High-performance photocatalytic reduction of Cr(VI) using a retrievable Fe-doped WO3/SiO2 heterostructure

The enhancement of the photocatalytic performance of pristine WO3 was systematically adjusted due to its fast recombination rate and low reduction potential. A designed heterostructure photocatalyst was necessarily synthesised by Fe3+ metal ions doping into WO3 structure with and composition modification. In this study, we synthesised a retrievable Fe-doped WO3/SiO2 heterostructure using a surfactant-assisted hydrothermal method. This heterostructure was then employed as an effective photocatalyst for the removal of Cr(VI) under visible light irradiation. Enlarged photocatalytic reduction was observed over a synergetic 7.5 mol% Fe-doped WO3/SiO2-20 nanocomposite, resulting in dramatically increased activity compared with undoped WO3 and SiO2 nanomaterials under visible light illumination within 90 min. The presence of 7.5 mol% Fe3+ ion dopant in WO3 optimised electron–hole recombination, consequently reducing WO3 photocorrosion. After adding SiO2 nanoparticles, the binary WO3-SiO2 nanocomposite played roles as both adsorbent and photocatalyst to increase specific surface area. Thus, the 7.5 mol% Fe-doped WO3/SiO2-20 nanocomposite catalyst had more active sites on the surface of catalyst, and enhanced photocatalytic reduction was significantly achieved. The results showed 91.1% photocatalytic reduction over the optimum photocatalyst, with a photoreduction kinetic rate of 21.1 × 10–3 min−1, which was approximately four times faster than pristine WO3. Therefore, the superior optimal photocatalyst demonstrated reusability, with activities decreasing by only 9.8% after five cycles. The high photocatalytic performance and excellent stability of our photocatalyst indicate great potential for water pollution treatments. Supplementary Information The online version contains supplementary material available at 10.1186/s11671-023-03919-0.


Introduction
Stainless steel manufacturing has seen a rapid increase in the use of chromium (Cr).Most stainless steels contain approximately 18% Cr to harden and toughen the alloys and increase their corrosion resistance, especially when used at high temperatures [1].This has, however, led to Cr contamination, affecting living organisms in both soil and aqueous environments [2].Several methods have been employed to remove Cr 6+ from industrial wastewater, including chemical and electrochemical precipitation, filtration, and ion exchange [3], prior to waste discharge.Among these, advanced oxidation processes (AOPs) are used in conjunction with semiconductor photocatalysts such as TiO 2 , ZnO, and WO 3 .
These photocatalysts have been widely considered viable in the treatment of less hazardous organic pollutants, such as azo dyes, and heavy metal contaminants in wastewater effluents.Notably, WO 3 , an n-type semiconductor with a band gap energy of 2.9-3.2eV [4], is widely used in photocatalysis owing to its low costs, eco-friendliness, excellent physicochemical stability, electrochromic properties, and ease of synthesis [5][6][7].Therefore, its use in visible-light-harvesting photocatalysis has become a major focus of research [8,9].Various types of controlled WO 3 morphologies incorporating single transition metal dopants or two metals as co-dopants can be obtained via different synthetic techniques-these semiconductors (offering different band gap energies) can be combined with supporting materials, such as mesoporous SiO 2 nanoparticles (NPs), to form heterostructures that raised the light-harvesting ability of the photocatalyst [10].In the past decade, several researchers have reported on the superior performance of metal-ion-doped WO 3 photocatalysts and (material-supported) W-/metal oxide-based heterostructures in the photocatalytic degradation of wastewater pollutants, including various organic dyes [11][12][13][14] and toxic Cr 6+ contaminants, [15][16][17][18], hydrogen production [19,20], and enhanced gas-sensing performance [21].Considering the above-mentioned literature sources and the advantages and drawbacks of photocatalysts based on WO 3 , metal doped WO 3 , and heterojunctions with SiO 2 , this work aimed to improve upon the WO 3 photocatalyst, implementing heterojunction modifications of the WO 3 semiconductor by blending with SiO 2 NPs and further doping with Fe 3+ ions to improve the photocatalytic reduction efficiency of Cr 6+ .Notably, SiO 2 NPs display high purity and high specific surface areas of 175-225 m 2 g −1 , not only facilitating the adsorption of organic compounds but also assisting in the dispersal of metal oxide particles [22].Therefore, the SiO 2 -supported W-based or WO 3 heterostructure photocatalyst in our above-mentioned research has shown superior performance over other photocatalysts in both energy and environmental applications [23], such as the photocatalytic degradation of herbicides and organic dyes and the removal of Cr 6+ via photoreductive conversion into the less toxic Cr 3+ [24].Recently, nanosized SiO 2 has been utilised in nanomaterial fabrication and semiconductor modification.Further, SiO 2 NPs have been used as a support material for W-based photocatalysts due to their cost-effectiveness, easy availability, high mechanical stability, and transparency in the excitation spectral range of WO 3 [X].To the best of our knowledge, our present work is novel in terms of the iron dopant percentage/loading and the combination of a SiO 2 support with high-purity WO 3 , yielding a reproducible method of photocatalyst synthesis for the photoreduction of the hazardous Cr 6+ to the less toxic Cr 3+ [25].The superior reusability of the optimal catalyst after five cycles reveals its high efficiency and stability.Moreover, the Fedoped WO 3 /SiO 2 heterostructure photocatalyst was synthesised via the notably inexpensive CTAB-assisted hydrothermal technique, exhibiting enhanced photocatalytic potential for wastewater treatment under visible light irradiation from a very low-intensity and low-cost halogen lamp.
Thus, the heterostructure Fe-doped WO 3 /SiO 2 nanocomposites were synthesised via hydrothermal treatment with Fe 3+ doping in this study.The weight ratio of Fe-doped WO 3 to SiO 2 was varied.The physicochemical properties of the prepared samples were characterised to describe the effects of the promoted photocatalytic removal of Cr 6+ .Additionally, the photocatalytic reduction of a toxic Cr 6+ solution over the prepared retrievable nanocatalyst was evaluated under visible light illumination (λ > 400 nm), which determined the kinetic rate of Cr 6+ photoreduction.In addition, the photocatalytic reduction efficiencies (%PE) and kinetic rate (k t ) values were also evaluated and compared under the effect of catalyst dosage and the pH of the suspension.Furthermore, the cycling ability of the optimum photocatalyst was also investigated.

Synthesis of Fe-doped WO 3 /SiO 2 heterostructure NPs
Pure WO 3 , Fe-doped WO 3 , and Fe-doped WO 3 /SiO 2 heterostructures were prepared via a surfactant-assisted hydrothermal and impregnation procedure.First, monoclinic WO 3 nanostructures were synthesised using a CTAB-assisted hydrothermal method.Briefly, a certain amount of Na 2 WO 4 ⋅2H 2 O was dissolved in DI water with constant stirring to obtain a clear solution (A).Subsequently, a 0.17 M aqueous CTAB solution was prepared and slowly added to solution (A).The conc.HCl solution was then applied dropwise for pH adjusted to 3.0 ± 0.3 using under vigorously stirring.Thereafter, the mixture was transferred into a Teflon-lined stainless-steel autoclave and kept at 200 °C for 12 h.The precipitate was washed several times and dried at 80 °C for 12 h using an air oven.Finally, the synthesised powder was calcined at 600 °C.The Fe-doped WO 3 NPs were prepared via wet impregnation using Fe(III)Cl 3 metal ions precursor, with doping concentrations of 2.5, 5.0, 7.5, and 10.0 mol% of Fe 3+ contents.The solutions with different concentrations of Fe 3+ solution were each added to continuously ground, dried, and sintered at high temperature [26].Initially, the optimum Fe-doped WO 3 sample was evaluated via the photoreduction of Cr 6+ prior to prepare the Fe-WO 3 /SiO 2 nanocomposite.The 7.5 mol% Fe-doped WO 3 sample displayed the highest photoreduction efficiency as shown in the supporting information (Fig. S1).Therefore, it was used in the further fabrication of the Fe-WO 3 / SiO 2 heterostructure catalyst.Figure 1 schematically demonstrates the synthesis process of the 7.5% Fe-WO 3 /SiO 2 -20 composite catalyst nanoparticles.The nominal weight ratio of 7.5 mol% Fe-doped WO 3 :SiO 2 was set at 50:50 and 80:20 wt%.The samples were prepared by directly mixing stoichiometric amounts of Fe-doped WO 3 and commercial SiO 2 nanopowder (99%, Sigma-Aldrich).The stoichiometric SiO 2 powder was sonicated in 90 mL of DI water and then directly mixed with 7.5 mol% Fe-doped WO 3 with stirring to obtain a homogeneous mixture with pH adjust of 3.0 by using HCl solution.The mixture was poured into a 350-mL Teflon-line autoclave and kept at 200 °C for 12 h.The precipitates were separated, milled to acquire a dried powder.The calcination characteristics of both ratios of the Fedoped WO 3 /SiO 2 nanocomposites were investigated at same ambient mention above.The obtained photocatalysts were denoted to SiO 2 , WO 3 , 7.5% Fe-WO 3 , 7.5% Fe-WO 3 /SiO 2 -50, and 7.5% Fe-WO 3 /SiO 2 -20.

Photocatalytic reduction experiments
Following the standard procedure, 0.10 g of the catalyst was suspended in 100.0 mL of 20.0 mg•L −1 Cr 6+ solution (ICP grade).The solution's pH was then adjusted to 3.0 using 0.10 M HClO 4 and 0.10 M NaOH.Prior to light exposure, the mixture was continuously stirred in the dark for 30 min to reach adsorption-desorption equilibrium.Subsequently, the solutions were irradiated using a 50-W halogen lamp (12 V, Phillips, Netherlands) with a light intensity of 364 W.m −2 .At specific time intervals, 4.0 mL of the reaction suspension was withdrawn and filtered through a 0.22-μm micro-syringe nylon membrane (Pro Filter, China).The concentration of Cr 6+ after reduction was determined colourimetrically using 1,5-diphenylcarbazide (99.0%,Loba Chemie) by measuring the characteristic UV-Vis absorption peak at 554 nm, detected by a PG-92 + UV-Vis spectrophotometer (Germany).The %PE was calculated using Eq. 1 in our previous work [27]: where C 0 is the initial concentration (mg L −1 ), and C t is the concentration at time (mg L −1 ).The photocatalytic kinetic reaction was investigated using the pseudo-first-order kinetic model of Eq. 2 as follows: where k t represents the apparent pseudo-first-order rate constant (min −1 ).Furthermore, the reusability of superior nanocomposite catalyst compared with other NPs catalysts was also evaluated towards the photoreduction of Cr 6+ solution for 5 cycles under the same ambient.In each cycle, to investigate the stability of the binary NPs catalyst.The non-magnetic sample was centrifuged the supernatant after first cycle run, sedimented, and washed by.After each cycle, the catalyst particles were recovered by centrifugation and then washed with 95% ethanol and hot water DI water (70 °C) for 3 several times [28].The photocatalyst was then dried at 60 °C for 12 h in an air oven and then ground to a fined power.Subsequently, the catalyst was immersed and dispersed into a fresh Cr 6+ solution prior to the next run.After 5 cycle tests, the dried precipitate photocatalyst had demonstrated the stability and recyclability against the chemical and photocorrosive property.The structural and phase crystallinity of reused catalyst was confirmed by the FE-SEM and XRD characterisation.In addition, Inductively Coupled Plasma-optical emission spectroscope (ICP-OES; PerkinElmer-4300 DV) was confirmed to determine Fe ions leaching in the supernatant Cr 6+ solution.
(1) The deviation values of the Fe-WO 3 samples decreased towards lower angles compared with the pristine WO 3 pattern.This could be attributed to the ionic radii of the lattice coordinated Fe 3+ and W 6+ ions and the substitution of the latter with the former because the ionic radii of Fe 3+ (64 pm) and W 6+ (62 pm) were very similar [31].In the meantime, the two broad peaks of the nanocomposite catalysts (20% and 50%) greatly collapsed, causing the (202) peak's strength to decrease by 10%.Furthermore, when SiO 2 was added, the XRD diffractogram of Fe-WO 3 /SiO 2 -20 showed noticeably less crystallinity than the pristine monoclinic WO 3 diffraction pattern.As 20% and 50% SiO 2 loading into the WO 3 crystal structure occurred, it was evident that the normalised intensities of various peaks collapsed and disappeared, generating the emerging phase.The presence of SiO 2 NPs combination was noticeable in the X-ray diffraction patterns of the Fe-WO 3 , Fe-WO 3 /SiO 2 -20, and Fe-WO 3 /SiO 2 -20 composite samples, which demonstrated a decrease in crystallinity degree and a lower peak intensity in the overall diffraction peaks [32].Consequently, XRD diffraction peak might be attributed to the emergence of recrystallisation between amorphous SiO 2 NPs and WO 3 .Moreover, it supposes that the peaks disappeared, the intensities decreased, and that broadening peaks at 2θ of approximately 23.9° and 33.42°, respectively, formed and initiated the combination of two broadening peaks (Fig. 2b and c).This possibility of SiO 2 covering intensity reduced, and some peaks disappear and might be formed to the new phase.Secondly, there is the possibility for SiO 2 to impregnate the WO 3 surface because the amorphous SiO 2 particles might interact or disturb the lattice planes of WO 3 crystal structure [33].The crystallite sizes of the obtained samples calculated from Scherer's equation [34]  It is evident that Fe-WO 3 /SiO 2 -20 has a drastically smaller scale compared to pure WO 3 .This observation supported the above assumption that the loading of Fe 3+ ions and SiO 2 can inhibit WO 3 grain growth, thereby resulting in a smaller crystalline particle size [35].
A small shift at the absorption onset and broadening of the absorption tail towards visible light region were observed upon doping WO 3 /SiO 2 with Fe 3+ .The KM absorption spectra were calculated using Eq.3: where R ∞ is the absolute diffuse reflectance for an infinitely thick sample.The UV-Vis DRS spectrum of amorphous SiO 2 exhibited very weak absorption intensity (Fig. 3a inset), whereas the ionic WO 3 NPs samples displayed high absorption intensity, as shown in Fig. 3a.The band gap energy (E g ) of the catalysts can be obtained as illustrated in Fig. 3b and calculated using Eq. 4 as our latest work as follows: where α is linear absorption coefficient, h is the Planck's constant, υ is light frequency, A is the proportionality constant, and E g is the band gap energy.The power of parenthesis is ½ for direct band gap WO 3 semiconductor, respectively, reported in Table 1.
Attribution to intrinsic to the band gap absorption can be determined by a linear extrapolation to the energy axis as shown in Fig. 3b.From the band gap energy values obtained by the intercept X-Y of the DR-UV-Vis spectra (Fig. 3b) were    [38].The results suggested that the optimum loading of SiO 2 was 20 wt%, as the large specific surface area, the larger pore volume than pure WO 3 , and the small particle size could enhance the adsorption of Cr 6+ .Thus, the raising Cr 6+ photocatalytic removal performance had revealed [39].

Morphology and microstructure
The morphologies and microstructures of selected nanomaterials were imaged as (i) SEM and EDS as shown in Fig. 5a-e and (ii) TEM micrographs in Fig. 6a-i, respectively.The microstructures of the samples are shown in Fig. 5.An agglomeration of small SiO 2 NPs is distinguished in the SEM image in Fig. 5a, whereas irregular rod-like WO 3 NPs, with an average diameter of 50 nm, can be seen in Fig. 5b. Figure 5e displays the EDS spectra of the 7.5% Fe-WO 3 /  2a.By contrast, WO 3 (Fig. 5d) exhibited a rod-like shape, with particle sizes in the range of 60-80 nm.Referring to Fig. 6d-f, the polycrystalline characteristic of WO 3 was confirmed from the SAED ring pattern and lattice fringes of 0.21, 0.20, and 0.30 nm belonging to the (020), (200), and (110) crystal planes of monoclinic WO 3 , respectively.Pore size is a component of scaffolds, with the pores providing a region to which cells can adhere or become attached (EDS) [40].Furthermore, TEM and HRTEM micrographs, as well as SAED patterns (Fig. 6), were obtained to reveal the particle sizes and internal microstructures of the samples (Fig. 6a-f ).Regarding 7.5% Fe-WO 3 /SiO 2 -20 NPs (Fig. 6g-i) and SiO 2 embedded in irregular WO 3 NPs, electron diffraction patterns (SAED) revealed the particle sizes and internal microstructures of the NPs of WO 3 , with an average diameter of 10-20 nm.[43].A similar negative shifted of the O 1s peak was also observed attributed to the rise of the lowerlevel occupancy of oxygen conduction band [44].Figure 7c represents the single Si 2p peaks at 103.9 eV for pure SiO 2 , corresponding to Si 4+ [45].The lattice oxygen species at ca. 531-532 eV [46] and the adsorbed water/hydroxyl species at ca. 532-533 eV [47]  ) in Fe 2 O 3 [50], respectively.The B.E. shift of XPS results could be ascribed to the divergent values between of Fe and W electronegativity (EN) [51].For the EN of Fe and W comparison, compared with the Fe-O-Fe bond, bonding between Fe and O could have been slightly enhanced in the Fe-O-W bond, hence resulting in the increase the redshift B.E. of the Fe 3+ peaks.Conversely, the bonding between W and O was slightly decreased in Fe-O-W opposed to W-O-W.From aforementioned, XPS peak shifts slightly because W 6+ cations were replaced by Fe 3+ .Thus, Fe-O-W bond was formed.Particularly, the metal doping and intercalated doping in WO 3 express enhanced crystal structure in preference to surface modification [52].

Photocatalytic reduction activities and kinetics studies
The photocatalytic reduction of the concentrations was performed by varying the overall synthesised photocatalysts.The SiO 2 , WO 3 , 7.5% Fe-WO 3 , 7.5% Fe-WO 3 /SiO 2 -20, and 7.5% Fe-WO 3 /SiO 2 -50 heterostructures were obtained over 50 ppm of the Cr 6+ concentration with a photocatalyst dosage of 1.0 g•L −1 and a pH of 3.0 under 90 min of visible light illumination at room conditions.Figure 8a represents the photoreduction of Cr 6+ efficiencies over visible light illumination over overall catalysts, and the first-order rate constants (k t ) were calculated and are similarly displayed in Fig. 8b.Moreover, the effect of photocatalyst dosage, pH of the suspension was also evaluated via the optimal result from %PE by using various type of photocatalyst.Furthermore, the usability test of photoreduction via the superior PE photocatalyst had studied together.The deactivation of used catalyst was confirmed by XRD patterns and FE-SEM after 5 cycles run.
The evaluation of the %PE and k t of the Cr 6+ reduction with the error bars of the synthesised photocatalysts is illustrated in Fig. 8a and b, respectively.Prior to %PE determination, 7.5% Fe-doped WO 3 /SiO 2 -20 demonstrated the highest Cr 6+ removal activity of 91.1% within 90 min.The improved photocatalytic efficiency of the WO 3 /SiO 2 nanocomposite was due to the presence of SiO 2 NPs.As shown in Fig. 8a, it could be assumed from the evidence that by adding SiO 2 to the composite, SiO 2 acted as an adsorbent, which sufficiently enlarged the surface area of WO 3 , representing the main role of SiO 2 as a high-surface-area catalyst support to generate well-dispersed Fe-WO 3 NPs.The increased surface area of WO 3 can facilitate the adsorption of organic pollutants through the formation of more useful adsorption-degradation sites [53].Additionally, an Fe-WO 3 /SiO 2 nanocomposite is a powerful photocatalyst for separating photogenerated electrons and holes, which is significant for photocatalytic activity.However, further adding SiO 2 reduced the activity to 81.0%, presumably since the composite catalyst contained silicon as the main element.Silicon can significantly decrease the crystallinity of WO 3 in composite materials by acting as a WO 3 grain growth inhibitor [54].The calculated results of the first-order rate constant (k t ) versus irradiation time are shown in Fig. 8b and listed in Table 2.The largest rate constant of 22.5 × 10 -3 min −1 was observed for the 7.5% Fe-WO 3 /SiO 2 -20 nanocomposite, which was larger than that of 7.5% Fe-WO 3 without SiO 2 (10.9 × 10 -3 min −1 ).Thus, the 7.5 mol% Fedoped WO 3 -20 catalyst was chosen for further investigation of the effects of various parameters on photoreduction efficiency.Furthermore, an evaluation of the photocatalytic reduction of the Cr 6+ solution influenced by two factors,

Effect of the photocatalyst dosage
The photocatalytic reduction of Cr 6+ was conducted with the 7.5% Fe-WO 3 /SiO 2 -20 photocatalyst by varying the catalyst dosages at 0.25, 0.5, 1.0, and 1.5 g L −1 , and the rate constant (k t ) was also evaluated.As the % PE and k t results in Fig. 9a-b illustrate, the %PE of the Cr 6+ solution (50 ppm) decreased in the following order: 1.0 g L −1 (91.2%) > 1.5 g L −1 (86.8%), > 0.5 g L −1 (60.2%), and > 0.25 g L −1 (38.9%).The results revealed that with catalyst loading increments from 0.25 to 1.0 g, an increase in %PE was found, and after further increasing catalyst loading to 1.5 g•L −1 , % PE was reduced to 86.6%.This was due to the increase in the catalyst content, which reached a higher quantity of active sites on the photocatalyst surface for the photoreduction reaction.However, upon increasing the catalyst loading to 2.0 g L −1 , the photodegradation performance precipitously decreased due to the agglomeration and sedimentation of the catalyst particles, which caused an increase in the particle size and decrease in specific surface area, thus leading to a decrease in the number of active sites [55].The excess catalyst content may have led to opaqueness and turbidity, possibly blocking the irradiation light, and increasing light scattering of the catalyst particles [56].

Effect of the pH of the suspension
The photocatalytic reduction performance of Cr 6+ with the 7.5% Fe-WO 3 /SiO 2 -20 photocatalyst by changing the pH of the suspension to 3.0, 5.0, 7.0, and 9.0 was investigated, as shown in Fig. 10a-b.The optimum pH was 3.0, as it reached the highest reduction activity of 91.9% and 22.3 × 10 -3 min −1 kt compared with suspensions with higher pH.The zeta potential value versus the pH of the catalyst suspension is displayed in Fig. S2.It can be seen that the PZC value of the 7.5% Fe-WO 3 /SiO 2 -20 photocatalyst suspension was approximately − 0.37 mV at pH 5.3.The catalyst surface revealed a negative charge below pH 5.3, and the electrostatic repulsion between the negative charge of the catalyst particles and the positive Cr 6+ ions suggested the improvement of photocatalytic reduction activity [57].Consequently, the surface of the nanocomposite was negatively charged at pH 3; therefore, the adsorption and enhanced photocatalytic reduction of the Cr 6+ ions were strongly beneficial because of the strong electrostatic repulsion and charge attraction between the charges of the photocatalyst particles and the Cr 6+ cations [58].Therefore, the optimum condition of photocatalytic reduction of 50 ppm of the Cr 6+ solution with the 7.5% Fe-WO 3 /SiO 2 -20 photocatalyst and 1.0 g L −1 catalyst loading at pH 3 illustrated the greatest efficiency for 90 min of visible light illumination.
The result from the cycling runs of the photoreduction of the Cr 6+ solution with the 7.5% Fe-WO 3 /SiO 2 -20 nanocomposite under visible light irradiation and a catalyst loading of 1.0 g L −1 are demonstrated in Fig. 11a.The %PE demonstrated reduced only 9.3% after 5 cycle runs but fresh and but the spent-catalysed and WO 3 products were also compared five times of 7.5% Fe-WO 3 /SiO 2 -20 sample a prepared fresh sample including the change XRD patterns crystallinity and 3D microstructure result from FE-SEM results.The XRD pattern of 5-spent time catalyst was demonstrated the good crystallinity, and XRD pattern of the used 7.5% Fe-WO 3 /SiO 2 -20 nanocomposite was found to decrease in the (202) peak, with an intensity of ~ 14% as shown in Fig. 11b.Meanwhile, Fig. 11c shows a minor change in noise pattern was observed the of the agglomeration of SiO 2 NPs together with the rod-like WO 3 like the fresh one in comparison with the un-catalysed nanocomposite.
However, the 5-spent times runs of the optimal catalyst observed a minor change of the photoreduction efficiency.There result indicates the 7.5% Fe-WO 3 /SiO 2 -20 does not suffer from the photocorrosive and chemical corrosive properties during the reduction towards Cr 6+ reactions.Moreover, the leaching of Fe ions from the 7.5% Fe-WO 3 / SiO 2 -20 was not found in the supernatant of Cr 6+ solution after determined by ICP-OES analysis [31].These results demonstrate that the heterostructure photocatalyst exhibits stability against both photocorrosion and chemical corrosion in acidic conditions.Consequently, it offers superior stability and efficient reusability for removing Cr 6+ effluent by using a retrievable Fe-doped WO 3 /SiO 2 -20% nanocomposite catalyst.This enhanced performance can be attributed to the inhibition of electron-hole recombination through the Fe 3+ dopant and an increase in the active Based on the results presented in Fig. 12, electron generation was observed to increase at the conduction band (CB) of the 7.5% Fe-doped WO 3 /SiO 2 -20% heterostructure.Inhibiting electron-hole recombination intensified the photoactivity of the WO 3 catalyst.This, in turn, resulted in enhanced photoreduction and modified photocatalysis efficiency [59].Table 3 compares the photocatalytic reduction of Cr 6+ or organic dye activity of the prepared Fe-doped WO 3 and WO 3 /SiO 2 in various nanocomposites was compared with previous reports [13,54,[60][61][62][63].
The photocatalytic reduction performance of a photocatalyst depends on the intrinsic properties of NPs in the photocatalyst, including the band gap energy of each photocatalyst, light source, type of reactant, and the initial concentration of Cr 6+ dye [65].Song et al. [61] had explained that the generated electrons could be excited within the visible light spectrum, contributing to the practical enhancement of photocatalytic performance, as shown in Eqs.5-8: The potential mechanism for e − /h + pairs separation at the 7.5% Fe-WO 3 -SiO 2 -20 surface under visible light irradiation is displayed in Fig. 12.As the results from calculated E g result from Fig. 3b, the conduction band (CB) and valence band (VB) potentials of WO 3 and SiO 2 can be determined using Eq. 9 in our previous work: where χ is the absolute electronegativity of the semiconductor (χ is 6.59 and 9.20 eV for WO 3 and SiO 2 , respectively), E C is the scaling factor relating the hydrogen electrode scale (NHE, bulk) to the absolute vacuum scale (AVS) (ca.4.5 eV vs. NHE), and E g is the band gap of Fe-WO 3 /SiO 2 -20 (2.70 eV) and SiO 2 (4.19 eV) obtained in this study.Using the equations mentioned above, the CB position and VB edges of Fe-WO 3 /SiO 2 -20 were calculated to be 0.74 and 3.44 eV, respectively.For SiO 2 , the VB and CB positions potentials were determined to be 2.60 and 6.79 eV, respectively.However, photoreduction was only achievable at the CB of Fe-WO 3 /SiO 2 -20 due to visible light generated only a single WO 3 semiconductor, which aligned the band gap energy.Moreover, the oxidation of h + could not oxidised with the hydroxide ions and water molecules in the aqueous Cr 6+ solution.Therefore, the potential of the accumulated electrons on CB of WO 3 was located at a lower band position than the reduction potential of O 2 /O 2 • (0.33 V vs. NHE) [66].The results indicated an increase in the generation of electrons in the CB of the heterostructure and highlighted its crystallinity, adequate morphology, specific surface area, and extrinsic adsorption capacity [36].The optimum loading of SiO 2 in Fe-doped WO 3 involved a concentration of 20 wt%.SiO 2 supporter acted great supporter to WO 3 NPs alike the type III heterojunction, and migration of charge carriers follows a similar pathway as in type II, resulting in no transference.However, SiO 2 could help to trap Cr 6+ ions on the interface and around the large surface area and high pore volume consisting of active surface under acidic condition.The photoinduced electrons initially reacted with the Cr 6+ present near the periphery of the nanocomposite catalyst, as shown in Eq. 8.As shown in Eq. 6, these highly reactive protons and electrons cumulatively reduce Cr 6+ to Cr 3+ .The purpose of the possible mechanism of the photocatalytic reduction of Cr 6+ by the 7.5% Fe-WO 3 /

Fig. 2 a
Fig. 2 a X-ray diffraction (XRD) patterns of overall nanocatalyst samples, b magnification of the XRD patterns in the 2θ range of 22.0°-25.0°,and c XRD patterns in the 2θ range of 30.0°-38.0°

Fig. 8 a
Fig. 8 a Photoreduction of Cr 6+ over overall catalyst and b first-order rate constant (k t ) of over samples

Fig. 9 a
Fig. 9 a Effect of the photocatalyst dosage on photoreduction of Cr 6+ efficiency over 7.5% Fe-WO3-SiO2-20 heterostructure photocatalyst b pseudo-first-order rate constant of (a)

Fig. 10 a
Fig. 10 a Effect of the pH of suspension on photoreduction of Cr 6+ efficiency over 7.5% Fe-WO 3 -SiO 2 -20 heterostructure photocatalyst b pseudo-first-order rate constant of (a)

Table 1
the undoped WO 3 had the lowest value of 7.68 m 2 •g −1 among all the prepared NP products.However, the SSA BET of 7.5% Fe-WO 3 /SiO 2 -50 became smaller than that of 7.5% Fe-WO 3 /SiO 2 -20.As shown in Table1, the crystallite size of pure WO 3 was larger than either 7.5% Fe-WO 3 , Fe-WO 3 /SiO 2 -20, or Fe-WO 3 /SiO 2 -50.After doping with Fe 3+ , the SSA BET of 7.5% Fe-WO 3 improved slightly to 8.39 m 2 g −1 .Incorporating 20 wt% of SiO 2 into Fe-WO 3 significantly increased the SSA BET to 50.38 m 2 g −1 ; however, further addition of SiO 2 to 50 wt% decreased the SSA BET to 21.29 m 2 g −1 .It might be possibly due to SiO 2 's ability inhibited WO 3 grain growth and disturbing the crystalline structure of monoclinic WO 3 resulting in smaller crystalline particles upon loading Fe 3+ and SiO 2

Table 3
Comparison of physiochemical properties and activities of photoreduction of Cr 6+solution and dyes, with different types and light sources of metal doped WO