In Situ Hydrothermal Synthesis of Ni1−xMnxWO4 Nanoheterostructure for Enhanced Photodegradation of Methyl Orange

The monoclinic nanocrystalline Ni1−xMnxWO4 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 Ni1−xMnxWO4 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 Ni1−xMnxWO4 is highly efficient (94.51%) compared with undoped NiWO4 (65.45%). The rate of photodegradation by Ni1–xMnxWO4 (0.067) was found to be 1.06 times higher than the undoped NiWO4 (0.062).


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  (023), which were found to have good agreement with the lattice structure of MnWO 4 with standard JCPDS no. 72-0478. The XRD results suggested that Mn 2+ has been successfully incorporated in the NiWO 4 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]: 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 NiWO 4 was 32 ± 1.05 nm, while for Ni 1−x Mn x WO 4 , it was 26 ± 0.65 nm. The outcomes indicated a contraction in the crystallite size in NiWO 4 upon Mn 2+ incorporation, suggesting an improvement in the optical activity of the material as well.
for the monoclinic wolframite structure of NiWO4 with standard JCPDS no. XRD spectra of synthesized Ni1−xMnxWO4 also exhibited maximum peaks except for the appearance of some new Miller indices values, namely (121 and (023), which were found to have good agreement with the lattice structu with standard JCPDS no. 72-0478. The XRD results suggested that Mn 2+ has fully incorporated in the NiWO4 lattice; moreover, there is an increase in th peaks after Mn incorporation, indicating an increase in the crystallinity of crystallite size was determined using the Debye-Scherrer equation given as where λ is the wavelength of X-ray source, K is a shape factor and is usuall corresponding angle, and β is the breadth of the observed diffraction line at sity maximum. The results obtained using Equation (3) disclosed that the p NiWO4 was 32 ± 1.05 nm, while for Ni1−xMnxWO4, it was 26 ± 0.65 nm. The o cated a contraction in the crystallite size in NiWO4 upon Mn 2+ incorporatio an improvement in the optical activity of the material as well.

Functional Group Studies
The functional group study of the materials was done using FTIR spe the results are given in Figure 2. From the FTIR spectra, NiWO4 shows charac at 3414, 1630 cm −1 belonging to stretching and bending vibration bands of (surface adsorbed water), 879 and 830 cm −1 antisymmetric stretching vibra W-O bonds, 706, 622 cm −1 stretching and bending vibrations of W-O bond tahedron, and 528, 434 cm −1 stretching vibrations of Ni-O bonds from the Ni [23,27]. The FTIR spectra of the as-synthesized Ni1−xMnxWO4 NC exhibited th with shifted values, which is due to the orbital hybridization and change o vironment of NiWO4 by Mn. The band at 706 cm -1 vanished completely, s acquirement of the portion associated with the [WO6] octahedron by Mn 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, NiWO 4 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 [WO 6 ] octahedron, and 528, 434 cm −1 stretching vibrations of Ni-O bonds from the NiO 6 octahedron [23,27]. The FTIR spectra of the as-synthesized Ni 1−x Mn x WO 4 NC exhibited the same bands with shifted values, which is due to the orbital hybridization and change of chemical environment of NiWO 4 by Mn. The band at 706 cm -1 vanished completely, suggesting the acquirement of the portion associated with the [WO 6 ] octahedron by Mn 2+ [37].

Optical Studies
The optical properties of the synthesized material were investigated UV-Vis spectra given in Figure 3. The UV-Vis spectra of NiWO4 exhib very low intensity at 278 nm, suggesting that the material is only UV-lig corresponding transition is due to a charge transfer process between Ni 2 hedron [WO6] clusters [39]. The UV spectra of Ni1−xMnxWO4 NC exhibit 286 nm, 355 nm, and 685 nm, suggesting the UV and visible light activi sized material. So, the incorporation of Mn in the NiWO4 lattice enlarged ing capacity; moreover, the peaks that appeared are due to Ni 2+ (d-d) to o clusters CT and secondly Ni 2+ (d-d) to octahedron [WO6] clusters to Mn creased its absorption intensity to visible light from the UV region bandgap (Eg) value of the synthesized material can be calculated using given as [41]: where B is a constant (dependent on the nature of the material), α is the ficient of the material (calculated from the absorption spectra), ν is the radiation, h is the Plank's constant, and Eg is the energy of the band gap of n (coefficient of transition) decides the type of energy bandgap, i.e., n sponds to allowed indirect energy bandgap, while n = 1/2 for allowe bandgap. Using Equation (4), the value of energy band gap (Eg) was calc for NiWO4 and 2.13 eV for Ni1−xMnxWO4 NC. The contraction in Eg valu ration of Mn in the NiWO4 lattice resulted in improved optical and phot ties.

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 NiWO 4 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 Ni 2+ (d-d) and octahedron [WO 6 ] clusters [38]. The UV spectra of Ni 1−x Mn x WO 4 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 NiWO 4 lattice enlarged its light absorbing capacity; moreover, the peaks that appeared are due to Ni 2+ (d-d) to octahedron [WO 6 ] clusters CT and secondly Ni 2+ (d-d) to octahedron [WO 6 ] clusters to Mn 2+ 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]: 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 (

SEM-EDX-Mapping Analysis
The surface morphology, elemental composition, and their distribution in the cr lattice were assessed by SEM-EDX analysis. Figure 4a represents the SEM image of tine NiWO4, which shows an aggregation of tiny interconnected particles, wherea SEM image in Figure 4b represents a porous morphology with interconnected tiny p cles. The EDX analysis given in Figure 4c

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 NiWO 4 , 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

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   For further insight into the morphology, including crystallite shape and size, TEM-SAED analysis was used. Figure 5a,b represents the TEM image of Ni1−xMnxWO4 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).  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−x Mn x WO 4 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).

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 Ni1−xMnxWO4 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 (pHpzc) value, which is found to be 5.8 for NiWO4 and 6.2 for Ni1−xMnxWO4 NC ( Figure S2). So, at pH < pHpzc, 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 > pHpzc, 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 [42]. The outcomes

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 NiWO 4 and 98.32% for Ni 1−x Mn x WO 4 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 NiWO 4 and 6.2 for Ni 1−x Mn x WO 4 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 • O 2 − 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−x Mn x WO 4 NC possessed better photocatalytic efficiency as compared with pristine NiWO 4 . Thus, Mn decoration in the NiWO 4 lattice, leading to the formation of the heterostructure, resulted in an enhancement of photocatalytic efficiency towards MO degradation.

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 Ni1−xMnxWO4 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 [43].

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 NiWO 4 and Ni 1−x Mn x WO 4 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 • O 2 − radicals) and thus cause a decrease in photocatalytic efficiency [42].  Figure 8a,b represents the degradation of MO in the presence of pristine NiWO4 and the as-synthesized Ni1−xMnxWO4 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 Ni1−xMnxWO4 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 [12,44]:

Kinetics of Photodegradation and Effect of Irradiation Time
where C0 and Ct are the concentration of MO at the initial state (t = 0) and specific time intervals (t) and kapp 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 (5) and the results obtained are given in Table 1 and Figure 8c. It was  Figure 8a,b represents the degradation of MO in the presence of pristine NiWO 4 and the as-synthesized Ni 1−x Mn x WO 4 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 NiWO 4 and 99.06% for Ni 1−x Mn x WO 4 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]:

Kinetics of Photodegradation and Effect of Irradiation Time
Molecules 2023, 28, 1140 10 of 18 where C 0 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 and Figure 8c. It was found that the rate of degradation of MO was higher for Ni 1−x Mn x WO 4 (0.067 min −1 ) as compared with pristine NiWO 4 (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 R 2 = 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 NiWO 4  found that the rate of degradation of MO was higher for Ni1−xMnxWO4 (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 R 2 = 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 t1/2 = ln 2/kapp, and the values obtained are 11 min for NiWO4 and 10.34 min for Ni1−xMnxWO4 NC.

Detection of ROS
The scavenger's test was performed to find out the primary ROS involved in the photodegradation of MO by Ni1−xMnxWO4 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 [45]. 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

Detection of ROS
The scavenger's test was performed to find out the primary ROS involved in the photodegradation of MO by Ni 1−x Mn x WO 4 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 • O 2 − ) 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−x Mn x WO 4 NC.  Based on the trapping experiments, the plausible general mechanism of MO degradation is given by Equations (6)(7)(8)(9)(10)(11)(12)   Based on the trapping experiments, the plausible general mechanism of MO degradation is given by Equations (4-10) [1,4,8]:  Figure 10 represents the mechanism of photodegradation of MO by Ni1−xMnxWO4 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 (6). These photogenerated electrons (e -) in CB interact with surface-adsorbed O2 molecules and oxidize to superoxide radicals ( • O2 − ) Equation (8). The holes interact with hydroxyl anions (OH − ) to produce hydroxy radicals ( • OH) Equation (7). The O2 molecule reacts with H + ions and produces H2O2, which further reacts with e -(CB) generating super reactive • OH radical reaction (9-12); these are responsible for the degradation of MO under visible light source.

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 Ni1−xMnxWO4 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 Ni1−xMnxWO4 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 Ni1−xMnxWO4 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.

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−x Mn x WO 4 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−x Mn x WO 4 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.

Photocurrent Measurement
Furthermore, to investigate the photophysical behaviors of NiWO4 and Ni1−xMnxWO4 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 Ni1−xMnxWO4 NC under visible light irradiation, which suggests that the separation efficiency of charger carriers in Ni1−xMnxWO4 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−x Mn x WO 4 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.

Photocurrent Measurement
Furthermore, to investigate the photophysical behaviors of NiWO 4 and Ni 1−x Mn x WO 4 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 NiWO 4 is found to be lower than Ni 1−x Mn x WO 4 NC under visible light irradiation, which suggests that the separation efficiency of charger carriers in Ni 1−x Mn x WO 4 NC is higher than NiWO 4 . The photocatalytic experiments and optical studies are also found to be in close agreement with the photocurrent data which concludes that Ni 1−x Mn x WO 4 NC has higher photocatalytic efficiency than the NiWO 4 . The photocurrent for NiWO 4 and Ni 1−x Mn x WO 4 NC was found to be 0.032 µA and 0.035 µA, respectively. The doping of Mn in the NiWO 4 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−x Mn x WO 4 NC compared with pristine NiWO 4 [45,46].

Photocurrent Measurement
Furthermore, to investigate the photophysical behaviors of NiWO4 and Ni NC under visible light irradiation, photocurrent measurements were taken int ation. The obtained results are given in Figure 12. It can be seen that the ph response of NiWO4 is found to be lower than Ni1−xMnxWO4 NC under visible l ation, which suggests that the separation efficiency of charger carriers in Ni NC is higher than NiWO4. The photocatalytic experiments and optical studi found to be in close agreement with the photocurrent data which conc Ni1−xMnxWO4 NC has higher photocatalytic efficiency than the NiWO4. The ph for NiWO4 and Ni1−xMnxWO4 NC was found to be 0.032 μA and 0.035 μA, re The doping of Mn in the NiWO4 solid matrix leads to the creation of mobile o cancies, which prompted the separation efficiency of charge carriers; consequen photocurrent was observed for Ni1−xMnxWO4 NC compared with pristine NiW

Comparison with Literature
The present study was compared with the reported studies in the literature and the results are given in Table 2. 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.

Synthesis of NiWO 4 and Ni 1−x Mn x WO 4 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 (NO 3 ) 2 .6H 2 O and 1 mole of MnCl 2 .4H 2 O 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 Na 2 WO 4 ·2H 2 O 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 Mn 2+ in the NiWO 4 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 NiWO 4 was prepared without adding MnCl 2 .4H 2 O.

Characterization Techniques
The synthesized NiWO 4 and Ni 1−x Mn x WO 4 nanocomposites were characterized by various instruments to verify the synthesis of the material. The crystal structure of the material and lattice deformation upon Mn 2+ 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).

Photocatalysis Process
The photocatalytic efficiency of the as-synthesized NiWO 4 and Ni 1−x Mn x WO 4 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 dis-persed 10 mg each of NiWO 4 and Ni 1−x Mn x WO 4 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 adsorptiondesorption 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): where C 0 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−x Mn x WO 4 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.

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):

Conclusions
In the present study, we accomplished the one-pot hydrothermal synthesis of a Mndecorated 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 NiWO 4 lattice. The material Ni 1−x Mn x WO 4 showed enhanced photocatalytic degradation of MO (99.06%) as compared with pristine NiWO 4 (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 (NiWO 4 ) to 3.33 eV (Ni 1−x Mn x WO 4 ). The SAED analysis also simulated, with XRD Miller indices data, the monoclinic wurtzite structure of the synthesized Ni 1−x Mn x WO 4 nanocomposite material. The kinetic data was best adjusted to the L-H pseudo first-order kinetic model with R 2 = 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.