Hydrothermal Synthesis, Characterization and Exploration of Photocatalytic Activities of Polyoxometalate: Ni-CoWO 4 Nanoparticles

: This study demonstrated the hydrothermal synthesis of bimetallic nickel-cobalt tungstate nanostructures, Ni-CoWO 4 (NCW-NPs), and their phase structure, morphology, porosity, and optical properties were examined using X-ray Diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), Scanning electron microscopy- energy dispersive X-ray spectroscopy (SEM-EDS), high resolution Transmission electron microscopy (HR-TEM), Brunauer-Emmett-Teller (BET) and Raman instruments. It was found that as-calcined NCW-NPs have a monoclinic phase with crystal size ~50–60 nm and is mesoporous. It possessed smooth, spherical, and cubic shape microstructures with deﬁned fringe distance (~0.342 nm). The photocatalytic degradation of methylene blue (MB) and rose bengal (RB) dye in the presence of NCW-NPs was evaluated, and about 49.85% of MB in 150 min and 92.28% of RB in 90 min degraded under visible light. In addition, based on the scavenger’s study, the mechanism for photocatalytic reactions is proposed.


Introduction
Among the various catalytic reaction, photocatalysis (activation of catalyst by light absorption of suitable wavelength/bandgap energy) has drawn much attention because photo-irradiation of the catalyst not only increases the reaction process, but also reduces the initiation path and activation energy without its involvement as reactants and products [1]. The photocatalysis process is a useful, advantageous, and economical low-cost process as well as a sustainable solution to eliminate various toxic chemicals from the environment and water such as removal/mineralization of dyestuffs and clean energy production including hydrogen fuel by water splitting [1][2][3]. In the photocatalytic process, photocatalysts generate active species according to their bandgap ranges in the presence of a light source (ultraviolet/visible light), which aids the mineralization of dyes into CO 2 and H 2 O using an advanced oxidation route.
Various semiconductor photocatalysts have been developed by research groups to keep zones of the Earth and atmosphere clean: terrestrial (forest, grassland, desert) and aquatic (biotic and abiotic) ecosystems [1,[3][4][5]. In this study, our research group reported semiconducting photocatalytic materials, which make different dyes active [6][7][8][9]. For the complete elimination of dye contaminants, researchers have mainly focused on developing special features of semiconductors for better degradation performance. The best-fitting parameters for the dye degradation are that the photocatalyst must have a low recombination rate of charge carriers (electron-hole pairs), extended wavelength/narrow bandgaps, high rate of electron transfer, and large facility for the photocatalytic reaction/chemisorption on its surface. These features can be achieved using various strategies such as morphology

Characterization
The crystallographic structure of fine powders was investigated via X-ray diffraction analysis in the range of 10 • -80 • (2θ angle rotation) using an X-ray diffractometer (Shimadzu 7000S, Shimadzu Analytical, Kyoto, Japan) equipped with CuK α radiation (λ = 0.154 nm). The infrared spectrum was recorded using a FTIR spectrometer (3000 Hyperion Microscope with Vertex 80 FTIR System; Bruker, Karlsruhe, Germany). The Raman spectrum was conducted with the use of a Renishaw micro-Raman spectrometer (RE-04) equipped with a solid-state laser diode-pumped at 514 nm. The morphological and elemental analyses were examined using scanning electron microscopy-energy dispersive x-ray spectroscopy (SEM-Hitachi S3400, Sydney, Australia; operating voltage = 10 kV) Additionally, to obtain more information about the microstructure of the material, transmission electron microscopy (TEM) was performed on an FEI Tecnai G2, F30 electron microscope with an operating voltage of 200 kV and resolution of 2.0 Å. The average crystallinity of the material was studied using the selected area electron diffraction (SAED) technique. The surface area and pore volume of the material were determined by nitrogen adsorption-desorption isotherm measured at 77 K on a BET surface analyzer (Micromeritics ASAP 2010, Norcross, GA, USA). Before each measurement, the sample was degassed at 200 • C under vacuum for 2 h before N 2 adsorption. Surface area and pore volumes were calculated using the Brunauer-Emmet-Teller (BET) equation and BJH methods, respectively.

Photocatalytic Measurement
Cationic (MB) and anionic (RB) dyes were used to evaluate the photocatalytic reactivity of the as-calcined NCW-NPs in response to visible light. The experiments were conducted as follows: 50 mg catalyst was placed in 5 ppm aqueous solution of dyes in a 100 mL beaker. Before illumination, the suspension was stirred in the dark for 1 h to ensure the establishment of an absorption-desorption equilibrium of dyes on the catalyst surface. Subsequently, the suspension was irradiated under a 300 W Xe lamp with a filter (λ > 420 nm) as a cutoff, about 10 cm away from the beaker. At given intervals, 2 mL of the suspension was extracted and centrifuged to separate the NPs. UV-Vis absorption spectra of the supernatant were then measured using a spectrophotometer (Jasco-770 Spectrometer). The characteristic absorptions of RB (λ = 547 nm) and MB (λ = 663 nm) were chosen as the controlled parameter, and the photocatalytic degradation were performed at the where A o and A t symbolize the initial absorbance and absorbance at time t of the chosen dyes, respectively. The simplified pseudo-first-order kinetic model of Langmuir-Hinshewood equation was used to estimate the rate constant for the degradation, in other words, where t is degradation time (min) and k is the rate constant (min −1 ). According to Equation (2), the rate constant k is the slope of fitting curves, when plotting ln(A o /A t ) vs. t. In addition, trapping experiments were performed for the determination of the photocatalytic reaction mechanism using various scavengers: tertiary butyl alcohol (TBA), disodium ethylenediamine tetra-acetic acid (EDTA), ascorbic acid (AA), and dichromate (K 2 Cr 2 O 7 ). The method is similar to the above-mentioned photocatalytic activity test with the addition of 10 mmol/L of trapping agents. The scavengers were added before the addition of the catalyst into the RB dye solution.

X-ray Diffraction
X-ray diffraction patterns of the as-synthesized and calcined powder along with their Rietveld refinements are shown in Figure 1a-c. Figure 1a shows the XRD pattern of both samples. We observed that the as-synthesized diffraction pattern showed several peaks and reflected the presence of the mixed-phase, CoWO 4 , and Co 3 O 4 but no existence of NiO ( Figure 1b). However, when it was calcined at 600 • C, peaks were suppressed and showed sharp crystalline planes in the as-calcined sample (Figure 1c). The intensity of nanoparticles increased after calcination. From this pattern, it can be indexed to the monoclinic phase with space group P2/a of the fabricated Ni-CoWO 4 , consistent with the JCPDS card no. 015-0867 [32]. The intensity of the peaks assigned to the phases demonstrates the dimensions (FWHM) of the Ni-CoWO 4 nanostructures. Table 1 shows the lattice parameters and crystallite size of both samples. The crystallite sizes of both samples were calculated using the Debye-Scherrer equation [33].
Crystals 2021, 11, x FOR PEER REVIEW 4 of 14 spectra of the supernatant were then measured using a spectrophotometer (Jasco-770 Spectrometer). The characteristic absorptions of RB (λ = 547 nm) and MB (λ = 663 nm) were chosen as the controlled parameter, and the photocatalytic degradation were performed at the range of 200-800 nm for 90 and 150 min for the RB and MB, respectively. The degradation efficiency was calculated using the following formula: where Ao and At symbolize the initial absorbance and absorbance at time t of the chosen dyes, respectively. The simplified pseudo-first-order kinetic model of Langmuir-Hinshewood equation was used to estimate the rate constant for the degradation, in other words, where t is degradation time (min) and k is the rate constant (min −1 ). According to Equation (2), the rate constant k is the slope of fitting curves, when plotting ln(Ao/At) vs. t. In addition, trapping experiments were performed for the determination of the photocatalytic reaction mechanism using various scavengers: tertiary butyl alcohol (TBA), disodium ethylenediamine tetra-acetic acid (EDTA), ascorbic acid (AA), and dichromate (K2Cr2O7). The method is similar to the above-mentioned photocatalytic activity test with the addition of 10 mmol/L of trapping agents. The scavengers were added before the addition of the catalyst into the RB dye solution.

X-ray Diffraction
X-ray diffraction patterns of the as-synthesized and calcined powder along with their Rietveld refinements are shown in Figure 1a-c. Figure 1a shows the XRD pattern of both samples. We observed that the as-synthesized diffraction pattern showed several peaks and reflected the presence of the mixed-phase, CoWO4, and Co3O4 but no existence of NiO ( Figure  1b). However, when it was calcined at 600 °C, peaks were suppressed and showed sharp crystalline planes in the as-calcined sample (Figure 1c). The intensity of nanoparticles increased after calcination. From this pattern, it can be indexed to the monoclinic phase with space group P2/a of the fabricated Ni-CoWO4, consistent with the JCPDS card no. 015-0867 [32]. The intensity of the peaks assigned to the phases demonstrates the dimensions (FWHM) of the Ni-CoWO4 nanostructures. Table 1 shows the lattice parameters and crystallite size of both samples. The crystallite sizes of both samples were calculated using the Debye-Scherrer equation [33].

FT-IR Analysis
FTIR spectroscopic analysis was performed to examine the chemical bonds and functional groups in the materials. The spectrum of calcined NCW-NPs is depicted in Figure 2. A wide and shallow peak observed in the range of 3000-3420 cm −1 can be ascribed to the O-H stretching mode of adsorbed water molecules. The peak obtained at 2310 cm −1 was related to the CO 2 , whereas an intense peak was observed at 1632 cm

FT-IR Analysis
FTIR spectroscopic analysis was performed to examine the chemical bonds and functional groups in the materials. The spectrum of calcined NCW-NPs is depicted in Figure  2. A wide and shallow peak observed in the range of 3000-3420 cm −1 can be ascribed to the O-H stretching mode of adsorbed water molecules. The peak obtained at 2310 cm −1 was related to the CO2, whereas an intense peak was observed at 1632 cm

Raman Spectroscopy
The calcined nanocrystal was characterized using a nondestructive Raman spectroscopy technique to understand the structural defects. Figure 3 shows the Raman spectrum for the fabricated NCW-NPs. The peaks appeared at 102, 142, 208, 290, 350, 416, 537, 694, and 889 cm −1 positions corresponding to the spectrum of bimetallic Ni-CoWO4 nanoparticles. The sharp band at 889 cm −1 was allocated to the symmetric mode of terminal W=O. The weak bands at 694 cm −1 and 537 cm −1 were antisymmetric and symmetric modes of O-W-O bridgings associated with tungstate anions. In-plane deformation and rotational mode of W-O bonds were observed at 416 cm −1 and 350 cm −1 , respectively. The band at 102 cm −1 and 142 cm −1 was attributed to the lattice vibration. Weak bands at 208 and 290 cm −1 were associated with Ni-O and Co-O stretching, respectively. All Raman bands were in agreement with the reported results [31,38,39]. Two more band signals are shown at a higher wavenumber (Figure 3 inset) because of the presence of the hydroxyl group, agreeing with the IR results [39]. Thus, the fabricated

Raman Spectroscopy
The calcined nanocrystal was characterized using a nondestructive Raman spectroscopy technique to understand the structural defects. Figure 3 shows the Raman spectrum for the fabricated NCW-NPs. The peaks appeared at 102, 142, 208, 290, 350, 416, 537, 694, and 889 cm −1 positions corresponding to the spectrum of bimetallic Ni-CoWO 4 nanoparticles. The sharp band at 889 cm −1 was allocated to the symmetric mode of terminal W=O. The weak bands at 694 cm −1 and 537 cm −1 were antisymmetric and symmetric modes of O-W-O bridgings associated with tungstate anions. In-plane deformation and rotational mode of W-O bonds were observed at 416 cm −1 and 350 cm −1 , respectively. The band at 102 cm −1 and 142 cm −1 was attributed to the lattice vibration. Weak bands at 208 and 290 cm −1 were associated with Ni-O and Co-O stretching, respectively. All Raman bands were in agreement with the reported results [31,38,39]. Two more band signals are shown at a higher wavenumber (Figure 3 inset) because of the presence of the hydroxyl group, agreeing with the IR results [39]. Thus, the fabricated bimetallic structured material exhibited a monoclinic phase and correlated with the X-ray diffraction pattern. bimetallic structured material exhibited a monoclinic phase and correlated with the x-ray diffraction pattern.  Figure 4 represents the low and high magnification images of the fabricated NCW-NPs. From the low magnification image (Figure 4a), it appears that nanoparticles formed a cluster of spherical and cauliflower-like morphology with the combination of several nano-and microparticles. The NCW-NP structures were analyzed with a magnified image (Figure 4b). From the image, it was observed that the nanoparticles were closely packed, and the estimated diameter of each cluster of spherical NPs ranged from 10 to 12 μm. The elemental analysis of nanoparticles was observed via EDX spectroscopy and is shown in Figure 4c. It can be seen that carbon, cobalt, nickel, tungsten, and oxygen peaks appeared in the EDX spectrum. A small peak of C was also seen in the spectrum, which relates to carbon tapes used in holding the sample during the EDX analysis. No impurity peak was detected in the EDX spectrum, their atomic ratios of carbon (11.84%), oxygen (65.40%), cobalt (5.78%), nickel (6.23%), and tungsten (10.75%) confirm that the synthesized nanoparticles were bimetallic NiCoWO4 in nature  Figure 4 represents the low and high magnification images of the fabricated NCW-NPs. From the low magnification image (Figure 4a), it appears that nanoparticles formed a cluster of spherical and cauliflower-like morphology with the combination of several nano-and microparticles. The NCW-NP structures were analyzed with a magnified image (Figure 4b). From the image, it was observed that the nanoparticles were closely packed, and the estimated diameter of each cluster of spherical NPs ranged from 10 to 12 µm. The elemental analysis of nanoparticles was observed via EDX spectroscopy and is shown in Figure 4c. It can be seen that carbon, cobalt, nickel, tungsten, and oxygen peaks appeared in the EDX spectrum. A small peak of C was also seen in the spectrum, which relates to carbon tapes used in holding the sample during the EDX analysis. No impurity peak was detected in the EDX spectrum, their atomic ratios of carbon (11.84%), oxygen (65.40%), cobalt (5.78%), nickel (6.23%), and tungsten (10.75%) confirm that the synthesized nanoparticles were bimetallic NiCoWO 4 in nature.

Morphological Analysis
The morphology of the NCW-NPs with their crystalline character was analyzed using TEM and the SAED facility. Figure 5a,b shows the low and high magnification TEM images, which represent several spherical and monoclinic/cubic shaped NPs with smooth surfaces arranged on the whole surfaces, indicating that the nanoparticles were not in aggregated forms. This is because during sample preparation for TEM analysis, it was first sonicated for 10 min, appearing non-aggregated. By TEM analysis, it was confirmed that during sonication, the particle well separated because it showed no broken or rough surfaces. Some porous structures formed by the network of the interlinked nanoparticles were observed from these TEM images. The average size of each NP was in the range of 50-60 nm. The material crystallinity was confirmed from the HR-TEM (Figure 5c) and SAED (Figure 5d) patterns, which depict that the prepared material is highly crystalline with a lattice distance of~0.342 nm, and can be recognized to the (011) plane in inset IFFT reflection of the red circle. The SAED patterns showed that the fabricated material is polycrystalline and inconsistent with the XRD, complimenting the characterization and formation of the Ni-CoWO 4 nanoparticles.  The morphology of the NCW-NPs with their crystalline character was analyzed using TEM and the SAED facility. Figure 5a,b shows the low and high magnification TEM images, which represent several spherical and monoclinic/cubic shaped NPs with smooth surfaces arranged on the whole surfaces, indicating that the nanoparticles were not in aggregated forms. This is because during sample preparation for TEM analysis, it was first sonicated for 10 min, appearing non-aggregated. By TEM analysis, it was confirmed that during sonication, the particle well separated because it showed no broken or rough surfaces. Some porous structures formed by the network of the interlinked nanoparticles were observed from these TEM images. The average size of each NP was in the range of 50-60 nm. The material crystallinity was confirmed from the HR-TEM ( Figure 5c) and SAED (Figure 5d) patterns, which depict that the prepared material is highly crystalline with a lattice distance of ~0.342 nm, and can be recognized to the (011) plane in inset IFFT reflection of the red circle. The SAED patterns showed that the fabricated material is polycrystalline and inconsistent with the XRD, complimenting the characterization and formation of the Ni-CoWO4 nanoparticles.

BET Surface Area
The BET surface area for the calcined polycrystalline was conducted to evaluate their area, volume, and pore-size distribution. Figure 6 shows the nitrogen adsorptiondesorption isotherm at 77 K and the corresponding pore-size distribution curves of the fabricated nanoparticles. The BET isotherm was recognized as type IV isotherm and H2type hysteresis, confirming the mesoporous structures [40]. The surface area was found to be 20.19 m 2 /g. The porosity of the NCW-NPs was determined from the desorption branch of the pore-size distribution curve (inset of Figure 6) and shows the sharp distribution in the mesoporous region. The average pore diameter and pore volume determined using the BJH method was found to be 2.22 nm and 0.075 cm 3 /g, respectively. Based on the analysis, it may be assumed that the formed nanoparticles exhibit good porosity, which is one of the key factors for photocatalytic degradation.

BET Surface Area
The BET surface area for the calcined polycrystalline was conducted to evaluate their area, volume, and pore-size distribution. Figure 6 shows the nitrogen adsorptiondesorption isotherm at 77 K and the corresponding pore-size distribution curves of the fabricated nanoparticles. The BET isotherm was recognized as type IV isotherm and H2type hysteresis, confirming the mesoporous structures [40]. The surface area was found to be 20.19 m 2 /g. The porosity of the NCW-NPs was determined from the desorption branch of the pore-size distribution curve (inset of Figure 6) and shows the sharp distribution in the mesoporous region. The average pore diameter and pore volume determined using the BJH method was found to be 2.22 nm and 0.075 cm 3 /g, respectively. Based on the analysis, it may be assumed that the formed nanoparticles exhibit good porosity, which is one of the key factors for photocatalytic degradation.

Photocatalytic Evaluation
To evaluate the photocatalytic activities of the Ni-CoWO4 nanoparticles, the photocatalytic decomposition of MB and RB under a 300 W Xe lamp with a 420 nm cutoff filter irradiation was selected as a model dye. Figure 7a,b shows the typical absorption spectra of the MB and RB aqueous solution (5 ppm) in the presence of the photocatalyst (50 mg) under visible-light illumination at ambient temperature and different time intervals. We observed that the intensity of the characteristic major absorption peaks of Figure 6. BET isotherms and inset corresponding to BJH pore-size distribution curves for the calcined Ni-CoWO 4 NPs.

Photocatalytic Evaluation
To evaluate the photocatalytic activities of the Ni-CoWO 4 nanoparticles, the photocatalytic decomposition of MB and RB under a 300 W Xe lamp with a 420 nm cutoff filter irradiation was selected as a model dye. Figure 7a,b shows the typical absorption spectra of the MB and RB aqueous solution (5 ppm) in the presence of the photocatalyst (50 mg) under visible-light illumination at ambient temperature and different time intervals. We observed that the intensity of the characteristic major absorption peaks of dyes (MB: 663 nm and RB: 547 nm) decreased as the exposure time extended. The peak intensity of RB (Figure 7b) decreased faster over time, about 80% degraded in 30 min and solutions became colorless after the required times. Hence, a decrease in the dye's concentration/absorption intensity indicated that the fabricated tungstate is a photoactive material toward RB. Moreover, the degradation efficiency of the dyes was evaluated using Equation (1) and is shown in Figure 7c. It was observed that RB was more degradable compared to MB in the presence of a catalyst. About 92.29% and 48.73% for RB and MB were mineralized, respectively, which demonstrated more photocatalytic degradation of RB due to interaction between the fabricated catalyst and anionic RB dye. Furthermore, the presence of a second Ni metal in Ni-CoWO 4 , obtained by the hydrothermal approach, enhanced the surface area for the photocatalytic degradation. During the photocatalytic process, both transition metals exhibited variable oxidation states due to charge transfer and provide more accessibility of active sites in fabricated NCW-NPs. These results overcome the limitations of poor performance of monometallic tungstate, as previously reported [10][11][12]15,16].  The comparison of the degradation efficiency of our fabricated tungstate with the previously reported polyoxometalates is tabulated in Table 3 [18,[41][42][43][44][45][46]. The present NCW-NPs showed comparable or higher photocatalytic degradation of MB, depending on the preparation and experimental conditions. From Figure 7d, we observed that the kinetics of the photocatalytic degradation of the dyes followed the Langmuir-Hinshelwood first-order kinetics model, which is described in the experimental section. The rate constants, by linear extrapolations, and half-life (t 1/2 ), were calculated and are summarized in Table 2. The rate constant value of the catalyst was better for RB compared with MB, which was around seven times (3.198 × 10 −2 min −1 ) higher than MB (0.437 × 10 −2 min −1 ). The comparison of the degradation efficiency of our fabricated tungstate with the previously reported polyoxometalates is tabulated in Table 3 [18,[41][42][43][44][45][46]. The present NCW-NPs showed comparable or higher photocatalytic degradation of MB, depending on the preparation and experimental conditions. To understand the photocatalysis mechanism of the NCW-NPs, the active species generated during the photocatalytic degradation were identified through radical and holetrapping experiments. TBA, EDTA, AA, and K 2 Cr 2 O 7 acted as the scavengers for hydroxyl radicals ( • OH), holes (h + ), superoxide radicals ( • O 2 -), and electrons (e -), respectively, and the obtained data in terms of degradation efficiency are represented in Figure 8a. From Figure 8a, it was observed that the degradation efficiency of RB without scavengers was 92.29%. However, the degradation efficiency of RB decreased slightly from 92.29% to 90.15% with the addition of EDTA, AA, and dichromate and considerable with the addition of TBA. This phenomenon suggests that hydroxyl radicals ( • OH) played a major role in the degradation mechanism of RB, whereas other scavengers showed no effect.
To further the photocatalytic mechanism of the fabricated NPs by generating hydroxyl radicals, a photoluminescence probing technique was conducted with commonly used coumarin [47]. In this experiment, 5 mg of the photocatalyst was dispersed in 100 mL of 10 −3 M (1 mM) coumarin solution. The solution was kept for adsorption-desorption equilibrium between the photocatalyst and coumarin before irradiation. This reaction was irradiated under visible light, for every 10 min, 2 mL of irradiated sample was withdrawn and the photoluminescence spectrum was measured. In the presence of the catalyst, non-luminescent coumarin reacted with ( • OH) and was converted to highly fluorescent 7-hydroxyl coumarin on exposure to visible light. The photoluminescence spectra of NCW-NPs/coumarin solution under visible-light irradiation are displayed in Figure 8b, where the emission peak of coumarin (λ max = 454 nm) increases continuously with irradiation time. In this way, we can detect the formation of photo-generated ( • OH) by monitoring the fluorescence intensity changes of the solution, concluding that the hydroxyl radical is a reactive species during the decomposition process.
adsorption-desorption equilibrium between the photocatalyst and coumarin before irradiation. This reaction was irradiated under visible light, for every 10 min, 2 mL of irradiated sample was withdrawn and the photoluminescence spectrum was measured. In the presence of the catalyst, non-luminescent coumarin reacted with ( • OH) and was converted to highly fluorescent 7-hydroxyl coumarin on exposure to visible light. The photoluminescence spectra of NCW-NPs/coumarin solution under visible-light irradiation are displayed in Figure 8b, where the emission peak of coumarin (λmax = 454 nm) increases continuously with irradiation time. In this way, we can detect the formation of photo-generated ( • OH) by monitoring the fluorescence intensity changes of the solution, concluding that the hydroxyl radical is a reactive species during the decomposition process. Therefore, based on the findings of the active species, we can conclude that photocatalytic degradation begins with the generation of a (e − -h + ) pairs. When a semiconductor photocatalyst is illuminated by light, with energy equal to or greater than the bandgap energy, the valence band (VB) electrons (reducing agent) can be excited to the conduction band, leaving a positive hole (oxidizing agent) in the valence band, as illustrated in Figure 9. Then, these photo-induced holes in VB of NCW-NPs react with the pollutant or H2O/surface − OH, leading to the formation of hydroxyl radicals ( • OH), which decompose RB to CO2 and H2O molecules. Therefore, a hydroxyl radical is recognized to be the most powerful oxidizing species in our case and can attack organic pollutants present at or near the surface of the photocatalyst. Therefore, based on the findings of the active species, we can conclude that photocatalytic degradation begins with the generation of a (e − -h + ) pairs. When a semiconductor photocatalyst is illuminated by light, with energy equal to or greater than the bandgap energy, the valence band (VB) electrons (reducing agent) can be excited to the conduction band, leaving a positive hole (oxidizing agent) in the valence band, as illustrated in Figure 9. Then, these photo-induced holes in VB of NCW-NPs react with the pollutant or H 2 O/surface − OH, leading to the formation of hydroxyl radicals ( • OH), which decompose RB to CO 2 and H 2 O molecules. Therefore, a hydroxyl radical is recognized to be the most powerful oxidizing species in our case and can attack organic pollutants present at or near the surface of the photocatalyst.

Conclusions
In this study, we reported on the hydrothermal synthesis of bimetallic Ni-CoWO4 nanoparticles (NCW-NPs), and their formation was confirmed using XRD, IR, Raman, SEM-EDX, and HR-TEM. The fabricated nanoparticles showed better crystalline properties after calcination at 600 °C for 6 h. The morphology investigated by SEM revealed that several nanoparticles combined in the form of spherical, smooth particles with an average diameter of a spherical cluster of 10-12 μm. Elemental analysis was conducted using EDX attached to the SEM, which showed a purity and relative percentage proportion of Ni, Co, W, and O in the NCW-NPs. Furthermore, its

Conclusions
In this study, we reported on the hydrothermal synthesis of bimetallic Ni-CoWO 4 nanoparticles (NCW-NPs), and their formation was confirmed using XRD, IR, Raman, SEM-EDX, and HR-TEM. The fabricated nanoparticles showed better crystalline properties after calcination at 600 • C for 6 h. The morphology investigated by SEM revealed that several nanoparticles combined in the form of spherical, smooth particles with an average diameter of a spherical cluster of 10-12 µm. Elemental analysis was conducted using EDX attached to the SEM, which showed a purity and relative percentage proportion of Ni, Co, W, and O in the NCW-NPs. Furthermore, its microstructure was confirmed via HR-TEM analysis, which showed that the average particle size of each NP was about 50 nm, well-arranged spherical and in a cubic shaped structure. The fringe distance between the two lattices was about 0.342 nm. The Raman peaks for the Ni-O, Co-O and W=O of the material were consistent with the HR-TEM and XRD. The obtained results from the characterization and photolytic activity test indicate that the fabricated NCW-NPs expressed better photocatalytic activity for RB compared to MB because of the synergistic effect and excellent redox ability of electron-hole pairs. As such, the processed materials can be used in large-scale industrial applications.