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Research Article pubs.acs.org/journal/ascecg

Synthesis, Characterization, and Photocatalytic Application of Iron Oxalate Capped Fe, Fe−Cu, Fe−Co, and Fe−Mn Oxide Nanomaterial Kasturi Sarmah and Sanjay Pratihar* Department of Chemical Sciences, Tezpur University, Napaam, Assam-784028, India

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ABSTRACT: A major challenge in the field of dye degradation and its remediation is the scarcity of active catalysts that are inexpensive, easily accessible, suitable for large-scale applications, and able to complete mineralization of dyes. In this regard, an easy, sustainable, scalable, and environmentally benign chemical method has been developed to synthesize four different selective orthorhombic iron(oxalate) capped Fe, Cu, Co, and Mn doped heterobimetallic oxide nanomaterials from the redox reaction between [Fe(ox)−Fe(0)] and the corresponding CuSO4, CoNO3, and KMnO4 in water. The variable band gaps, morphologies, surface charges, and surface areas of the four synthesized materials have been successfully utilized for large scale visible light promoted complete mineralization of various dyes into the corresponding CO2, NO3−, and SO42−. The photodegradation mechanism by taking methylene blue as a representative dye suggested that photogenerated holes and free hydroxyl radicals (OH°) are mainly responsible for its skeletal decomposition. DFT study further suggested that the protonated species of MB, which have lower HOMO LUMO energy gap and more surface absorption affinity compared to its neutral species, will favor the degradation process and thus is evidence of its peak shifting and enhanced photodegradation rate in acidic pH. The photocatalytic activities of four synthesized materials were checked with phenol and observed degradation rate decreases in the order of UV > yellow LED > visible light. Toward the large scale degradation of dyes, iron(oxalate) capped Fe−Mn oxide [Fe(ox)Fe-MnOx] nanomaterials show the highest activity among the four synthesized materials. Finally, aqueous reaction medium, easy and scalable synthesis, large scale photodegradation of various dyes and their removal via complete mineralization, and efficient recycling of the catalyst make the protocol economical and sustainable. KEYWORDS: Photo catalysis, iron oxalate, nanomaterial, phenol degradation

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INTRODUCTION The impending global energy crisis has prompted intense research into the development of artificial sustainable energy conversion systems that can mimic natural photosynthesis to directly harvest and convert solar energy into usable or storable energy resources for the treatment of environmental pollutants or wastewater.1−10 The wastewater coming from the textile, paper, plastics, tannery, and paints industries is often rich in color, containing various dyes and chemicals, which can cause a deterioration in water quality and sometimes causes food chain contamination, resulting in adverse effects on biodiversity.11−16 To date, various physical and chemical approaches, including adsorption, reverse osmosis, chlorination, ozonolysis, coagulation, visible or UV light photocatalysis, and oxidation processes, have been reported for wastewater treatment.17−22 However, to take the necessary step from academic research to industrial application of a wide range of catalytic transformations, the catalytic processes have to be sustainable with regard to parameters such as solvent, energy source, separation, and the nature of the catalyst.23−31 In this regard, the efficient utilization of solar energy is one of the major goals in modern © 2016 American Chemical Society

science and engineering that will have a great impact on technological applications. Toward this, various metallic, bimetallic, and metal doped oxide semiconductors of Ti, Au, Ag, Mn, V, Ni, W, Zn, and Sn were successfully utilized for different types of photochemical reactions of environmental interest.32−35 Toward wastewater treatment, a possible solution for the industrial application of catalysts can be the increased utilization of heterogeneous catalysts on biorelevant metals,36−38 such as iron, cobalt, copper, and manganese, which would be beneficial with respect to catalyst recycling and engineering.39,40 In this regard, we have reported an efficient, sustainable, cost-effective synthetic procedure for the gram scale production of selective orthorhombic iron(oxalate) capped Fe(0) [Fe(ox)−Fe(0)] nanomaterial using sodium borohydride (NaBH4) reduction of iron(II) salt in the presence of oxalic acid at room temperature in water without the use of high temperature calcination.41,42 Further, the synthesized orthoReceived: July 19, 2016 Revised: November 17, 2016 Published: November 30, 2016 310

DOI: 10.1021/acssuschemeng.6b01673 ACS Sustainable Chem. Eng. 2017, 5, 310−324

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ACS Sustainable Chemistry & Engineering

orthorhombic Fe(C2O4)·2H2O and Fe3O4 (JCPDS card 851436) (Figure 1b).48 On the other hand, the XRD spectrum of [Fe(ox)Fe-CuOx] nanomaterial shows the presence of orthorhombic Fe(C2O4)·2H2O, Fe3O4, and CuO material (Figure 1c). The Fe(ox)Fe-CoOx nanomaterial characterization by X-ray diffraction (XRD) confirms the presence of orthorhombic Fe(C2O4)·2H2O, Fe3O4, and CoO, as the peaks in XRD are in good agreement with the reported XRD pattern of Fe(C2O4)· 2H2O, CoO, and Fe3O4 (Figure 1d). However, the Fe(ox)FeMnOx nanomaterial characterization by X-ray diffraction (XRD) suggests the amorphous nature of the material.49 FT-IR Analysis. The FTIR spectra of Fe(C2O4)·2H2O show two peaks at 1640 and 1362 cm−1 for typical metal carboxylate. The corresponding shift of ν(COO−) bands, Δ, between free oxalic acid and Fe(C2O4)·2H2O was found to be 60 cm−1, which directly suggests the bidentate coordination mode of oxalate in Fe(C2O4)·2H2O. The other two peaks at 1320 and 820 cm−1 in Fe(C2O4)·2H2O are assigned to the C−O and C− C stretching vibrations of coordinated oxalic acid. When we compare the FTIR spectra of synthesized [Fe(ox)−Fe(0)] nanomaterial with Fe(C2O4)·2H2O, both the spectra well matched with each other, which confirms the presence of iron oxalate in synthesized [Fe(ox)−Fe(0)].50 The presence of both oxalate and Fe3O4 in [Fe(ox)−Fe3O4] nanomaterial was also confirmed from the corresponding peaks at 1412 and 1655 cm−1 for oxalate coordination and another two peaks at 415 and 563 cm−1 due to Fe−O symmetric bending and Fe−O−Fe stretching vibrations. On the other hand, the synthesized [Fe(ox)Fe−CuOx] nanomaterial showed peaks at 502, 819, 1319, 1360, and 1691 cm−1. Interestingly, the carbonyl stretching frequency in [Fe(ox)Fe−CuOx] slightly shifted to a higher region, which may be ascribed due to the relatively weak coordination of oxalate in [Fe(ox)Fe−CuOx] compared to its parent material (Figure 2). The FT-IR spectra of the other two synthesized [Fe(ox)Fe−CoOx] and [Fe(ox)Fe− MnOx] nanomaterials show the presence of oxalate in the material, as most of the observed peaks are well matched with Fe(C2O4)·2H2O and [Fe(ox)−Fe(0)] nanomaterial (Figure 2). Morphology Analysis. The surface morphologies of four synthesized nanomaterials were analyzed from the representative HR-SEM images (Figure 3). The porous morphology connected with several nanoribbon-like structures was observed in Fe(ox)-Fe3O4. On the other hand, a flower-like morphology was observed in both the Fe(ox)Fe-CuOx and Fe(ox)Fe-CoOx nanomaterials. However, the HR-SEM image of Fe(ox)FeMnOx nanomaterial shows a particle-like nature of the material (Figure 3). The HR-TEM images of four synthesized materials were recorded and depicted in Figure 4. The initial nanoribbon morphology of Fe(ox)-Fe(0) has been retained in the Fe(ox)Fe3O4, which consists of Fe(ox) template bound Fe3O4 nanoparticles.51 The calculated lattice spacing between two planes in Fe(ox)-Fe3O4 is found to be ∼0.24 nm.52 When Cu is doped with Fe(ox)-Fe3O4, the morphology of the Fe(ox)FeCuOx nanomaterial shows a porous layer structure. Similarly, TEM micrographs of Fe(ox)Fe-CoOx and Fe(ox)Fe-MnOx nanomaterial shows a porous morphology in the material. To further confirm the elemental compositions, the EDS analysis of the selected region for four synthesized nanomaterials was done in HR-SEM. The EDS spectrum confirms the presence of carbon, oxygen along with iron, Cu, Mn, and Co metals in the synthesized material (Figure S1).53

rhombic iron(oxalate) capped Fe(0) [Fe(ox)−Fe(0)] nanomaterial was successfully utilized for the formation of iron(oxalate) capped Fe−Cu, Fe−Co, and Fe−Mn bimetallic oxide material in gram scale from the reaction between [Fe(ox)−Fe(0)] and corresponding metal salt in water. The as obtained iron(oxalate) capped Fe−Cu, Fe−Co, and Fe−Mn bimetallic nanomaterials were successfully utilized for the large scale photocatalytic degradation and complete mineralization of various dyes.

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RESULTS AND DISCUSSION Synthesis and Characterization. The synthesis of orthorhombic iron(oxalate) capped Fe(0) [Fe(ox)−Fe(0)] nanomaterial was done from the reduction of Mohr’s salt [(NH4)2SO4FeSO4] with NaBH4 in the presence of oxalic acid (H2C2O4) at room temperature in water. Next, to utilization reactive Fe(0) for the reduction of other metals, three metal precursors, viz. CuSO4, CoNO3, and KMnO4, were chosen (Scheme 1). Gradual addition of aqueous CuSO4 solution to as Scheme 1. Synthetic Route of the Nanomaterial

synthesized [Fe(ox)−Fe(0)] leads to a color change from blackish to reddish brown with spontaneous reduction of Cu2+ to Cu0 by the sacrificial oxidation of Fe0/Fe2+ (EFe2+/Fe0 −0.44 V and ECu2+/Cu0 +0.34 V versus SHE).43 On the other hand, addition of aqueous CoNO3 solution to as synthesized [Fe(ox)−Fe(0)] leads to a color change from blackish to yellow with spontaneous reduction of Co2+ to Co0 by the sacrificial oxidation of Fe0/Fe2+ (EFe2+/Fe0 −0.44 V and ECo2+/Co0 −0.29 V versus SHE) and afforded a yellow [Fe(ox)Fe-CoOx] nanomaterial, whereas black color [Fe(ox)Fe-MnOx] nanomaterial was observed after the addition of KMnO4 to [Fe(ox)− Fe(0)] in water from the sacrificial oxidation of Fe0/Fe2+ (EFe2+/Fe0 −0.44 V and EMnO4−/Mn2+ +1.49 V versus SHE).44−47 However, initial black color [Fe(ox)−Fe(0)] nanomaterial transformed into light brown after keeping it in open air for 72 h due to the oxidation of reactive Fe(0) to corresponding Fe3O4. After the completion of the reaction, all the materials were collected via centrifugation and washed several times with distilled water to remove unreacted metal salt and dried in an oven at 80 °C for 8 h and used for further characterization. PXRD Analysis. The XRD spectra of all the synthesized materials were recorded in the 2θ range of 10−90. Although the presence of orthorhombic Fe(C2O4)·2H2O in [Fe(ox)− Fe(0)] nanomaterial was confirmed from the comparative XRD pattern of standard Fe(C2O4)·2H2O, we failed to detect the existence of Fe(0) in the material because both the peaks for Fe(ox) and Fe(0) overlapped each other (Figure 1a). However, [Fe(ox)−Fe(0)] material is found to be active for the reduction of methylene blue to its corresponding leuco methylene blue. Interestingly, the XRD analysis of the brown material, transformed after the MB reduction, suggested the presence of both orthorhombic iron oxalate and magnetite Fe3O4, as the XRD peak position and intensity are well matched with 311


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Figure 1. XRD spectrum of (a) Fe(ox)-Fe(0), (b) Fe(ox)-Fe3O4, (c) Fe(ox)Fe-CoOx, and (d) Fe(ox)Fe-CuOx.

Figure 2. FTIR spectrum of all the synthesized nanomaterial.

X-ray Photoelectron Spectral Analysis. To know the oxidation state and chemical composition of the synthesized

nanomaterials, X-ray photoelectron spectral (XPS) analyses were carried out. The XPS measurements were done with an Al 312


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presence of both Fe and Mn was observed in the XPS plot. The binding energy of Mn2p3/2 centered at 641.9 eV and of Mn2p1/2 at 653.9 eV and the peak-to-peak separation (12 eV) of the XPS spectrum are consistent with the reported data for elementary MnO. Further, the presence of a satellite feature (∼648.0 eV) in the material is also evidence of MnO, which is not present in either Mn2O3 or MnO2.56 The XPS spectrum of Fe(ox)Fe-CoOx nanomaterial showed peaks centered at 781.0 and 796.5 eV for Co2p3/2 and Co2p1/2 (Figure 5), which indicates the presence of Co3O4, as both the binding energy and peak-to-peak separation (15.4 eV) are consistent with the reported data for elementary Co3O4.57 Furthermore, the presence of two satellite peaks at ∼787.5 and 803.4 eV also justifies the existence of Co3O4. The elemental compositions of four materials calculated from the XPS analysis were provided in Table 1. Surface Area, Charge, and Optical Property Analysis. To know the surface characteristic of synthesized nanomaterial, the nitrogen adsorption−desorption isotherms and the pore size distributions of all four synthesized materials were recorded and provided in Table 2. The surface area of Fe(ox)-Fe3O4 is found to be 55.2 m2 g−1, which increases upon the incorporation of another transition metal. Among all four materials, Fe(ox)Fe-MnOx shows the highest surface area with lowest pore volume and pore radius, while the highest pore volume and pore radius are found in the case of Fe(ox)FeCoOx nanomaterial. So, the observed porous nature of the synthesized nanomaterial from BET analysis is also consistent with the observed morphology from HR-SEM and HR-TEM analyses. Next, the optical properties of four synthesized nanomaterials were investigated by DRS experiment. The band gaps of the synthesized nanomaterials were calculated from the plot of (αEp)2 versus Ep based on the direct transition and the extrapolated value of Ep at α = 0. The observed band gap of Fe(ox)-Fe3O4 is found to be 2.0 eV, whereas an enhancement in band gap was observed in both the Fe(ox)Fe-CuOx and Fe(ox)Fe-CoOx nanomaterials. On the other hand, among all four synthesized nanomaterials the lowest band gap of 1.7 eV was observed in Fe(ox)Fe-MnOx (Table 2). To obtain insight into the surface charge of all the synthesized nanomaterials, all the powdered samples suspended in nanopure water and their zeta potentials were measured. The zeta potential of the Fe(ox)Fe-MnOx nanomaterial suggested a slightly negative charge at the material surface and is found to be −10.2 mV. According to the Derjaguin−Landau−Verwey−Overbeek (DLVO) model, agglomeration of uncapped nanocrystals depends upon the repulsive interaction arising from the electrostatic force and the van der Waals force of attraction.58−61 The NP with larger zeta potential will generally reduce hydrodynamic size because higher surface charges will influence less electrostatic repulsive force among them. So, smaller hydrodynamic size is expected in the case of higher surface charge. In fact, the small particle size in Fe(ox)Fe-MnOx nanomaterial revealed from HR-SEM and HR-TEM images as well as measured average hydrodynamic size in the range of 10−100 nm is further evidence of the higher negative charge in the material. On the other hand, the relatively lower surface negative charge in the other three synthesized materials in comparison to Fe(ox)Fe-MnOx is further evidence of their flower-like morphology. Application of the Synthesized Material in Photocatalytic Degradation of Phenol. Phenols and related aromatic hydrocarbons are common in refining and petro-

Figure 3. HR-SEM images of Fe(ox)Fe3O4 (a); Fe(ox)Fe-CuOx (b); Fe(ox)Fe-MnOx (c); and Fe(ox)Fe-CoOx (d).

Figure 4. HR-TEM images of Fe(ox)Fe3O4 (a); Fe(ox)Fe-CuOx (b); Fe(ox)Fe-MnOx (c); and Fe(ox)Fe-CoOx (d).

α X-ray source, and the calibration of the binding energy was made against the C1s peak. The XPS spectra of all the materials exhibit distinct peaks at 283.7 and 531.9 eV for C1s and O1s, respectively. These two peaks further confirm the presence of oxalic acid in all four synthesized nanomaterials. The XPS spectra of Fe(ox)-Fe3O4 show peaks at 724.7, 710.8, and 56.1 eV for corresponding Fe2p1/2, Fe2p3/2, and Fe3p, respectively, and match well with the standard XPS pattern of Fe3O4.54 Further, the absence of a satellite peak of Fe2p3/2 in Fe(ox)-Fe3O4 is also evidence of the existence of Fe3O4 in the material. In the case of Fe(ox)Fe-CuOx nanomaterial, the presence of both Fe3O4 and oxalate was confirmed from the XPS analysis. The peaks for the corresponding Cu2p3/2 and Cu2p1/2 are found at 934.1 and 954.0 eV, which further suggests the presence of CuO in Fe(ox)FeCuOx nanomaterial because the binding energy and peak-topeak separation (19.9 eV) of the XPS spectrum are consistent with the reported data for elementary CuO.55 On the other hand, two satellite peaks at 943.6 and 962.4 eV further confirm the existence of CuO in the material. A similar observation was observed in the case of Fe(ox)Fe-MnOx nanomaterial: the 313


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Figure 5. XPS of Fe(ox)Fe3O4 (a); Fe(ox)Fe-CuOx (b); Fe(ox)Fe-MnOx (c); and Fe(ox)Fe-CoOx (d).

synthesized material, phenol was chosen as a model colorless dye and its degradation monitored in a UV−vis spectrophotometer. Initially, a stock solution of phenol was prepared with a concentration of 100 mg L−1. After that, a 20 mL stock solution was taken in a 50 mL round-bottom flask, and to this 40 mg of each of the synthesized materials was added and stirred for 1 h under dark. The reaction was continued under different light sources (UV, yellow LED, and visible) with a fixed intensity of 298 mW/m2. The progress of the reaction was monitored from the corresponding change in the UV−vis absorbance of the reaction mixture with time. The degradation % versus time plot for four synthesized materials under different light sources has been plotted in Figure 6. Among three light sources, fast degradation of phenol was observed in the presence of UV light, while the slowest degradation was found in visible light, which suggests the importance of a particular fixed energy light source. The higher activity of all the material in the UV light source compared to yellow LED may be ascribed due to rapid electron transfer from

Table 1. Elemental Composition (atom%) of the Synthesized Nanomaterials #

Fe (At%)

C (At%)

O (At%)

Other metal (At%)

Fe(ox)-Fe3Ox Fe(ox)Fe-CuOx Fe(ox)Fe-MnOx Fe(ox)Fe-CoOx

18.6 11.3 5.2 12.3

31.9 38.7 29.6 37.5

49.5 41.7 54.3 42.9

8.2 (Cu) 10.8 (Mn) 7.2 (Co)

chemical wastewater, which are toxic, and thus a necessary step is required for their degradation.62−65 In this regard, photocatalytic degradation of phenol will be a cost-effective method for their removal. To date, numerous reports on the photocatalytic activity of various synthesized materials are in the literature, where colored dye is used as a representative substrate in most of the cases. Later on, it has been found that photosentization of dyes may operate under visible light, which may mislead and will provide inconclusive activity data for new materials.66,67 Thus, to check the photocatalytic activity of the

Table 2. Surface Area, Pore Radius, Pore Volume, and Band Gap of All the Synthesized Samples Surface area (m2 g−1) Pore Radius (Å) Pore Volume (cc g−1) Band Gap (eV) Zeta Potential (mV)

Fe(ox)-Fe3O4

Fe(ox)Fe-CuOx

Fe(ox)Fe-MnOx

Fe(ox)Fe-CoOx

55.2 18.332 0.071 2.0 −2.6

78.4 19.263 0.141 2.2 −1.1

155.4 16.263 0.092 1.7 −10.2

139.9 43.516 0.263 2.2 −1.6

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Figure 6. % of degradation of phenol under UV-light, yellow light, and visible light by Fe(ox)Fe3O4 (a); Fe(ox)Fe-CuOx (b); Fe(ox)Fe-MnOx (c); and Fe(ox)Fe-CoOx (d).

the valence to the conduction band in a higher energy UV light source as compare to yellow LED. Further, to check the comparative activity of the material in different light sources, absorbance versus time plots were recorded, which shows a profile of exponential nature and indicates the pseudo-firstorder reaction. Furthermore, a straight line plot between Ln(A) vs time confirms the pseudo-first-order kinetics.68 The observed pseudo-first-order rate constant values further indicate higher reactivity in UV light compared to yellow LED and visible light in all the cases. On the other hand, irrespective of the light sources used for the study, the degradation reactivity decreases in the order of Fe(ox)Fe-MnOx > Fe(ox)-Fe3O4 > Fe(ox)FeCoOx > Fe(ox)Fe-CuOx (Table 3). Application of the Synthesized Material in Photocatalytic Dye Degradation. Inspired by the phenol degradation study, next we sought to check the activity of our synthesized material for colored dyes, because they are often found in industrial wastewater and are generally toxic and resistant to destruction by biological treatment methods. The activity of the synthesized nanomaterials was determined from both the % of degradation versus time as well as rate kinetics data using methylene blue (MB) as a representative case for the study. For the kinetics measurement, 5 mL of 10−4 (M) MB was taken in a vial and to it 5 mg of representative synthesized material was added. The progress of the reaction was monitored from the gradual decrease of absorbance with time.69 The corresponding plot between change in absorbance and time shows a profile of exponential nature, which indicates

Table 3. Pseudo-First-Order Rate Constant for Synthesized Nanomaterial Promoted Phenol Degradation under Different Light Sourcesa Rate Constant (min−1) UV Light Fe(ox)Fe-MnOx Fe(ox)Fe-CoOx Fe(ox)Fe-CuOx Fe(ox)-Fe3O4

58.9 42.5 21.6 45.1

× × × ×

−4

10 10−4 10−4 10−4

Yellow LED

Visible Light

−4

12.1 × 10−4 23.6 × 10−4 9 × 10−4 26.5 × 10−4

41.1 27.7 13.3 29.4

× × × ×

10 10−4 10−4 10−4

a

Reaction conditions: 40 mg of the corresponding catalyst was added into 20 mL of phenol solution (concentration 100 mg L−1) in a roundbottom flask and stirred for 1 h under dark, and then corresponding light was provided.

the pseudo-first-order reaction. Furthermore, the pseudo-firstorder rate constant was determined from the slope of the straight line plot between Ln(A) vs time.70 In terms of reaction rate, degradation under UV light shows the highest reactivity, while the least reactivity was found in the case of visible light for all the samples (Table 4). A similar observation was also found with the % of degradation versus time plot. To check further the need for light, the degradation experiment of MB with all four synthesized materials was done under dark. In all the cases, a slight decrease in absorbance of methylene blue was observed after keeping the dye solution with synthesized material for 12 h, which is due to the adsorption of dye on the surface of the material (Figure 8).71 So, the experiment under 315


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ACS Sustainable Chemistry & Engineering Table 4. Pseudo-First-Order Rate Constant for Synthesized Nanomaterial Promoted Degradation of Methylene Blue under Different Light Sourcea Rate Constant (min−1) Fe(ox)Fe-MnOx Fe(ox)Fe-CoOx Fe(ox)Fe-CuOx Fe(ox)-Fe3O4

UV Light

Yellow LED

Visible Light

10−3 10−3 10−3 10−3

23 × 10−3 3.2 × 10−3 2.4 × 10−3 6 × 10−3

17 × 10−3 2.6 × 10−3 2 × 10−3 3 × 10−3

36 6 4 10

× × × ×

a

Reaction conditions: 5 mg of the corresponding nanomaterial was added to 5 mL of 10−4 M methylene blue solution and kept under the corresponding light source.

dark indeed suggests the need for light for the electron transfer from the valence band to the conduction band for the photodegradation of MB. Next, to check the reactive species involved in the photodegradation process, Fe(ox)Fe-MnOx was chosen as model catalyst for further experiment, as it has better catalytic activity in comparison to the other three catalysts.

Figure 7. Fe(ox)Fe-MnOx promoted photodegradation of MB under different conditions: visible light (A); H2O2 (B); MeOH/H2O (1:20) (C); MeOH/H2O (1:20) and H2O2 (D); iPrOH/H2O (1:20) (E); i PrOH/H2O (1:20) and H2O2 (F); benzoquinone (G); EDTA (H).

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MECHANISM Detection of Reactive Oxidant Species. For this, a series of experiments with Fe(ox)Fe-MnOx promoted degradation of MB was set with different scavenger molecules for trapping the actual radical or hole involved in the process. Generally, the reactive species, such as hydroxyl radicals (OḢ ), superoxide radical anions (O2−), and holes (h+), are expected to be involved in the photocatalytic dye degradation processes.72−74 The agents, methanol (MeOH/H2O = 1:4), 2-propanol (2propanol/H2O = 1:20), and benzoquinone (0.1 mmol) were added to the MB solution, and its photodegradation with Fe(ox)Fe-MnOx was analyzed with a UV−vis spectrometer. No significant change in the degradation rate was observed when benzoquinone was added to the reaction mixture. As benzoquinone is an O2− radical quencher, thus involvement of superoxide radical anions can be ruled out. However, in the presence of methanol or 2-propanol, the rate of degradation drastically decreases. The reductions of rate constant in the presence of hydroxyl radical scavengers suggest that hydroxyl radicals are the key active oxidant that is responsible for the degradation in all the cases. In contrast, EDTA was used as a hole scavenger to check its role in the photodegradation.75,76 It had also significantly reduced the degradation rate of MB, confirming that the holes were the dominant active species. From these results, we can conclude that the photocatalytic process was mainly governed by direct holes (h+) and free hydroxyl radical (OH°). Further, to know the involvement of hydroxyl radical (OḢ ), the Fe(ox)Fe-MnOx promoted visible light photodegradation of MB was done in the presence of hydrogen peroxide (H2O2). Interestingly, enhancement in the photodegradation rate of MB was observed in H2O2 compared to only visible light, because of no more (OH°) radical in H2O2 for the possible Fenton process. Again, in the presence of OH° radical scavengers such as methanol or 2-propanol, a 50% decrease in degradation rate was observed, which also confirms the involvement of OH° radical in the photodegradation process (Figure 7). Moreover, detection of hydroxyl radical was carried out by the benzoic acid hydroxylation method. For this, benzoic acid was added to a aqueous solution of Fe(ox)FeMnOx and stirred for 5 min at room temperature. After that, 100 μL of H2O2 was added and stirred for another 5 min. Then FeCl3·6H2O was added in the reaction mixture and monitored

with UV−vis spectroscopy. Gratifyingly, the yellow color reaction mixture turned dark brown and finally deep red with the generation of one broad band at 520 nm, which indicated the formation of FeIII complex with salicylic acid.77 So, the above experiment also suggested the formation of active hydroxyl radical via the possible Fenton reaction between H2O2 and Fe(ox)Fe-MnOx. Role of Catalyst−Dye Interaction and Detection of Degraded Product. Photocatalysis aims at mineralization of poisonous dyes to CO2, H2O, and inorganic compounds or at least their transformation into biodegradable or harmless products. In this regard, few studies with few semiconducting materials were done for the destructive oxidation of various dyes to their corresponding CO2, H2O, and nontoxic inorganic compounds.78−80 In a Fe(ox)Fe-MnOx promoted photodegradation study of MB, the decrease in the 663 nm band intensity of the MB may be explained either by the reduction of MB to leuco MB (LMB) or by decomposition of MB. In our case, we did not observe any peak at 256 nm under deaerated conditions, which ruled out the possible reduction process of MB to LMB. On the other hand, Fe(ox)Fe-MnOx promoted decomposition of MB may take place via either the oxidative demethylation of the aromatic moiety from the dye skeleton or cleavage of the C−N bond of the aromatic moiety of MB. In oxidative demethylation, a gradual blue shift of the MB peak absorbance at 662 nm is expected. But in our case, we did not observe any blue shifting of the MB absorbance peak at 662 nm, which ruled out the possibility of oxidative demethylation of the dye skeleton (Figure S10). However, a flat decrease in the absorption maxima of MB in the presence of Fe(ox)FeMnOx material demonstrates the skeletal degradation of MB. This skeletal degradation of MB may occur via C−N bond cleavage at the central ring of the MB. To check further, a Fe(ox)Fe-MnOx promoted degradation study was done at different pH. First, the UV−vis spectrum of an aqueous solution of MB was recorded at different pH.81−83 Interestingly, a red shift of the MB absorbance peak was observed in acidic pH. Although we did not observe any shift in the absorbance peak of MB, absorbance at 662 nm decreases at alkaline pH. In highly alkaline solution (pH = 13), a rapid decrease in absorbance at 662 nm was observed along with enhancement at 316


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Figure 8. UV−vis spectrum of an aqueous solution of methylene blue at different pH (a); % of degradation of MB under dark (b); % of degradation versus pH plot in Fe(ox)Fe-MnOx promoted degradation of MB under visible light (c); different protonated species and their calculated HOMO LUMO energy level at the B3LYP/6-31+G* level of theory (d).

606 nm, which corresponds to the dimerization of MB.84 Further, to know the actual species responsible for peak shifting of MB at acidic pH, a theoretical calculation was done with plausible protonated species of MB. Protonation in MB may occur either in two NMe2 groups or at the central nitrogen atom of MB or both in tandem. All the plausible protonated species of MB were optimized at the B3LYP/6-31+G* level of theory, and their stability was calculated from the corresponding free energy of formation (ΔG) of the product from the free energy difference between product and reactant with zero point energies (ZPEs) and thermal corrections at 298 K. In terms of ΔG, MB-I is more stable compared to MB-II, which indicates the greater protonation tendency at the central nitrogen compared to the other two NMe2. On the other hand, a decrease in the HOMO LUMO energy gap was found in the protonated species, which also justifies the red shift in the UV− vis spectrum of MB in acidic solution. When we compared the Fe(ox)Fe-MnOx promoted visible light degradation rate at different pH, a gradual enhancement in degradation rate was observed with decreasing pH.85−88 The higher degradation rate in acidic solution may be explained from both the generation of protonated species and its higher surface adsorption toward a negatively charged nanomaterial surface. To check the adsorption capacity of protonated MB, an aqueous solution of MB and a fixed amount of Fe(ox)Fe-MnOx material was kept at different pH under dark for 10 h. After that, the absorbance of each of the solutions was recorded with UV−vis spectroscopy. Interestingly, more adsorption of MB in the nanomaterial surface was observed in acidic pH compared to neutral or alkaline medium, which is due to the greater interaction between two oppositely charged protonated MB

species and the nanomaterial surface. At the same time, the lower HOMO LUMO energy gap of protonated species compared to MB will also drive the degradation process because of rapid transfer of electrons in the former compared to the latter (Figure 8). Next, to check the photodegraded product of MB, the reaction mixture was analyzed in an ion chromatogram after completion of the reaction. Interestingly, we observe the presence of ions such as nitrate, sulfate, and chloride, which were initially absent in the dye or nanomaterial solution.89 So, the presence of the above-mentioned ions is also evidence of skeletal decomposition of methylene blue. The decomposition of MB may occur via three different paths. First, Cl− present in the MB structure ionized first during the dissolution of MB and exists in the detached state. Then, the N−CH3 bond will break down, as it has the lowest bond energy of 70.8 kcal/mol, and simultaneously, oxidation of CH3 to HCHO or HCOOH occurs. Then, skeletal degradation through C−S and C−N bond cleavage to its corresponding small molecule will occur. Finally, the oxidation of the corresponding small molecule will lead to the generation of CO2, H2O, Cl−, SO42−, and NO3− (Figure 10).90−92 The degraded product in Fe(ox)Fe-MnOx promoted degradation of MB in the presence of yellow LED and visible light was also analyzed with ion chromatography. In both cases, the presence of ions indicates the total mineralization of the MB dye into their corresponding ions and CO2 and H2O (Figure 9 and 10). Thus, detoxification of wastewater by complete mineralization of dye can be done with Fe(ox)Fe-MnOx promoted photocatalysis rather than chemical transformation. The abovementioned experiments suggest that hydroxyl radical is mainly responsible for the degradation of organic dyes (Figure 9). 317


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dye. The Fe(ox)Fe-MnOx promoted degradation experiments of all the four cationic dyes were carried out with 10−4 M dye solution with 5 mg of catalyst. The progress of the photodegradation of the representative dye was measured from the decrease of their corresponding absorbances. For the study, the absorbances at 664 nm for MB, 621 nm for MG, 585 nm for CV, and 583 nm for MV were used. When we compared the Fe(ox)Fe-MnOx catalyst activity with all the four dyes, the highest degradation rate was achieved with MV. The Fe(ox)Fe-MnOx promoted visible light photodegradation rate decreases in the order MG > MB > MV > CV (Table 5). Figure 9. Schematic view of dye degradation.

Table 5. Pseudo-First-Order Rate Constant for Each Dye Degraded by Fe(ox)Fe-MnOxa Dye

Rate Constant (min−1)

Methylene Blue Melachite Green Crystal Violet Methyl Violet

64 × 10−2 76 × 10−2 47.06 × 10−2 47.98 × 10−2

Reaction conditions: 5 mL of 10−4M of representative dye solution is taken, and to it 5 mg of the catalyst is added, and then the reaction mixture is kept under a corresponding light chamber. a

Figure 10. Ion chromatogram for Fe(ox)Fe-MnOx promoted photodegradation of methylene blue under yellow light (a) and visible light (b).

Reusability of the Material. For practical applications of such heterogeneous catalysts, the lifetime of the catalyst and its reusability are very important factors. To check this, a set of experiments was done for the visible light photodegradation of MB using all the four synthesized nanomaterials as a catalyst. After the completion of the first reaction, the catalyst was recovered via centrifugation, washed with water for 3−4 times, and dried in an oven at 80 °C for 12 h. Then, a new reaction was performed with fresh MB solution under the optimized reaction conditions. The kinetics of each of the reactions was monitored from the gradual decrease of the absorbance (A) of MB at 664 nm. The rate constant of each of the reactions was observed from the corresponding slope of the Ln(A) vs time plot. In the first cycle, all four materials are active toward the visible light photodegradation of MB. However, in terms of photodegradation rate, Fe(ox)Fe-MnOx is most reactive and Fe(ox)-Fe3O4 is least reactive. In the second cycle, the reactivity of all the four materials drops down slightly, as we achieved lesser rate constant in all the four cases (Figure 11). However, in the fourth cycle, the rate of photodegradation was very less, and complete degradation did not occur except for Fe(ox)FeCoOx. The Fe(ox)Fe-CoOx material is found to be active for the degradation of MB even after the 10th cycle. So, from the reusability point of view, Fe(ox)Fe-CoOx is the best material for the photodegradation of dye. After the completion of the reaction, four materials were collected in the second and fourth cycles and used for ICP-OES measurements to check the leaching of metal. When we compare the metal content of used material (after fourth cycle) with its parent material, significant leaching of both the metals was observed in most of the cases. However, the leaching of metal in Fe(ox)Fe-CoOx is found to be less compared to the other three materials, which further justifies its better reusability compared to the other three nanomaterials (Table 6).95 Large Scale Application and Processing Cost of the Catalyst. Next, the activity of four synthesized nanomaterials was checked to see if the process could be scaled up for the large scale photodegradation application of dyes and its possible

Initially, light induced excitation of an electron from the valence band to the conduction band will produce electron−hole pairs in the material. After that, the dissociation of water adhering to the surface of a semiconductor nanomaterial by a photogenerated hole will lead to the generation of highly reactive hydroxyl radicals (OH−). So, the rate of the overall reaction is directly dependent upon the rate of electron−hole pair recombination, which will control the reactive free radical generation. In the case of doping, the band gap of the new material either increases or decreases, depending upon the extent of doping and the nature of the dopant, which will directly affect the electron−hole pair recombination rate. In our case, doping of MnO to the Fe(ox)-Fe3O4 leads to a decrease in its band gap, whereas an enhancement of the band gap was observed in the corresponding Fe(ox)Fe- CuOx and Fe(ox)FeCoOx materials after the doping of CuO and Co3O4 in the parent material. The higher photocatalytic reactivity in the case of Fe(ox)Fe-MnOx may be attributed to the delay in electron− hole pair recombination. The lower band gap and higher surface area of Fe(ox)Fe-MnOx are also responsible for its high photocatalytic activity compared to other materials. On the other hand, the lower activity in the case of Fe(ox)Fe-CuOx and Fe(ox)Fe-CoO x may be attributed to the faster recombination of the electron−hole pair.93,94 Substrate Scope in Fe(ox)Fe-MnOx Promoted Degradation of Dyes. To further check the versatility of the most active Fe(ox)Fe-MnOx catalyst, photodegradation experiments with cationic, neutral, and anionic dyes were attempted. Interestingly, the photodegradation activity of Fe(ox)FeMnOx catalyst was found with cationic dyes such as methylene blue (MB), melachite green (MG), crystal violet (CV), and methyl violet (MV). No photodegradation activity of Fe(ox)FeMnOx for dyes such as rose Bengal, methyl orange, methyl red, and congo red was found to be observed under visible light. The inactivity may be suggested to be due to the lack of binding between the negatively charge surface and anionic or neutral 318


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Figure 11. Rate constant vs number of cycles for all the synthesized materials up to the 4h cycle (a), and rate constant vs number of cycles for Fe(ox)Fe-CoOx up to the 10th cycle (b).

In all the cases, the highest reactivity was achieved in the first cycle and reactivity decreased with increasing number of cycles (Table 8). However, in the case of Fe(ox)Fe-CoOx, the largest

Table 6. Metal Content in Synthesized Nanomaterial from ICP-OES Measurement Before use Fe Fe(ox)Fe-MnOx Fe(ox)-Fe3O4 Fe(ox)Fe-CoOx Fe(ox)Fe-CuOx

a

9.0 16.0 7.2 7.6

M

2nd cycle b

4.1 4.8 4

4th cycle

Fe

M

Fe

M

8.2 14.2 6.3 6.1

3.2

5.3 10.2 5.8 4.8

2.1

4.1 3.1

Table 8. Large Scale Photodegradation of Four Different Dyes with Four Different Synthesized Nanomaterialsa #

3.2 2.7

a

Metal content (in wt %) in the sample was determined using ICPOES analysis after digesting the sample with HCl. bM represents the corresponding transition metal (Mn, Cu, or Co) in the sample.

industrial utilization. Initially, large scale synthesis of all the materials was attempted. Gratifyingly, four nanomaterials could be easily synthesized even up to 25 g scale in a single batch without compromising any reduction in their conversion compared to small scale reaction. For the testing and development of the active catalyst for large scale industrial application, the final processing costs of the catalyst must fall within the allocated budget. The synthetic costs of the four materials have been calculated and provided in Table 7. Next, Table 7. Synthetic Cost for 1 g of All the Heterobimetallic Nanomaterials Material

Cost in Indian Rupee

Cost in Dollar

Cost in Euro

Cost in Yen

Fe(ox)-Fe3O4 Fe(ox)Fe-CuOx Fe(ox)Fe-CoOx Fe(ox)Fe-MnOx

1.1 1.3 1.5 1.3

0.016 0.020 0.022 0.019

0.015 0.018 0.020 0.018

1.9 2.5 2.7 2.4

Cycle 1

MB MV CV MG

80 15.5 45 100

MB MV CV MG

35 10.7 27 50

MB MV CV MG

32 9.5 25 45

MB MV CV MG

40 11.9 30 65

Cycle 2

Cycle 3

Fe(ox)Fe-MnOx (1 g) 45 30 10.7 5.9 20 10 60 30 Fe(ox)Fe-CuOx (1 g) 25 10 7.1 3.5 15 8 30 20 Fe(ox)-Fe3O4 (1 g) 25 10 5.9 3.6 15 10 32 20 Fe(ox)Fe-CoOx (1 g) 30 27 8.3 5.6 20 15 50 30

Cycle 4−8

Total

5 2.4 5 10

160 34.4 80 200

4 1.2 4 5

74 22.5 55 105

5 1.2 7 4

72 19.2 57 101

24 3.2 15 30

121 29.0 80 175

a

Reaction condition: In a 1 L beaker, 1 g of the catalyst was added to 500 mL of water, and to it the desired amount of dye was added. The reaction mixture was stirred until complete discoloration occurred. After that, again the desired amount of dye was added, and the process was continued until degradation stopped. After this, the catalyst was recovered and sonicated for 30 min and washed 2−3 times and dried at 80 °C for 6 h. Again the process was repeated until the catalyst lost its activity.

large scale photodegradation with these materials was checked by using four representative dyes, such as MB, CV, MV, and MG. For this, 1 g of material was added in 1 L of water, and then the appropriate amount of dye was added to it and stirred at room temperature for the required time. After the completion of the first reaction (checked vide UV−vis absorbance), nanomaterial was recovered by centrifugation and washed with water for 3−4 times and dried in an oven at 80 °C for 12 h. Then, a new reaction was performed with fresh dye solution under the optimized reaction condition, and thus, the cycle was repeated.

scale activity of the catalyst was observed up to the eighth cycle with reduction in reactivity after proceeding to each cycle. In terms of reusability, Fe(ox)Fe-CoOx was found to be best. However, in terms of total amount of degraded dye, Fe(ox)FeMnOx showed the highest reactivity compared to all the three materials. With respect to dye, the highest degradation in the case of MB and MG was achieved, while the lowest activity was found in the case of MV. 319


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ACS Sustainable Chemistry & Engineering Large Scale Application of the Nanomaterial for the Mixture of Dyes. Next, to check the photodegradation activity of four synthesized materials for a mixture of dyes, four dyes were mixed in equimolar proportions, and the reaction kinetics was monitored from the gradual decrease of the absorbance (A) at 584 nm (Figure 12). When judged in terms of their pseudo-

Table 10. Large Scale Photodegradation of Four Synthesized Materials for the Photodegradation of Mixtures of Four Cationic Dyes Fe(ox)Fe-MnOx Fe(ox)Fe-CoOx Fe(ox)Fe-CuOx Fe(ox)-Fe3O4

first-order rate constant, Fe(ox)Fe-MnOx showed the highest reactivity, while Fe(ox)-Fe3O4 is the least reactive (Table 9). Table 9. Pseudo-First-Order Rate Constants (k, s−1) of Four Synthesized Materials for the Photodegradation of Mixtures of Four Cationic Dyesa Rate constant (s−1)

Fe(ox)Fe-MnOx Fe(ox)Fe-CoOx Fe(ox)Fe-CuOx Fe(ox)-Fe3O4

2.42 10 × 10−3 6.04 × 10−4 2.23 × 10−4

a

Reaction condition: 5 mg of each catalyst was added to 5 mL of the synthetic dye having concentration 10−5 M, and then the reaction mixture was kept in the corresponding light chamber until degradation occurred.

Next, the study was extended for large scale degradation of mixtures of dyes. For this, 1 g of material was added in 1 L of water, and then appropriate amounts of equimolar mixtures of four dyes were added to it and stirred at room temperature for the required time. After the completion of the first cycle (checked vide UV−vis absorbance), a new reaction with fresh dye solution was done with recovered catalyst under optimized reaction conditions, and thus, the cycle was repeated. The observed large scale degradation for a mixture of dyes with all the four synthesized materials was summarized in Table 10. In terms of total amount of dye, Fe(ox)Fe-MnOx is best, while Fe(ox)-Fe3O4 is least reactive.

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Cycle 2

Cycle 3

Cycle 4−8

Total

80 55 45 40

40 32 25 30

25 27 15 15

4 15 5 4

149 129 100 89

vigorous stirring. During the addition, the color of the solution slowly turned yellow and then green, and finally black iron nanoparticles began to appear in the solution. The whole set up was kept overnight with stirring, and after 12 h, a dark brown material was obtained. This material was collected by centrifuging, and 495 mg of the material was obtained after keeping the material at 80 °C in an oven for 8 h. General Procedure for the Synthesis of Fe(ox)Fe-CuOx, Fe(ox)Fe-CoOx, and Fe(ox)Fe-MnOx Nanomaterials. At first in a 500 mL conical flask, 850 mg of Mohr’s salt, (NH4)2SO4·FeSO4, and 252 mg of oxalic acid were dissolved in 150 mL of distilled water. Then, a solution of 600 mg of NaBH4 was prepared in 50 mL of distilled water. Then, NaBH4 solution was added dropwise to the previously prepared solution under vigorous stirring. After the NaBH4 addition, the color of the reaction mixture slowly turned yellow and then green, and finally, black iron nanoparticles began to appear in the solution. After that, the corresponding metal salt solution was added to the reaction mixture and stirred at room temperature for 30 min. Then, the reaction mixture was heated to 80 °C and stirred at that temperature for another 12 h. During this period, a colored material was obtained, which was centrifuged, washed with distilled water for several times, and dried in an oven at 80 °C for 8 h. The metal salts used in different cases were 800 mg of CuSO4·5H2O for 540 mg of brown Fe(ox)Fe-CuOx; 1 g of Co(NO3)2·6H2O for 570 mg of yellow Fe(ox)Fe-CoOx. and 1 g of KMnO4 for 560 mg of black Fe(ox)FeMnOx nanomaterial. Large Scale Synthesis of Fe(ox)-Fe3O4, Fe(ox)Fe-CuOx, Fe(ox)Fe-CoOx, and Fe(ox)Fe-MnOx Nanomaterials. In a typical procedure, 34 g of Mohr’s salt and 10 g of oxalic acid were added to 400 mL of distilled water to prepare a solution in a 1 L beaker. A solution of 30 g of NaBH4 was prepared in 200 mL of distilled water and added dropwise to the earlier prepared solution under vigorous stirring. After the completion of the reaction, the reaction mixture was kept under stirring at room temperature for 12 h. During this, the black colored solution slowly turns yellow-brown, and finally brown material was isolated in 22 g amount after centrifuging and oven drying at 80 °C for 8 h. For the synthesis of Fe(ox)Fe-CuOx, 25 g of CuSO4·5H2O dissolved in 100 mL of distilled water was added to the reaction mixture, followed by the immediate formation of a black colored Fe(ox)-Fe(0) nanomaterial, and stirring at room temperature for 45 min. After that, the reaction mixture was kept at 80 °C under stirring for 12 h. After that, the reaction mixture was centrifuged, washed several times with distilled water to remove unreacted metal salt, and collected after keeping it in an oven at 80 °C for 8 h. Finally, 25 g of yellow-brown material was collected from the reaction. The same procedure was applied with 29 g of Co(NO3)2·6H2O and 16 g of KMnO4 for the synthesis of the corresponding Fe(ox)Fe-CoOx and Fe(ox)Fe-MnOx nanomaterials. The collected materials after the reactions in both the Fe(ox)Fe-CoOx and Fe(ox)Fe-MnOx cases were found to be 27 and 24 g, respectively. Degradation of Phenol with Fe(ox)-Fe3O4, Fe(ox)Fe-CuOx, Fe(ox)Fe-CoOx, and Fe(ox)Fe-MnOx Nanoparticles. At first 100 mL of an aqueous homogeneous solution of phenol with concentration 100 mg/L was prepared separately for the study. Then 20 mL of the phenol solution was mixed with 40 mg of each of the synthesized nanomaterials in different reaction chambers and kept at dark for 1 h at room temperature with continuous stirring. Then, the reaction chamber was kept under different light sources. The progress

Figure 12. Degradation of synthetic dye by Fe(ox)Fe-MnOx under visible light.

Sample

Cycle 1

EXPERIMENTAL SECTION

Typical Procedure for the Synthesis of Fe(ox)-Fe3O4 Nanoparticles. In a typical procedure, 850 mg of Mohr’s salt, (NH4)2SO4· FeSO4, and 252 mg of oxalic acid were added to 150 mL of distilled water to prepare a solution in a 500 mL conical flask. A solution of 600 mg of NaBH4 was prepared in 50 mL of distilled water. Then, NaBH4 solution was added dropwise to the earlier prepared solution with 320



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of each of the reactions was followed from a steady decrease of the absorbance maxima at a specified band position of the phenol. Degradation of Dye with Fe(ox)-Fe3O4, Fe(ox)Fe-CuOx, Fe(ox)Fe-CoOx, and Fe(ox)Fe-MnOx Nanoparticles. At first 50 mL of an aqueous homogeneous solution of methylene blue, crystal violet, melachite green, and methyl violet (10−4 M) dye was prepared separately for the study. Then, 5 mL of the representative dye solution was mixed with 7 mg of each of the synthesized nanomaterials in different vials and kept at room temperature under focused visible, yellow LED, and UV light. The progress of each of the reactions has been followed from a steady decrease of the absorbance maxima at a specified band position of the representative dye. General Procedure of Large Scale Degradation of Different Dyes, and Synthetic Mixtures of Dye with Fe(ox)-Fe3O4, Fe(ox)Fe-CuOx, Fe(ox)Fe-CoOx, and Fe(ox)Fe-MnOx Nanomaterials. At first 1 g of representative nanomaterial was taken into 500 mL of water and stirred for 5 min at room temperature. Then, the required amount of dye was added to the solution and its photocatalytic degradation was studied visually from the fading of the original dye color as well as UV−vis monitoring of the solution. After the completion of the first cycle, catalyst was recovered from the reaction mixture by centrifugation and washing with water for 3−4 times and drying in oven at 80 °C for 6 h. Then, a new reaction was performed with fresh dye solution under the optimized reaction condition and thus the cycle was repeated. For the degradation of the synthetic mixture of four dyes, the above-mentioned procedure was applied with an equimolar mixture of four dyes.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b01673. Procedural, spectral, and reaction kinetics data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] or [email protected]. ORCID

Sanjay Pratihar: 0000-0002-0229-735X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support of this work by DST-New Delhi (to SP for INSPIRE grant no. IFA-12/CH-39) is gratefully acknowledged. We are highly grateful to Santi M. Mandal, IIT Kharagpur, for all the help regarding XPS analysis.

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REFERENCES

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CONCLUSIONS In summary, a green and sustainable approach has been designed for the large scale production of four different selective orthorhombic iron(oxalate) capped Fe, Cu, Co, and Mn doped heterobimetallic oxide nanomaterials without the use of high temperature calcinations. The variable band gap, morphology, surface charge, and surface area of the four synthesized materials have been successfully applied for the visible light photodegradation of various dyes. The mechanistic study from in situ experimental evidence suggests that complete mineralization of dye into corresponding CO2, NO3−, and SO42− took place through photogenerated holes and free hydroxyl radical (OH°) via skeletal decomposition of dye. The higher enhanced photodegradation rate as well as peak shifting of methylene blue (MB) in acidic pH is justified with theoretical studies and suggested to be suitable for more surface absorption between protonated MB and the catalyst surface and lowering of the HOMO LUMO energy gap in protonated species compare to MB. Among four synthesized nanomaterials, the highest photodegradation activity toward various dyes is achieved with iron(oxalate) capped Fe−Mn oxide [Fe(ox)Fe-MnOx] nanomaterials. The synthesized material is also found to be suitable for the degradation of phenol under different light sources. Thus, the method developed here is favorable for the scalable synthesis of nanomaterials and provides large scale application of photodegradation of various dyes and is expected to be useful in many other applications.

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COMPUTATIONAL DETAILS

All the calculations were performed using the Gaussian09 suite of programs.96 Geometries of all the molecules were optimized using the B3LYP functional and 6-31+G* basic set. The free energy of formation of the product was calculated from the free energy difference between product and reactant with zero-point energies (ZPE) and thermal corrections at 298 K. 321


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ACS Sustainable Chemistry & Engineering Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P. J.; Dannenberg, J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision D.02; Gaussian, Inc.: Pittsburgh, PA, 2009.

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