\mathrm{C}}_0}\right)\times 100$$

(11)

where C₀ is the initial concentration of the MO solution and C_t is the concentration of the MO solution at specific times. A selectivity test was performed by taking 10 mg of Ni_1−xMn_xWO₄ NC with various dyes, such as bromophenol (BP), methyl orange (MO), Congo red (CR), malachite green (MG), crystal violet (CV) and methylene blue (MB). The results are given in Figure S1, which suggest that the synthesized nanocomposite material is most sensitive towards MO, showing a photocatalytic efficiency of 94.51%. Therefore, photocatalytic experiments were conducted for MO degradation by varying the reaction parameters, such as irradiation time, pH of the MO solution, catalyst dose, and concentration of the MO solution, to optimize the photocatalytic efficiency of the synthesized material. Thus, based on the outcomes of the experiments, the kinetics and mechanism of photocatalysis was predicted.

3.5. Reusability and TOC Test

To check the stability of the synthesized material towards the treatment of MO in wastewater, a photocatalytic test was performed in cyclic mode. In cycle 1, the photocatalyst was dispersed in the MO solution under optimized reaction conditions; after the completion of the reaction, it was separated out by centrifugation, washed and dried, and then again dispersed in the MO solution for cycle 2. In this way, the same material underwent 6 cycles of the photocatalytic test to check its stability under the given conditions towards MO. Similarly, to observe the mineralization process of MO by the photocatalyst, the total organic carbon test was taken into consideration. As the irradiation time increases, the photocatalytic efficiency also increases, suggesting an increase in the mineralization process, which will reflect in a decrease in TOC value. The TOC (%) was calculated using Equation (12):

$$\mathrm{TOC}(\%)=\left(\frac{{\mathrm{TOC}}_0-{\mathrm{TOC}}_{\mathrm{f}}}{{\mathrm{TOC}}_0}\right)\times 100$$

(12)

4. Conclusions

In the present study, we accomplished the one-pot hydrothermal synthesis of a Mn-decorated NiWO4 nanocomposite material in a Teflon-lined autoclave at 120 °C for 24 h. Various analytical and spectroscopic tests such as XRD, FTIR, UV-Vis, SEM-EDX-mapping, and TEM-SAED supported the successful incorporation of Mn in the NiWO₄ lattice. The material Ni_1−xMn_xWO₄ showed enhanced photocatalytic degradation of MO (99.06%) as compared with pristine NiWO₄ (93.58%) at pH 5 using 10 mg of catalyst for 50 ppm dye concentration under 70 min of visible light irradiation. The enhanced photocatalytic efficiency belongs to improved optical properties by reducing the energy bandgap (Eg) from 3.49 eV (NiWO₄) to 3.33 eV (Ni_1−xMn_xWO₄). The SAED analysis also simulated, with XRD Miller indices data, the monoclinic wurtzite structure of the synthesized Ni_1−xMn_xWO₄ nanocomposite material. The kinetic data was best adjusted to the L-H pseudo first-order kinetic model with R² = 0.99. The rate of photodegradation was found to be 1.06 times higher than the pristine nanoparticles as ^•OH radicals play the primary reactive oxidant species. The outcomes of this study suggest that the material is highly stable and can be reusable for the mineralization of MO (67.75% TOC remove) and other organic pollutants under optimized conditions for environmental remediation without producing secondary pollution.