Facile synthesis of novel CoWO 4 /FeWO 4 hetrocomposite with efficient visible light photocatalytic degradation and hydrogen evolution aspects

Tungstate-based nanomaterials exhibit efficient photocatalytic performance and offer several advantages owing to their electrical and superior optical features, charge transport potentials, and superb corrosion resistance. The objective of the present study is to fabricate cobalt tungstate (CoWO 4 ), Ferric tungstate (FeWO 4 ) and CoWO 4 /FeWO 4 heterojunction composite photocatalysts using a hydrothermal route with various molar concentrations (2:1, 1:1, 1:2, 1:5). The model pollutant Methyl Orange (MO) and Congo Red (CR) azo dyes were degraded 98.26% and 99.61% in 150 min by the as-synthesized CoWO 4 /FeWO 4 at a molar concentration ratio of 1:2. A feasible photodegradation mechanism is purposed and the optimum values for different parameters are also evaluated by considering two different dyes as model organic pollutants. Hydrogen production efficiency reaches up to 36 μ molg − 1 h − 1 under visible light over 1:2 CoWO 4 /FeWO 4 . This work may open new possibilities for the use of CoWO 4 /FeWO 4 composite for potential applications such as the hydrothermal synthesis of composites and their photocatalytic wastewater remedy and as hydrogen evolution applications.


Introduction
Today's world is dominated by technology and is ruled by compatible electronic devices, necessitating the use of materials with adaptable properties [1].The term "semiconductor" emerges as a very popular category of material after mining the pages of history for this type of material.Nanotechnology, which deals with scales less than 100 nm, is a new and emerging field of technology in semiconductors science [2,3].Since Richard Feynman's first lecture on the subject in 1959, it has become a field of study from the previous century [4].Nanotechnology is the study of materials at the nanoscale 10 −9 [5].Materials have different characteristics at the bulk scale than they do at the nanoscale, where their surface-to-volume ratio increases with size [6].This surface-to-volume ratio feature, when utilized as a catalyst, boosts efficiency by a factor of a hundred when compared to bulk materials.In the energy sector, we can improve the efficiency of hydrogen production by using nanomaterials as a catalyst in the water-splitting process [7].Nanomaterials exhibit exceptional photocatalytic capabilities due to their large surface area, high aspect ratio, electrostatic, compressible without affecting the surface area, adjustable pore volume, magnetic, short intra-particle diffusion CONTACT Muhammad Munir Sajid m.munirsajid476@gmail.com; Haifa Zhai haifazhai@126.comSupplemental data for this article can be accessed online at https://doi.org/10.1080/16583655.2023.2265631.
This article has been corrected with minor changes.These changes do not impact the academic content of the article.
distance, hydrophobic and hydrophilic qualities, and so on.Nanotechnology for clean water, particularly for water pollution, is transforming the commercial environment in both developed and developing countries.Nanotechnology can assist us in resolving the current issue with traditional water filtration methods [8,9].Water microbes are just a thousand nanometres in size, however, conventional purifiers only lead to bacteria.Nanotechnology purifiers can deal with viruses and organic pollutants without the need for chemicals, high-temperature crushing, or electricity [10,11].In today's world, both industrialized and developing countries have two big basic problems; pollution and energy concerns caused by climate change and overpopulation; even the twenty-first century COVID-19 problem is tied to pollution [12].Water pollution, air pollution, land pollution, light pollution, and noise pollution are all kinds of pollution, but water and air pollution cause the most damage to our environment, climate, and all living things, and death from these two are quite prevalent [13].Without a solution to these essential problems, the survival of humanity on our planet would be very difficult.The "photocatalytic approach" yielded some groundbreaking findings for the previously unsolved problem [14].Photocatalytic techniques in the energy and pollution sectors provided us with an ecologically friendly, no-side-effect alternative for water and air purification [11].Numerous subdivisions, including those that deal with textiles, paint, ink, plastics, and cosmetics, employ several dyes as raw material which afterwords is expelled to canals and rivers causing risk for acoustics [15].During the dyeing process, nearly 15% of the world's yearly production of synthetic textiles (7,00,000 tonnes) of colours is lost in wastewater streams.Due to the presence of the chromophore −N = N-unit in their molecular structure, these dyes are categorized as azo compounds [16].Common information holds that when soluble azo dyes are ingested into a living organism, liver enzymes and intestinal flora convert them into corresponding aromatic amines, which can cause cancer in humans.The textile sector generates vast amounts of brightly coloured effluents, most of which are hazardous and difficult to remove using biological treatment techniques.To effectively remove the colour from textile effluents, a practical technology must be developed [17,18].Photocatalytic degradation by semiconductors is a unique, rapid, and effective technique for eliminating pollutants from water [8,19,20].The semiconductor comprises very active photocatalytic properties, such as its ability to degrade chemical compounds and its extreme hydrophilicity, antibacterial traits and has drawn a lot of interest recently [21,22].When a semiconductor is lighted it produces photoexcited electrons (e − ) and positively charged holes (h + ) in a procedure known as photocatalysis.When organic molecules are exposed to light, photoinduced hydroxyl radicals or photogenerated holes mix with them to create a range of hydroxylated reaction intermediates [23][24][25].The fabrication of visible-light-driven photocatalysts has recently drawn a lot of attention as a substitute for traditional wastewater treatment methods and photocatalytic hydrogen evaluation from water splitting [26][27][28].The incorporation of a heterostructure is an efficient and straightforward method to increase a photocatalyst's activity because heterojunctions have great potential for tailoring the desired electronic properties of photocatalysts and effectively separating the photogenerated electron-hole pairs [29][30][31].
In the early stages of cobalt analysis, cobalt was separated from silicate and other materials using cobalt sulphide's insolubility in an alkaline solution.Utilizing colourimetric methods to determine cobalt in basalts and other analogous rocks is difficult.With the space group P2/c and monoclinic geometry, CoWO 4 seems to be a monoclinic wolframite crystal, which has a narrow band gap of 2.41 eV [32].In the wolframite composition, each tungsten atom is surrounded by six oxygen atoms.Among other noteworthy properties, nanoscale transition metal tungstates have piezoelectric, laser, fluorescent, ferroelectric, and ferroelastic properties.
However, CoWO 4 nanostructures have several limitations, such as electron and hole recombination and poor electrical conductivity, which limit its impressive capabilities, such as the ability to separate electron-hole pairs, extend photoresponse, transfer oxygen, have good electrical conductivity, and have strong redox ability.A potential photocatalyst material is iron tungstate (FeWO 4 ), which has the wolframite structure, a narrow band gap of 2.17 eV, a high oxidation state of tungsten (W), crystal lattice defects, and a regular arrangement of hexagonally close-packed oxygen and iron cation in octahedral sites [13].FeWO 4 and CoWO 4 paired with the multivalence qualities might prevent the effects outlined above [12].As a result, we reported here the synthesis of CoWO 4 /FeWO 4 composite as a photocatalyst for wastewater remedy and photocatalytic hydrogen evaluation by water splitting.
According to our knowledge, based on the literature study, there is no study reference of CoWO 4 /FeWO 4 composite for photocatalytic degradation of organic pollutants and hydrogen evaluation by water splittingrelated applications.Now we have a general understanding, let us move on to the investigation of the synthesis of CoWO 4 /FeWO 4 and their effects on crystallinity, morphology, and photocatalytic activity for the degradation of organic pollutants MO and CR in visible light irradiation and Hydrogen evolution by photocatalytic water splitting.The recycling ability of the CoWO 4 /FeWO 4 (1:2) composite reveals the photocatalyst's excellent photostability.The optimum values for the photocatalytic application of CoWO 4 /FeWO 4 have been investigated and presented in this study.

Experiments
The following chemicals were used for the completion of research work without any further purification.Iron (III) nitrate nonahydrate [Fe (NO 3 ) 3 .9HAfter the heating reaction autoclave was put in the open air to attain ambient temperature.Finally, the collected precipitates were rinsed with deionized water and absolute ethanol thrice each and dried the obtained product in a heating vacuum oven for 8 h at 100°C.The dried sample was then ground using a mortar and pestle before characterization.The proposed synthesized schematic diagram is illustrated in Figure 1.

Characterization
X-ray diffraction using a Bruker D8 Advance diffractometer operating at 40 kV and 80 mA with a scanning speed of 2°min −1 from 20 to 70°, equipped with Cu-Kα radiation (λ = 1.5418Å) was used to confirm the crystal structure and crystallinity of the collected samples.The morphology of the samples was examined by field emission scanning electron microscopy (FESEM, Hitachi-S5500) and the elemental validity is confirmed, using energy-dispersive X-ray spectroscopy.Nicolet iS50 offered a Fourier Transform Infrared (FT-IR) spectrum in the 650-4000 cm −1 range for the linked functional group.For the assessment of the Brunauer-Emmett-Teller (BET) surface area of the powder samples, the N 2 adsorption-desorption isotherm curve at 77.55 K was obtained using a Micromeritics ASAP 2010 system.Photoluminescence (PL) scans were recorded using a Hitachi luminescence spectrometer (F-4500) at room temperature using a 532 nm He-Cd laser as the excitation source.UV-vis absorbance scans were executed between 400 and 800 nm using a Perkin Elmer Lambda-35 spectrometer [33][34][35][36][37].

Photocatalytic activity
To investigate the photocatalytic response of powdered samples to degrade dyes, a Xenon lamp with an accumulative intensity of 300W was utilized.For each experiment, a dye stock solution of 15 mg/L concentration was prepared, and 5 mg of the sample was added to the dye solution.Before being exposed to 400-800 nm visible light, the dye solutions were briskly mixed in the dark for 30 min.Before the experiment began, the pH of the liquids was held constant at 7. The reactor was below 30 cm from the lamp bulb.Subsequent light exposure of the dye and photocatalyst combination, samples (4 mL) were collected at regular intervals for optical density measurement.During the studies, a nitrogen cooling system HX-DC1006 was used to avoid thermal damage and keep the temperature at 0 °C.The solutions were then centrifuged at 5000 rpm for 3-5 min.A Perkin Elmer lambda 35 UV-Vis spectrometer in the 400-800 nm range was used to study the degradation of azo dye solutions [33,[38][39][40].
The synthesized CoWO 4 /FeWO 4 by facile hydrothermal composites were employed for the photocatalytic response.The UV-Vis spectrophotometer was applied for the measurement of absorption.The percentage of For quantitative analysis and to study the kinetics of dye degradation pseudo-first-order model was applied as represented in Equation 3 [43].This model is generally applied when pollutant concentration is very low [44,45].
Where C o and C t are the initial, at a later time "t" dyes concentrations and "k" is the pseudo-first-order rate constant.

Photocatalytic hydrogen production
An XPA-7 photocatalytic reactor was used for the photocatalytic hydrogen generation test.A 0.01 g dosage of the photocatalysts and 10.0 mL of a water solution containing 15% methanol were mixed in a 20 mL Quartz bottle sealed with a silicone rubber septum in the usual method.The sample solution was fully deaerated by evacuation and purged with nitrogen for 10 min before the photocatalysis experiment.The sample was then exposed to a Xenon lamp with an accumulative intensity of 300W for 8 h at room temperature while being constantly stirred.Gas chromatography (FULI 9750, TCD) was used to examine the produced gas.A chemical's apparent colour is directly influenced by its optical absorption or reflection.Absorption spectroscopy investigates transitions from the ground state to the excited state, as opposed to fluorescence spectroscopy, which analyses transitions from the excited state to the ground state [50].Molecules possessing bonding and non-bonding electrons (n-electrons) can drive these electrons to higher anti-bonding molecular orbitals by absorbing energy in the form of ultraviolet or visible light.The optical band gap energy is calculated through the Tauc relation as follows;

Result and discussion
Where A is a constant, hν is the photon energy, e.g. is the bandgap energy, and α is the absorption coefficient.The calculation formula is: Here, Ab is absorbance and, t is the thickness of the cuvette.The graph of (αhν) 2 versus hν is represented in Figure S1 (ESI  † ).

Photocatalytic activity
The absorbance of two different dyes, Methyl Orange (MO) and Congo Red (CR), was used to measure the photocatalytic degradation capability of the asprepared CoWO 4 /FeWO 4 composites.MO and CR are both azo dyes, which means they are benzene-derived dyes.These dyes are known to be harmful and are commonly used to colour leather, fabrics, and some foods [51,52].MO's usual absorbance peak is orientated at 464 nm, whereas CR's typical absorbance peak is increased at 496 nm [53].The peak intensity level decreased with time, as seen in Figures 5 and 6(a), and the bleaching of the dye solution indicates the dye breakdown [54].Under visible light for 150 min, the absorption of MO dye drops to 0.00026 gL −1 .Meanwhile, the CR was degraded by 99.61% in 150 min.In comparison to MO dye, CR dye decomposed more primitively.The photocatalytic experiment with both dyes was performed three times, showing the recyclability and stability of the CoWO 4 /FeWO 4 composite photocatalyst (Figures 5 and 6(b)).To understand the influence of visible light during the photocatalytic process, the degradation was studied without a photocatalyst.Figures 5 and 6   in the absence of a CoWO 4 /FeWO 4 composite photocatalyst, indicating that photolysis may be ignored for this investigation.Meanwhile, CoWO 4 /FeWO 4 have low photocatalytic activity in the absence of visible light radiation.Figures 5 and 6(a) exhibit a visual record of the concentration ratio vs irradiation time, indicating that the MO dye degradation includes almost 20% and the CR nearly 22%, respectively, in 150 min without light.As a result of the foregoing photocatalytic experiments, it is clear that visible radiation significantly reduces the degradation of both dyes.Because of the enhanced crystallinity, large specific surface area, and improved charges separation carrier lifespan, the photodegradation reaction of CoWO 4 /FeWO 4 of ratio 1:2 shown good activity.For kineticks study the scedio-firstorder model was applied and it gives the rate constant "K" values 0.0231 and 0.00918 min −1 for CR and MO dyes respectively.

Photocatalytic mechanism
Typically, three steps are involved in the photocatalytic mechanism: the first is dye diffusion, in which dye specks move from aqueous to the outside surface of the catalyst, the second is interior diffusion, in which dye corpuscles are adsorbed by the outer layer to the inside pores of the catalyst, known as intraparticle diffusion, and the third is dye particle interaction with the intrinsical pores active locations of the catalyst [42,[55][56][57][58][59]. Figure 7 depicts a schematic illustration of the photo-aided deterioration process derived from the preceding discussion.Once a photon with energy equal to or greater than the band gap energy of the CoWO 4 /FeWO 4 photocatalyst produces an electron-hole pair, the electron runs to the conduction band (CB) of FeWO 4 where it interacts with oxygen (O 2 ) molecules and produces superoxide radical, which further interacts with the H 2 O 2 to produce hydroxyl radical, while the hole in the valance band (VB) of CoWO 4 interacts with a water molecule (H 2 O) and produces hydroxyl radicals, which further interact with organic pollutants and decompose the pollutants into less hazardous materials and minerals [60,61].The equations below express the broad chemical reaction phenomena.
To validate the active species involved in dye photodecomposition, radical scavenger analysis was performed.T-butanol for • OH radical scavengers and ascorbic acid for • O 2 radical scavengers were chosen for these purposes.The photosensitive species scavenging tests were identical to the photodegradation studies.In these tests, t-butanol and ascorbic acid were added to the dye solution before the catalysts were added.As shown in Figure 8(a and b), the improver of t-butanol had a significant effect on the photo debasement of MO and CR over CoWO 4 /FeWO 4 , showing that • OH is an important active species.Nevertheless, both dye solutions' responses to photocatalytic degradation weaken when ascorbic acid is present, suggesting that oxygen only had a very minor role in the photodegradation process [62].These results clearly demonstrated that MO and CR were photodegraded by the major chemical • OH species.
The Brunauer EmmettTeller surface area (A BET ) value was calculated from the 1:2 CoWO 4 /FeWO 4 BET isotherm and was about 129 m 2 /g as shown in Figure 9.This was found to be greater than that of CoWO 4 (83 m 2 /g) and FeWO 4 (89 m 2 /g) due to the particle size.Large A BET and pore volume are related to a high number of surface active sites, which promote reactant transport and improve photocatalytic activity.As a result, the remarkable photocatalytic activity of the composite may be explained by the substantial A BET and pore volume measured for CoWO 4 /FeWO 4 .The CoWO 4 /FeWO 4 isotherms hysteresis loops are resembling to type VI at high relative pressure between 0.05 and 1.0 [63,64].The hysteresis loop's shape is related to type H 3 , indicating that slit-like cavities are frequently generated by the aggregation of plate-like particles.The existing nanosheet structure provides highly efficient paths for transporting reactants and a large number of active sites for photocatalytic reactions, as well as improving photo energy harvesting efficiency and electron-hole pair segregation, the result of which is increased photocatalytic activity [65].
In the photocatalytic process, photocatalysts are activated to produce electron-hole pairs immediately after light illumination.The primary determinant of photocatalytic efficiency is the lifespan or recombination rate of the photogenerated electron-hole pairs.The time required for a chemical reaction decreases with the speed of recombination [66].The recombination rate of the photogenerated electron-hole pairs is therefore investigated using PL spectra.The measurements of the PL spectra of CoWO 4 /FeWO 4 are used to examine the separation capacity of the photogenerated carriers in the heterostructures; the findings are  displayed in Figure 10.The as-prepared samples show a prominent emission peak at around 467 nm that may have resulted from band transitions' direct electron-hole recombination.However, the characteristic emission peak for CoWO 4 /FeWO 4 (1:2) at low intensity 467 nm implies that the recombination of photogenerated charge carriers is relentlessly restricted.The emission peak intensities of CoWO 4 after coupling with FeWO 4 dramatically lowered, demonstrating that the introduction of FeWO 4 significantly reduces the recombination of electron-hole pairs.One explanation might be that coupling with FeWO 4 could alter the path taken by photogenerated electrons to reach the CoWO 4 ground state as a result of interactions between CoWO 4 and FeWO 4 .Effective charge separation may boost the efficiency of interfacial charge transfer to the adsorbed substrates and extend the life of charge carriers, hence improving photocatalytic activity [67].CoWO 4 /FeWO 4 (1:2) composites have the lowest relative PL intensities, as seen in Figure 10.This indicates that they have a lower rate of electron-hole pair recombination, which contributes to their better photocatalytic activity, as shown in Figures 5 and 6.The PL results support the significance of the composites in preventing the recombination of electrons and holes and explain why CoWO 4 /FeWO 4 composites have improved photocatalytic activity.

Hydrogen production
In an XPA-7 photocatalytic reactor using methanol as a sacrificial agent and a 300 Xenon lamp light source with a cutoff of 420 nm illumination for 8 h, the photocatalytic water-splitting activity of the as-prepared catalysts was investigated.When compared to CoWO 4 and FeWO 4 , the CoWO 4 /FeWO 4 with a ratio of 1:2 composite exhibited more hydrogen under visible light (Figure 11(a)).The CoWO 4 /FeWO 4 sample with a ratio of 1:2 had the greatest photocatalytic activity.Under ideal reaction circumstances, the hydrogen production efficiency is 36 μmolg −1 h −1 under visible light irradiation.After five photocatalysis cycles, the CoWO 4 /FeWO 4 with a ratio of 1:2 the sample retained photocatalytic activity.This demonstrates its high chemical stability (Figure 11

Conclusions
In summary, FeWO 4 , CoWO 4 , and their composites FeWO 4 /CoWO 4 at various ratios were fabricated using a hydrothermal method.Through the use of XRD, SEM, UV-visible, FTIR, EDS, BET and PL spectroscopy, the manufactured materials were physiochemical characterized.According to the XRD data, FeWO 4 has an orthorhombic form, CoWO 4 is monoclinic, and FeWO 4 /CoWO 4 composites have higher crystallinity.SEM images show the irregular morphology of synthesized CoWO 4 /FeWO 4 composites.CoWO 4 , FeWO 4 , and their composites showed maximal absorption peaks in the visible spectrum at wavelengths of 525, 574, and 613 nm, respectively.The composite with a ratio of CoWO 4 /FeWO 4 (1:2) shows efficient degradation of CR dye under visible light in 150 min.The as-prepared photocatalyst also exhibited a hydrogen evolution rate production efficiency of up to 36 μmolg −1 h −1 under visible light over the 1:2 CoWO 4 /FeWO 4 composite.This may be attributed to the efficient separation of electron-hole between the heterostructure CoWO 4 /FeWO 4 composite.This work may open new possibilities for the use of CoWO 4 /FeWO 4 as a photocatalyst for wastewater remedy and hydrogen evaluation by water splitting under visible light in future.

Figure 1 .
Figure 1.Schematic illustration of the experimental setup for preparation of CoWO 4 /FeWO 4 composites at different ratios.

CoWO 4 and
FeWO 4 vibrational and stretching bands and these coincide with earlier reports that indicated the primary absorption bands of wolframite type structure.The stretching and bending vibrations of the water molecules O-H and H-O-H absorbed on the sample's surface are responsible for the peak at 3418.3 cm −1 [47-49].As a result, the outcomes of Figure 4 provide evidence supporting the creation of CoWO 4 /FeWO 4 as a product of the hydrothermal method.

Figure 7 .
Figure 7. Schematic diagram of the proposed mechanism involved in the photocatalytic processes of CoWO 4 /FeWO 4 .

Figure 8 .
Figure 8. Effects of different radical scavengers on (a) MO and (b) CR degradation over CoWO 4 /FeWO 4 under visible light Irradiation.

Figure 9 .
Figure 9. N 2 absorption-desorption curve for the Surface area (inset) differential pore size distribution.
2 O], Cobalt (II) chloride hexahydrate [CoCl 2 .6H 2 O], Sodium tungstate dihydrate [Na 2 WO 4 .2H 2 O], Deionized Water [H 2 O] and Polyethylene glycol [H(CH 2 CH 2 O) n OH].All the chemicals were of analytical grade (Merck, Germany), purchased from a local vendor (Azeem Scientific store, Pakistan), and used without further purification.The sample preparation was made by using the following formula for the concentration.