Rapid reduction of nitroarenes photocatalyzed by an innovative Mn3O4/α-Ag2WO4 nanoparticles

A novel photocatalyst based on the design of P-N heterojunction between hollow spherical Mn3O4 and nanorods shape of α-Ag2WO4 is synthesized using a sonication-deposition–precipitation route. The nanocomposite Mn3O4/α-Ag2WO4(60%) exhibits a great potential towards nitroarenes (including 4-nitrophenol, 4-nitro-aniline and 4-Nitro-acetanilide) reduction under visible light irradiation exceeding that of Mn3O4/α-Ag2WO4(40%) as well as their individual counterparts (3–5%). The Mn3O4/α-Ag2WO4(60%) catalyst exhibited an excellent photo-reduction activity comprised of 0.067 s−1 towards 4-nitrophenol (0.001 M) in only 60 s reaction time using NaBH4 (0.2 M). This was due to the successful formation of the Mn3O4/α-Ag2WO4 composite as validated by XRD, TEM-SAED, XPS, FTIR, UV–Vis diffuse reflectance and PL techniques. Decreasing the Eg value into 2.7 eV, the existence of a new (151) plane in the composite beside enhancement of the composite electrical conductivity (1.66 × 10–7 Ω−1 cm−1) helps the facile nitroarenes adsorption and hydrogenation. Transient photocurrent response and linear sweep voltammetry results prove the facilitation of photogenerated charge carriers separation and transport via improving electron lifetime and lessening recombination rate. The composite photocatalyst produced higher amounts of H2 production, when inserted in a typical reaction medium containing NaBH4, comprised of 470 µ mole/g exceeding those of the counterparts (35 µ mole/g). This photocatalyst is strikingly hydrogenated 4-nitrophenol under mild conditions (25 °C and 0.35 MPa pressure of H2) with magnificent rate constant equal 34.9 × 10−3 min−1 with 100% selectivity towards 4-aminophenol.


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| (2020) 10:21495 | https://doi.org/10.1038/s41598-020-78542-5 www.nature.com/scientificreports/ different organic pollutants was achieved 7,[24][25][26] . The respects and recognition of the serious demand to discover families of new materials other than the well-studied oxide semiconductors such as TiO 2 and WO 3 have achieved considerable momentum recently. Accordingly, significant attempts have been dedicated to the shift of the V B and C B edge locations of different semiconductors ("bandgap construction") to modify their interfacial energetics to the targeted photooxidation or photoreduction processes. Apparently, neither Ag 2 WO 4 nor Mn 3 O 4 is employed in nitroarenes photoreduction unlike metal oxides supported noble metals [27][28][29][30][31] , which use to suffer a rapid decrease in the activity due to metal aggregation and leachability in addition to the noble metal high prices that limits its usage on a large-scale.
Based on the above knowledge, there are yet no articles devoted on the combination of Mn 3 O 4 with Ag 2 WO 4 to configure their impact on the photocatalytic reduction of nitroarenes under visible light illumination. The heterojunction based on Ag 2 WO 4 /Mn 3 O 4 is expected to possess an excellent candidate capable of nullifying the photocorrosion of Ag 2 WO 4 ; based on the electron transfer from Mn 4+ /Mn 3+ and Mn 3+ /Mn 2+ moieties manipulated by structural changes 30,31 , amplify light harvesting as well as expanding the charges separation. In the present study, we have successfully utilized ultrasonication-deposition-precipitation route to fabricate Mn 3 O 4 /Ag 2 WO 4 nano-composite in the absence of any stabilizing agents with no agglomeration and high performance towards nitroarenes photo-reduction. From the practical point of view we used also molecular hydrogen as the cleanest and environmental friendly reducing agent for 4-NP and at room temperature to obviate energy consumption and suspicious safety problems. The morphology, crystal structure, surface properties and optical properties of the nano-composites and individual analogues have been thoroughly studied.

Results and discussion
Bulk, morphology and elemental composition. The  The XRD patterns of the Mn 3 O 4 /Ag 2 WO 4 composites were originated from the mixed phases of tetragonal Mn 3 O 4 and orthorhombic α-Ag 2 WO 4 . The two phase composition of Mn 3 O 4 and Ag 2 WO 4 in the nanocomposites have shown significant decrease in peaks intensities at the 40% Ag 2 WO 4 loading. On the other hand, a tremendous increase is obtained for the (220) plane of Ag 2 WO 4 at the 60% loading. Nonetheless, a new phase for Ag 2 WO 4 never obtained in its pure form existed at 2θ = 54.6° is depicted and ascribed to the (151) plane, developed from the evolution of the Ag 2 WO 4 hexagonal structure 7,24 . The measured crystallites size determined using the Scherrer's equation indicate average sizes of 28 nm and 34 nm for the Mn 3 O 4 and Ag 2 WO 4 , respectively. Whereas, the average crystallite size was 21 nm for Mn 3 O 4 /Ag 2 WO 4 (40%) and 53 nm for Mn 3 O 4 /Ag 2 WO 4 (60%), which exhibits larger d-spacing and minor shifts in peak positions to lower angles than the former. This provokes that a unit cell expansion is developed in Mn 3 O 4 /Ag 2 WO 4 (60%) with a strong strain owing to planar stresses resulted from stoichiometry alteration. Moreover, larger sizes could also cause such shifts to lower XRD angles.
The morphological structure of α-Ag 2 WO 4 analyzed by the TEM-SAED measurements; and shown in Fig. 2A, discloses the existence of nanorods shape of an average diameter of 37 nm and few micrometer length. A deposition of Ag nanospherical particles; of an average diameter of 18 nm, on the Ag 2 WO 4 nanorods was depicted although it is never seen by XRD results. The inset Figure associated to the selected area electron diffraction of α-Ag 2 WO 4 indicates few spots of diffraction allocated on concentric spheres. They are consistent with the planes of (121), (022) and (220) of Ag 2 WO 4 those were in harmony with its XRD pattern, depicted in Fig. 1 Fig. 2A) shows only the existence of Ag nanoparticles (100 at % Ag) point to the formation of Ag on the surface of Ag 2 WO 4 ; as shown in Fig. 2C. Figure 2D shows the image of hollow spherical-like shape of Mn 3 O 4 with average diameters of ~ 30 nm. The inset shows ring patterns of uniform structure with the plane lines (200), (103) and (211) correspond to Mn 3 O 4 . Figure 2E shows the image of Mn 3 O 4 /Ag 2 WO 4 (60%) that hold opposing shapes compared to the individual analogue. The Ag 2 WO 4 image in the composite resembles spheroidal-like structure whereas Mn 3 O 4 configures as rice beads wherein the former coating that of the latter. The particle diameter of the composite is in the span range of 25-35 nm. The SAED pattern reveals the presence of clear fringes with different spacing's correspond   (Fig. 3). The W4f 7/2 and 4f 5/2 peaks detected at 34.9 and 37.1 eV are typically correlated to W 4+ tungstate oxide structure 32 . The Ag3d 5/2 and 3d 3/2 detected at 367.8 and 373.8 eV revealed the presence of Ag + , comparable to those similarly seen in other oxide structures 33 .
Whereas, the small broad blue shaded peaks depicted at 368.8 eV and 375.1 eV implied the existence of Ag°2 7 . The binding energies noticed at 642.0 eV and 653.2 are attributed to the existence of Mn 3+ of Mn 3 O 4 34 . The O1s peak positioned at 531.2 eV is consistent with the lattice oxygen coped with those observed in both Mn 3 O 4 and Ag 2 WO 4 structures 33,34 . Hence, the structure of the Mn 3 O 4 /Ag 2 WO 4 (60%) nanocomposite is clearly identified and the amount of Mn is verified at 39.4 wt% whereas those of Ag o is quantitatively exhibited a value of 2%, to finally propose the existence of Ag o (2%)/Mn 3 O 4 (40%)/Ag 2 WO 4 (60%) elements in the composite.

Vibrational, electronic and conductivity characteristics.
To have an idea about the functional groups verified on the as-synthesized catalysts, FT-IR spectroscopic examination was carried out in the wavelength range of 4000-400 cm −1 (Fig. 4). The FT-IR spectrum of Mn 3 O 4 exposes distinctive bands at 493 and 610 cm −1 connected to the Mn-O stretching vibration modes in tetrahedral and octahedral positions, respectively 35 .
The weak absorption bands at 942 cm −1 and 1061 cm −1 are attributed to the C-C bond and CH 2 rocking in PVP pointing to the presence of template residuals. Whereas, the bands at 1628 cm −1 and 3400 broad cm −1 are related respectively, to water molecules stretching vibrations and hydrated free O-H groups 36 . The FTIR spectrum of Ag 2 WO 4 indicates a band at 608 cm −1 ; due to the stretching mode of W-O in WO 6   www.nature.com/scientificreports/ band structures of valence band and conduction band; apart from the individual analogue. This is expected to alter the behaviors of photo-generated charge carriers as well as the excitation processes 30,31 . The shift of the OH stretching band into lower wavenumbers (3388 cm −1 ) besides its broadening is correlated to strengthening of the hydrogen bonding interaction. These results are taken as a criterion for the composite formation.
To prove the mission of electrons during the photoreduction performance, the electrical conductivity of the assynthesized nanocomposite beside bare Mn 3 O 4 and Ag 2 WO 4 catalysts was evaluated at room temperature (Fig. 5 is effectively amplified the electronic conductivity compared to the pure correspondents. It also signifies the synergism between the latter components and verifies in addition the increase of the hopping rate in spite of the substitution of Ag 2 WO 4 into Mn 3 O 4 . This could give a clue about the good contact between the components forming the nanocomposite although of exceeding their crystallites size. The superiority of crystallites size of Mn 3 O 4 / Ag 2 WO 4 (60%) is going to decrease scattering of the free electrons if compared with the smaller crystallized ones based on the fact that conductivity is inversely proportional to the electron scattering γ(σ = N e e 2 = me * γ) 38 . The above results were also well confirmed from the resistance values, which were found to decrease in the sequence;   www.nature.com/scientificreports/ and 564 nm). Shifting the latter bands into higher frequencies in the nanocomposite spectrum beside its median behavior confirms the composite formation. Besides, it exhibits an increased light harvesting capacity over the range till 440 nm, after which Mn 3 O 4 took over till the end of the range (800 nm). The band gap energies were evaluated by fitting the absorption data into indirect transition via using the equation αhν = E d (hν − E g ) 2 where E g is the indirect band gap, E d is a constant, α is an optical absorption coefficient and hν is the photon energy 39 .
In this essence, the E g values were 2.2, 3.15 and 2.7 eV for Mn 3 O 4 , Ag 2 WO 4 and Mn 3 O 4 /Ag 2 WO 4 (60%) respectively. Apparently, the E g value of the nanocomposite lies in the midway between pure catalysts elaborating the exhibited interaction between them and the facile charge transfer.
Photoluminescence study. The photoluminescence emission spectra are composed of two broad bands, starting between 440-520 nm and 520-680 nm, and completed at the wavelength of 800 nm. Apparently, the PL first band (440-520 nm) indicates a decrease in PL emission intensities in the order Mn 3 O 4 < Mn 3 O 4 /Ag 2 WO 4 (60%) < Ag 2 WO 4 ( Fig. 7), typical to that elaborated in the optical band gap sequence. Apparently, tungstate luminescence is more intense and its maximum emission is localized at higher energy than Mn 3 O 4 advocating that the electron transfer from oxygen to Mn is easier than that with tungstate cation due to increasing the electronegativity of the latter cation 40 . This behavior was not in a good correlation with the sequence in crystallites diameter decrement. The major emission intensity was for the Ag 2 WO 4 first band; due to WO 6 group transition, although the composite spectrum first band shows a red shift compared with pure ones configuring the composite forming effect. Contrarily, the second broad band (yellow emission) of the composite spectrum; maximized at 593 nm, exhibits a major PL emission exceeding those of individual analogues, reflecting high charges recombination within this margin, unlike that of the first one depicted at 510 nm. This is attributed to the d-d transitions engaging Mn 3+ ions and to the abundant defect. Accordingly, the two different charge transfer transitions committed in the PL emission spectrum of Mn 3 O 4 /Ag 2 WO 4 (60%) revealed the presence of efficient controlled charge carrier separation (first band-quenched) as well as excessive charges transfer with limited opposition   Similarly, the photoreduction of 4-nitro-aniline (4-NA) into 4-amino-aniline (4-AA) on the most active catalyst Mn 3 O 4 /Ag 2 WO 4 (60%) proceeds to produce 4-AA in 180 s (Fig. 9A, B) with a rate constant of 0.011 min −1 . The UV-Vis spectra of 4-NA that diminish with time following the addition of NaBH 4 /catalyst demonstrate the developing of 4-AA at 290 nm (Fig. 9C).
The apparent increase in the reaction rate of Mn 3 O 4 /Ag 2 WO 4 (60%) towards 4-AP than 4-AA by 5 times is probably due to the solvation effect and electron density effect (electron donating effect) by which NH 2 is expected to increase the polarity; which works as electron rich sites interconnected to a position of conjugation, facilitating the reaction via the formed anions. Exposing the plane (151) on α-Ag 2 WO 4 when incorporated with Mn 3 O 4 ; never seen in the pure form, might increase the reactants absorbability facilitating the reduction consequences.
The comparison of the percentage photoreduction of 4-Nitro acetanilide (4-NAC) to 4-amino acetanilide (4-AAC), calculated from the decrease in the peak at 360 nm of 4-AAC in UV-Vis spectra, as a function of visible light irradiation is shown in Fig. 10A for the Mn 3 O 4 /Ag 2 WO 4 (60%) catalyst.
After 6 min of visible light exposure, a reduction conversion comprised of 100% was reached. The Langmuir-Hinshelwood (L-H) kinetic model calculated by outlining ln C o /C t versus the exposure time (t) as well as C t /C o with time indicates a rate constant value of 0.24 min −1 (Fig. 10B,C). Apparently, increasing the electronic density of the nanocomposite Mn 3 O 4 /Ag 2 WO 4 (60%) increases the catalytic photoreduction consequences towards all the nitroarenes, beside the appreciable absorption of light exhibited in the visible light range. Indeed, although the lifecycle of photo-generated electrons is not as high as Mn 3 O 4 ; as PL committed, the high electron mobility was the prime reason responsible for enhancing the photo-reduction performance followed by the recombination control of the photo-generated charge carriers.
Increasing the rate constant towards 4-NP photoreduction surpassing those of 4-NA and 4-NAC is greatly dependent on the concentration of nitrophenolate moieties 41 . The activity is well correlated to the reactants solvation. Apparently, hydrogen bonding in nitrophenol is more prominent than in 4-NA and 4-NAC, those possess higher hydrophobicity than 4-NP. Indeed, this hydrophobicity will affect the well dispersion of the catalyst and rather delays attainment of the reactants onto the catalyst active sites. It seems also that the reduction activity has nothing to do with the nitroarenes acidity, which is in the order; 4-NP (pKa = 6.90) < 4-NA (pKa = 1.0) < 4-NAC (pKa = 0.8) indicating that the greater tendency to dissociate protons the stronger is the acid, which specified at lower pKa value. Furthermore, the presence of -C=O-R group in 4-NAC decreases the rate of reaction relative to the H addition. These outcomes signify that Mn 3 O 4 /Ag 2 WO 4 (60%) offers the best active sites for both NaBH 4 and 4-NP and exhibits the greatest catalytic activity in contrast to Mn 3 O 4 /Ag 2 WO 4 (40%) and counterpart catalysts. pH effect and recyclability. Figure 11 displays the pH influence on the catalytic activity of Mn 3 O 4 / Ag 2 WO 4 (60%) towards 4-NP photoreduction. The photoreduction of 4-NP on Mn 3 O 4 /Ag 2 WO 4 (60%) is greatly dependent on the pH value; median pH is advantageous to an efficient magnificent reduction. A complete reduction of 4-NP is observed within only 60 s at pH equal 7 reflecting the highest K obs of 0.0675 s −1 . On the other hand, at pH 2 and 12 the photoreduction percentages reach 60% and 5%, respectively. This indeed is relied on the correlation between PZC of Mn 3 O 4 /Ag 2 WO 4 (60%) and pKa of 4-NP. The measured zero-point charge of Mn 3 O 4 /Ag 2 WO 4 (60%) was found at pH 7.9 (not shown), meaning that the catalyst surface is positively charged at pH < 7.9. Admittedly, 4-NP photoreduction executed over Mn 3 O 4 /Ag 2 WO 4 (60%) together with the existence of NaBH 4 obeyed Langmuir-Hinshelwood kinetics and adsorption is the leading action in the reduction pro-  www.nature.com/scientificreports/ cess. Thus, acquiring + ve charge on the nanocatalyst surface facilitates the adsorption of the negatively charged BH 4 − , causing an enhanced reduction conversion and superior rate constant of 4-NP photoreduction, under neutral condition. On the other hand, acquiring negative charge on the catalyst surface at pH equal 12 will induce repelling with the negative charges positioned on 4-NP and BH 4 − , with the catalyst surface thus decreasing seriously the adsorption and reduction rate. Slowing down the photoreduction reaction of 4-NP at pH equal 2 compared to that at 7 is probably due to the catalyst dissolution.
This retards the well adsorption-desorption of the nitroarenes on the catalyst surface and thus affects the rate. The stability and reusability of Mn 3 O 4 /Ag 2 WO 4 (60%) at 0.1 g amount is inspected for 4-NP (0.001 M) in 5 cycles at pH 7 without any treatment for the catalyst between the runs. The photoreduction activity decreased slightly from 100% into 84% after the fifth run indicating high efficiency and stability. This decrease in activity is probably due to accumulation of 4-aminophenol at the active sites of the Mn 3 O 4 /Ag 2 WO 4 nanocatalyst 33 .
H 2 production. The borohydride photo-hydrolysis to signify the amount of H 2 produced, which follows the reaction; BH 4 (aq) + 2H 2 O (l) → BO 2 (aq) + 4H 2 (g) (1), was analyzed on the different catalysts to monitor the amount of generated volume of H 2 as a function of time; which is similar to that taken place during the reduction performance of 4-NP. Figure 12a  The higher activity of the former photocatalyst is probably due to increasing the electron density; as evidenced by the electronic conductivity measurement, together with the expected improvement in the electron transfer that highly enhanced following light irradiation. Figure 12b shows the trapping experiment while performing the 4-NP reduction to investigate whether electrons are involved in this reaction or not. The used K 2 S 2 O 8 scavenger indicates a suppression of 48% manifesting that electrons are very important for the reduction process. However, the % of 52 dictates the involvement of some other species mostly as H 2 . Thus, photo-induced electrons and H 2 of strong reductive potentials are competently reduced 4-NP.

Mechanism for the 4-NP photocatalytic reduction. It is evident that the Mn 3 O 4 /Ag 2 WO 4 (60%) com-
posite has the greatest photo-current density. Because, this composite structure provides small diffusion distance for photo-motivated charges that are quickly transport into the catalyst surface. Moreover, the small electron transfer resistance of Mn 3 O 4 /Ag 2 WO 4 (60%) signifies a high-speed interfacial charge transfer, which may be responsible for the excellent performance of 4-NP reduction. Electrochemical impedance spectroscopy (EIS) was performed to study the transfer and separation of photo-excited carriers. As confirmed in Fig. 12C, the Nyquist plot of Mn 3 O 4 /Ag 2 WO 4 (60%) displays a much smaller semi-circle than those of Mn 3 O 4 and Ag 2 WO 4 , reflecting a smaller charge transfer resistance for the former comparatively.
In view of above results, we suggested a mechanism of Mn 3 O 4 /Ag 2 WO 4 (60%) in the photo-reduction of 4-NP with borohydride. The reaction of the latter with 4-NP to produce p-nitrophenolate anion is primarily attained via adsorption onto the positively charged surface of the composite. Consequently, p-nitrophenolate anions are reduced to 4-AP with borohydride. Under visible light illumination, electrons can pass from Mn 3 O 4 into Ag 2 WO 4 when they come into contact forming a Schottky barrier. The latter catalyst possesses a smaller work function (4.3 eV) relative to that of the former (4.5-6 eV) thus facilitating the electron transfer in the sequence from Mn 3 O 4 to Ag 2 WO 4 . The poor photocatalytic reduction-ability for 4-NP on the individual catalysts raises the negligible importance of the bulk morphology that results in high charge recombination rate. Besides, their negatively charged surfaces rule out the nitrophenolate anions from being well adsorbed. In addition, the amorphous nature of individual catalysts as well as the Mn 3 O 4 /Ag 2 WO 4 (40%) catalyst have shown negative effects on their activities. This is because they own less number of active coordinate, the requisite of high energy for creating e − -h + and the high opportunity for their recombination. Figure 12D shows The adsorption of both BH 4 and 4-NP ions on the composite surface is followed by H transfer from BH 4 to the composite forming hydridic moieties, which react with water molecules liberating H 2 molecules that can also reduce 4-NP. The deposited Ag nanoparticles on the surface of α-Ag 2 WO 4 can act as electron catch centers, and as a result retard the photogenerated charges recombination. Also, Ag nanoparticles can inhibit the transfer of holes from the V B to the interface of the photocatalyst and solution 42 . That is why we did not detect any oxidation products for 4-NP. Simultaneously, the V B potential of Mn 3 O 4 is negative compared to the redox potential of ·OH/OH − (2.38 V vs. NHE), showing that the photo-induced holes are not capable of oxidizing surface hydroxyls (and OH − ) or H 2 O in the degradation medium to provoke the formation of •OH radicals.
To further prove the capability of the Mn 3 O 4 /α-Ag 2 WO 4 catalyst in evoking the photo-generated charge carrier transport upon light irradiation, we traced transient photocurrent responses and linear sweep photovoltammetry measurements 43 . Figure 13A 44,45 . This is likely due to at such mentioned loading [Mn 3 O 4 /Ag 2 WO 4 (60%)], an increase in the separation efficiency of photo-generated charge carriers (electrons-holes) is improved 46 . Figure 13B displays the plots between photocurrent density and applied voltage attained under visible light irradiation. Evidently, the photocurrent density of the catalysts rises thru the forward bias voltage, revealing a characteristic n-type  www.nature.com/scientificreports/ semi-conductor 47,48 . It is shown that the Mn 3 O 4 /Ag 2 WO 4 (60%) catalyst exhibits the optimal photocurrent within the same designated voltage profile, typical to the results obtained from the photocurrent density-time curves.
To understand the effect of Ag NPs on catalytic performance, a chemical excavation approach via using a mild ethanolic nitric acid (~ 10% v/v) is employed to steadily remove Ag NPs from the Mn 3 O 4 /α-Ag 2 WO 4 (60%) surface. Accordingly, the correlated catalytic activity indicates the pertinent role of Ag NPs in the photoreduction process. This demonstrates that Mn 3 O 4 /Ag 2 WO 4 crystal renders the aid to promote 4-NP reduction, whereas Ag NPs assist the catalyst to modulate the charge transfer. Decreasing the induction time observed in Mn 3 O 4 /α-Ag 2 WO 4 compared with the individual analogue during the reduction of 4-NP reflects the higher diffusion of the reactants (NaBH 4 and 4-NP) on the former surfaces. This reveals that the formed Mn 3 O 4 /α-Ag 2 WO 4 surface owns a higher hydrophilic nature that facilitates the diffusion of the reactants as well as electrons and H 2 and thus modulates the catalytic reduction property. Comparative investigation of catalytic performances and kinetics details with the start-of-the-art catalysts accomplished under comparable experimental conditions for 4-NP reduction are displayed in Table 2. Evidently, the Mn 3 O 4 /α-Ag 2 WO 4 (60%) photocatalyst displays a significantly higher kinetic rate (0.067 s −1 ) than mostly used metals (Pd, Ag and Cu) in addition to some metal oxides (TiO 2 ), ferrites based catalysts and Au@Ag core-shell NPs [49][50][51][52][53][54][55][56][57] .

Conclusions
A novel Mn 3 O 4 /α-Ag 2 WO 4 (60%) heterojunction photocatalyst fabricated by a facile sonication-deposition-precipitation route was thoroughly characterized by XRD, TEM-SAED, XPS, FTIR, UV-Vis diffuse reflectance and PL techniques. Results showed that Mn 3 O 4 /α-Ag 2 WO 4 (60%) possessed the best photoreduction activity for nitroarenes (0.001 M); under visible irradiation, with a conversion efficiency reaching 100%, attained for example in 1 min reaction time for 4-NP with a kinetic rate equal 0.067 s −1 . The mechanism exploration indicates that the generated hydrogen and electrons reacts with 4-NP in presence of BH 4 − on the nanocomposite surface promoting the reduction process. Appropriately, the amalgamation of α-Ag 2 WO 4 with Mn 3 O 4 not only achieves spatial separation of photo-induced charge carriers, facilitated by the deposited Ag nanoparticles, but also boosts up the electronic conductivity. The composite Mn 3 O 4 /α-Ag 2 WO 4 (60%) is efficiently photocatalyzed the hydrogenation of 4-NP into 4-AP (34.9 × 10 -3 min −1 ) under mild conditions with excellent selectivity (100%) as well as it shows an improve hydrogen production under using NaBH 4 (470 µ mole/g) delineated under the solution condition. The Mn 3 O 4 /α-Ag 2 WO 4 (60%) photocatalyst exhibited a self-restoration ability providing a new perspective for application in the photocatalysis field. dispersed into 100 mL distilled water by ultrasonic irradiation for 10 min. Then, 0.210 g of silver nitrate was added to the suspension and stirred for 60 min. An aqueous solution of sodium tungstate Na 2 WO 4 .2H 2 O; prepared by dissolving 0.213 g in 20 mL water, was drop wisely added to the previous suspension followed by refluxing at 90 °C for 60 min. The resultant suspension was then centrifuged to collect the precipitate, which washed two times with water/ethanol solution and dried in an oven at 60 °C for 24 h. The pure Ag 2 WO 4 photocatalyst was typically fabricated as mentioned except the Mn 3 O 4 addition. The synthesis of Mn 3 O 4 /Ag 2 WO 4 -40% was attained via using the same procedure to give 40 wt. % of Ag 2 WO 4 relative to Mn 3 O 4 .

Reduction of nitroarenes.
To investigate the catalytic reduction of 4-nitrophenol, 4-nitro aniline and 4-nitro acetanilide, 100 mL of 0.001 M aqueous solution of the nitroaromatics was taken in a 250 mL beaker with 10 mL of 0.2 M NaBH 4 . This system was then illuminated by a visible light LED lamp of 50 W with a cut off filter (λ > 420 nm, 30 mWcm −2 ); to obviate the low emissions existed near to UV and IR margins, and fixed at a distance of 25 cm. The solution was subjected to a constant stirring. A desired amount of catalyst was added; so as to reaching 1 g/L, while stirring and continued at room temperature. The dark yellow color of the solution is progressively vanished with time, demonstrating the reduction of nitro aromatics. The reaction progress was checked via withdrawing samples from the reaction mixture at normal time intervals. The conversion of nitroaromatics to the corresponding aminoaromatics was checked by UV-Visible spectroscopy (6705 UV/Vis JENWAY). However, in the fast reduction of 4-nitrophenol, 3 mL of 0.001 M was placed in the quartz cuvette of the spectrophotometer with 3 mg catalyst and the absorbance is recorded regularly without stirring i.e. the catalyst is settle down at the cuvette bottom. The catalysts stability and reusability was examined after the reaction completion, via washing with distilled water and ethanol in sequence to remove the nitroaromatics adsorbed on the surface. The catalyst recycling test was accomplished 5 times.
The photocatalytic hydrogenation of nitrophenol was conducted in a 75 mL sealed glass autoclave with a quartz window for light irradiation. A typical reaction process is described as follows: 0.001 M nitrophenol and 100 mg of catalysts were dispersed in 20 mL of neat ethanol, and the suspension was then sealed in an autoclave under 0.35 MPa of H 2 with stirring. The mentioned lamp was also employed as the light source at the same light intensity and at room temperature. After reaction, the collected products following 1.0 h reaction time were analyzed by gas chromatography-mass spectroscopy (GC-MS-Bruker, Germany) technique.

Scientific Reports
| (2020) 10:21495 | https://doi.org/10.1038/s41598-020-78542-5 www.nature.com/scientificreports/ Hydrogen generation. The catalytic hydrolysis of NaBH 4 (200 mg in 100 mL deionized H 2 O) was carried out at ambient temperature that never exceeds 25 °C by wetting the photocatalyst in water inside a vessel system that maintained stirring at 750 rpm. The liberated H 2 was measured using a water-displacement technique; which was completely insulated so as to guarantee that all the gas evolved are well stored, using an electronic balance with an accuracy of 0.01 g. The volume of H 2 produced was quantified as a function of time, with repeating each experiment not less than two times to guarantee the reproducibility. The calculated relative error was no more than 2%.
Characterization techniques. X-ray diffractions (XRD) provided with Ni-filtered copper radiation (λ = 1.5404 Å) was used to distinguish the crystal structure of the nanocomposites operated at 30 kV and 10 mA with a scanning speed of 2θ = 2.0°/min. The Fourier transform infrared (FT-IR) spectra are evaluated via a Perkin Elmer Spectrometer (RXI FT-IR), at a resolution of 1.0 cm −1 , within the region 400-4000 cm −1 using the KBr technique. Diffuse Reflectance Ultraviolet-visible spectroscopy (UV-vis DRS) of the nanocomposite and the individual catalysts is measured at r.t. using UV-vis JASCO spectrophotometer (V-570) in the range of 200-800 nm. The edge energies (Eg) of allowed transitions are determined by finding the intercept of the straight line in the low-energy rise of the plot using the relation αhʋ = A(hʋ − Eg) n . The photoluminescence (PL) emission spectra were measured following the excitation with a continuous-wave He-Cd laser (λ = 325 nm). X-ray photoelectron spectroscopy (XPS) spectra were measured by the Thermo ESCALAB 250XI photoelectron spectroscopy system using a monochromatic Al Kα source operated at 200 W. The TEM micrographs are obtained using an FEI; model Tecnai G20, Super twin, double tilt 1010, at a pick up voltage of 100 kV. The electrochemical impedance spectroscopy (EIS) studies were made using an EG&G PAR galvanostat/potentiostat, model type 273, with an amplitude of ± 5 mV in the frequency range 10 3 -10 −2 Hz. All photo(electro)chemical measurements of the films deposited on conducting glasses used as working electrode are conducted on an EG&G PAR potentiostat/galvanostat, model 273. A Pt electrode is used as a counter electrode and Ag/AgCl as the reference one, where the electrolyte was 0.5 M of Na 2 SO 4 aqueous solution. The turning of the incident light on and off consecutively was permitted for a period using the 150 W halide lamp (60 mW cm 2 ). The dc electrical-resistivity is measured with an electrical circuit as illustrated elsewhere using the equation σ dc = (l/A s ). (1/R dc ) 30,31 ; where A s is the cross-sectional area, R dc is the sample resistance and l is the length of the sample.