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Acrylate–based polymerizable sol–gel synthesis of magnetically recoverable TiO2 supported Fe3O4 for Cr(VI) photoreduction in aerobic atmosphere Swapna Challagulla, Ravikiran Nagarjuna, Ramakrishnan Ganesan, and Sounak Roy ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b01055 • Publication Date (Web): 01 Feb 2016 Downloaded from http://pubs.acs.org on February 2, 2016

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ACS Sustainable Chemistry & Engineering

Acrylate–based polymerizable sol–gel synthesis of magnetically recoverable TiO2 supported Fe3O4 for Cr(VI) photoreduction in aerobic atmosphere Swapna Challagulla‡, Ravikiran Nagarjuna‡, Ramakrishnan Ganesan* and Sounak Roy* Department of Chemistry, Birla Institute of Technology and Science (BITS) Pilani, Hyderabad Campus, Jawahar Nagar, Shameerpet Mandal, Hyderabad-500078, India

Abstract: TiO2 synthesized by polymerizable sol–gel approach is used to demonstrate the photoreduction of Cr(VI) in aqueous medium. Fe3O4 is chosen as the magnetically recoverable support, onto which, 10, 20, 30 and 50% of TiO2 have been dispersed following the polymerizable sol–gel approach. All the synthesized materials are characterized using XRD, SEM, diffuse reflectance spectroscopy and BET surface area measurement. Different holescavengers (oxalic acid, ammonium oxalate and ethanol) and an electron scavenger (potassium periodate) have been studied to compare the efficiency of Cr(VI) photoreduction in nitrogen as well as air atmosphere. With 5 mM of oxalic acid as the hole-scavenger, the TiO2/Fe3O4 materials have demonstrated superior activity to the non-supported bulk TiO2 towards Cr(VI) photoreduction in aerobic atmosphere. Particularly, 30% TiO2/Fe3O4 has shown the highest Cr(VI) photoreduction rate among the lot. The 30% TiO2/Fe3O4 catalyst has also demonstrated good recoverability as well as recyclability.

Keywords: TiO2, Fe3O4 support, sol–gel, Cr(VI) photoreduction, hole & electron scavengers

‡These authors contributed equally. *Corresponding Author(s) Email: [email protected]; [email protected]

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Introduction: In the backdrop of increasing environmental concern, water treatment is being considered as one of the frontier research areas.1–3 Water is mainly contaminated by the presence of various matters like dyes, active pharmaceutical ingredients, organic industrial effluents, and also heavy metals.4–6 Photocatalytic removal of organics and heavy metals is a promising approach as it offers several advantages.7,8 Some of them include easy recoverability of the photocatalyst, room temperature process and efficient energy management. For heavy metals, the hazardous metal ions are either reduced or oxidized and converted into non-hazardous species. One such example is chromium, whose most stable oxidation states are reported to be VI and III. Cr(VI), which is a thermodynamically stable state of chromium, is a highly hazardous species.9 It possesses acute toxicity as carcinogen and mutagen to the living being.10 It is also reported that Cr(VI) can easily penetrate the placenta and affect the fetus.11,12 On the other hand, Cr(III) is a non-harmful, essential metabolite for the function of insulin. Photocatalytic reduction of Cr(VI) to Cr(III), is therefore an effective strategy as it converts highly hazardous Cr(VI) into an essential and nonharmful species.13–15 Owing to the enormous potential for environmental remediation, there is a persistent quest for the design and development of new efficient photocatalytic materials. Several semiconducting materials such as TiO2, BaTiO3, SrTiO3, Bi2WO6 etc. have shown significant photocatalytic properties.16–24 Among them, TiO2 is by far the most popular candidate, on which, numerous works have been carried out such as tuning the band gap by doping, supporting onto different materials etc. so as to improve its efficiency.25–31 For example, He et al., reported photocatalytic reduction of Cr(VI) with TiO2 nanosheets.32 They have used the {001} facets of surface fluorinated anatase TiO2 in aqueous suspension to reduce Cr(VI). The approach of anchoring an active photocatalytic material like TiO2 on a support disperses the active material on its surface and thereby increases the active sites. In addition, the supports often synergistically act with the active material in enhancing the initial adsorption of the pollutants and therefore bring the pollutants to the near proximity of the active sites. In this line, we recently reported a new polymerizable sol–gel approach to synthesize TiO2 supported Zeolite-4A and demonstrated its application for dye degradation and nitroarenes reduction.33–35 This approach is suitable for large scale synthesis of state-of-the-art catalytic materials dispersed over a variety of supports to


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generate highly active heterojunctions. In addition, this approach also opens up the possibility of facile band gap engineering. Therefore, in this current work, we have extrapolated the polymerizable sol–gel approach to synthesize TiO2/Fe3O4 catalysts. Fe3O4 is chosen as the support since it is a magnetically recoverable and non-toxic material. Due to its biocompatibility, Fe3O4 has also been used in sensors and drug delivery. For sustainability and recyclability, such an easily recoverable support will be highly desirable. The recent trends show that a variety of functional materials has been anchored onto Fe3O4 in order to enhance the recyclability.36–38 A recent report by Yang et al., describes a three-component Fe3O4@rGO@TiO2 photocatalytic system, wherein, the role of Fe3O4@rGO in taking up the photogenerated electrons in the conduction band (CB) of TiO2 is discussed.39 Such a phenomenon of electron hopping from the CB of one semiconducting material to that of another semiconducting material would minimize the electron-hole pair recombination, which, in turn may enhance the photocatalytic efficiency of the overall system.40 In a CdS/α-Fe2O3 system, Zhang et al., have reported the synergistic role of heterojunctions towards the enhancement in photocatalytic activity.41 Thus, the Fe3O4 support chosen in this work is also aimed at enhancing the photocatalytic activity of TiO2 by minimizing the electron-hole pair recombination. This current work describes the synthesis and characterization of TiO2/Fe3O4 catalysts and their application for photoreduction of Cr(VI) that is considered to be an important environmental issue.

Materials and methods Materials. Titanium (IV) isopropoxide (TiPO), methacrylic acid (MAA), Fe3O4, potassium dichromate, benzoyl peroxide (BPO) and acetone were procured from Sigma Aldrich and used as-received. Ethanol, oxalic acid, ammonium oxalate, potassium periodate and 1,5diphenylcarbazide (DPC) were purchased from SD fine chemicals and used as-received. Synthesis of TiO2 and TiO2/Fe3O4 catalysts. TiO2 supported Fe3O4 catalysts were synthesized by acrylate–based polymerizable sol–gel approach, analogous to our previous report.33 Briefly, a polymerizable Ti-methacrylate complex was obtained by dropwise addition of MAA (2 eqvt.) to neat TiPO (1 eqvt.) that was kept stirring under inert atmosphere. Instantaneous yellow coloration was observed upon addition of MAA, indicating the formation



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of titanium dimethacrylate diisopropoxide complex. To formulate the polymerizable precursor solution, a 2 wt% of BPO (w.r.to MAA) in 50 µL of acetone was added to the Ti-methacrylate complex. Free-radical thermal polymerization at 110 °C of this polymerizable precursor solution and subsequent calcination at 450 °C yielded pristine TiO2. To synthesize TiO2/Fe3O4 catalysts, calculated amount of Fe3O4 nanoparticles was added as a support material to the polymerizable precursor solution as prepared above. About 50 µL of water was introduced to this mixture and shaken vigorously until it formed a homogeneous gel, inside which the Fe3O4 support was homogeneously dispersed. This gel was subjected to polymerization and calcination as mentioned above to obtain TiO2/Fe3O4 catalysts. Depending on the loading of Ti-methacrylate complex, catalysts containing different amount of TiO2 such as 10, 20, 30 and 50 wt% supported onto Fe3O4 were prepared. Characterization. X-ray diffraction (XRD) analysis of the synthesized catalysts was performed with Bruker AXS D8 Advance with Cu Kα radiation (λ = 1.5418 Å) at a scan rate of 1°/min. Field Emission Scanning Electron Microscopy (FE-SEM) fitted with energy dispersive spectroscopy (EDS) [Carl-Zeiss ULTRA-55] was employed to analyze the surface morphology and composition of the catalysts used in this study. Raman microscope (UniRAM 3300) was used to collect spectra of the catalytic materials at 532 nm wavelength. The core level X-ray photoelectron spectra (XPS) were collected using PHI 5000 Versa Prob II (FEI Inc.) to analyze the oxidation states of the constituent metal ions.The Brunauer-Emmett-Teller (BET) surface area of the synthesized catalysts was determined using nitrogen adsorption/desorption method in a Micromeritics ASAP 2020 surface area analyzer. Atomic absorption spectroscopy (AAS) (Shimadzu AA-7000) was used to quantify the total chromium content in the solution. Solid state UV measurements were performed on JASCO V-670 to study the band gap of the synthesized materials. JASCO V-650 UV-visible spectrophotometer was used to analyze the Cr(VI) concentration. Photocatalytic Studies. A cylindrical annular batch photoreactor fitted with a medium pressure mercury vapor lamp of 125 W was used for the photoreduction of Cr(VI). The lamp mainly had a broadband from 250 nm to 450 nm with useful maxima at 254, 312, and 365 nm for the photocatalytic reactions. The lamp was surrounded with a double-walled borosilicate immersion well and the set up was fitted inside a reaction vessel. To prevent IR radiation and to


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maintain constant temperature, water was constantly circulated around the lamp through the double-walled well. For each experiment, 100 mL of 20 ppm potassium dichromate solution and 30 mg of catalyst were taken in the reaction vessel. The pH of the solution was brought to 3.0 in the beginning of each experiment. Prior to the irradiation, the solution was incubated with the catalyst in dark for 15 minutes to attain adsorption-desorption equilibrium. The aliquots were collected at regular time intervals during the photoreduction. The samples were analyzed for Cr(VI) through complexation with DPC and the concentration was measured at 540 nm. The overall rate of the reaction was calculated below 10% conversion of Cr(VI). Results and discussion The pristine TiO2 and TiO2/Fe3O4 were synthesized using the acrylate–based polymerizable sol–gel route as per our previous reports.33–35 XRD was employed to characterize the as-procured Fe3O4, and 10, 20, 30 and 50% TiO2 supported Fe3O4 (Figure 1). The XRD profile of as-procured Fe3O4 corresponds to the inverse spinel phase with the space group Fd3തm (JCPDS No. 82-1533). When lower (10 and 20%) amounts of TiO2 are dispersed on Fe3O4, a prominent TiO2 phase was not visible. However, when the TiO2 loading was increased to 3050%, the anatase peak of TiO2 at 2θ=25.5° was noticeable. The broad anatase peak indicates the formation of highly dispersed nanocrystalline TiO2 over the Fe3O4 support. In all the TiO2/Fe3O4 catalysts, peaks correspond to α-Fe2O3 were observed (JCPDS No. 89-8103, and 89-8104). Interestingly, with the increase in the TiO2 loading, the intensity of the 2θ peaks correspond to αFe2O3 was also increased. We speculate that this could be due to the TiO2-mediated thermal oxidation of Fe3O4. Also, the synthesized TiO2/Fe3O4 catalysts were found to be brown in color, as opposed to the original black colored Fe3O4. Thus, we can conclude that TiO2 shares a heterojunction with α-Fe2O3, which is generated on the surface of Fe3O4 during the calcination step. FE-SEM was studied with a view to analyzing the morphology of the synthesized materials and the corresponding images are shown in Figure 2 (a-l). The pristine TiO2 was found to be having irregular shapes and the size of the particles was mostly in the range of 5 to 30 µm. A closer look at the particles revealed that the polycrystalline nature of TiO2 and the crystallite domains were in the size range of 10 to 50 nm. The commercial Fe3O4 particles were in octahedral shape, having the edges in the range of 200 to 300 nm. The tiny islands found on the



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surface of Fe3O4 in Figure 2d were due to the sputtered AuPt alloy and hence the TiO2/Fe3O4 catalysts were imaged without any metal sputtering to avoid this clustering. With the increase in TiO2 loading from 10-50%, there was a clear increase in the trend of TiO2 coated over the Fe3O4 support. At higher loadings, however, in addition to this coating, free TiO2 particles were also found. EDS and elemental mapping were performed on 30% TiO2/Fe3O4 to gain further insight on the elemental composition as well as dispersion, the results of which are shown in Figure 2 (m-p). The EDS spectrum revealed that the Ti:Fe ratio in the catalyst matches closely with that of feed ratio (Figure 2 (n)), while the elemental mapping analysis demonstrated a high dispersion of TiO2 over the Fe3O4 support (Figure 2 (o,p)). Raman spectroscopy was employed to analyze the pristine TiO2, calcined Fe3O4, and assynthesized 30% TiO2/Fe3O4 (Figure 3). A prominent peak at 134 cm-1 and weak peaks at 382, 500, and 618 cm-1 were observed in case of pristine TiO2 that correspond to the Raman-active modes of anatase phase with the symmetries of Eg, B1g, A1g, and Eg, respectively.42 Two more weak bands were observed at 430 and 593 cm-1, that can be assigned to the Eg and A1g Ramanactive modes of rutile phase.42 This is in accordance with the XRD analysis of pristine TiO2 that revealed the presence of both anatase as well as rutile phases. The calcined Fe3O4 support exhibited peaks at 215, 235, 281, 396, 483, 596, and 642 cm-1, which can be indexed to the Fe2O3 Raman-active modes of A1g(1), Eg(1), Eg(1), Eg(1), A1g(2), Eg(1), and Eu, respectively.43,44 The 30% TiO2/Fe3O4 composite catalyst possessed both the characteristic peaks of TiO2 as well as Fe2O3, which is again in accordance with the XRD observation that TiO2 shared an effective heterojunction with Fe2O3. Figure 4 shows the survey spectra of as-synthesized 30% TiO2/Fe3O4 and its core level photoelectron spectra of Ti(2p) and Fe(2p). The survey scan reveals the presence of constituent elements such as Ti, Fe and O.45-47 The Ti(2p3/2) and Ti(2p1/2) peaks were observed at 458.6 and 464.4 eV, respectively. This indicates the presence of Ti4+ in the catalytic material. The peaks at 710.7 and 724.3 eV were due to Fe(2p3/2) and Fe(2p1/2), respectively, which corresponds to Fe3+. The satellite peak of Fe(2p3/2) at ~718 eV further confirms that the iron on the surface is predominantly Fe3+, which is in accordance with XRD and Raman analyses. (K*E)1/2 vs E plots, derived from diffuse reflectance spectra of pristine TiO2, 10, 20, 30, and 50% TiO2/Fe3O4, calcined Fe3O4 and as-procured Fe3O4 are shown in Figure 5. Kubelka-


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Munk factor (K) is calculated by using the formula, K = (1-R)2/2R, where R represents the % reflectance and E stands for energy of the incident radiation. The sol–gel synthesized TiO2 shows the semiconducting band gap (Eg) of 3.2 eV. The as-procured Fe3O4 did not show any defined bandgap. The calcined Fe3O4, on the other hand, showed an Eg of 2.2 eV, which agrees well with the Eg of α-Fe2O3.48,49 In addition to the 2.2 eV, a shoulder is observed at ~1.9 eV. These transitions could be due to metal to ligand, ligand to metal charge transfers, and magnetically coupled Fe3+ adjacent site transitions.50 The 10-30% TiO2/Fe3O4 samples showed profiles exactly similar to that of α-Fe2O3. We did not observe any signature of TiO2 band gap in these samples. This could be attributed to the fine dispersion of TiO2 and large presence of α-Fe2O3. However, in 50% TiO2/Fe3O4, the spectrum clearly showed a shoulder at ~3.2 eV corresponding to Eg of TiO2, in addition to the Eg = 2.2 eV of α-Fe2O3. Table 1 shows the BET surface area measurements of the synthesized catalysts. The surface area of the pristine TiO2 was found to be 35.1 m2/g, while that of the calcined Fe3O4 was found to be 7.3 m2/g. With the increase in TiO2 loading from 10 to 50%, the surface area was found to be increasing from 10.9 m2/g to 59.3 m2/g. Such a remarkable increase in the surface area demonstrates high dispersion of the active material over the support, which is in line with the XRD observation. To gain further clarity, the surface area of 30% TiO2 and Fe3O4 physical blend was compared with 30% TiO2/Fe3O4 supported catalyst. The results revealed ~3 fold increase (11.9 m2/g to 32.6 m2/g) in surface area when TiO2 was anchored over the Fe3O4 support. Thus, the polymerizable sol–gel approach clearly enhances the dispersion of the active material over the support. Photocatalytic reduction of Cr(VI) by sol-gel synthesized pristine TiO2 with various hole scavengers under inert and aerobic conditions is plotted in Figure 6a and 6b, respectively. Ammonium oxalate, oxalic acid and ethanol have been chosen as the hole scavengers in this study. In all the photocatalytic reduction experiments, the Cr(VI) was incubated with the catalyst in dark for 15 min to equilibrate the adsorption/desorption process before the light was turned on. Irrespective of the reaction atmosphere, the initial adsorption of Cr(VI) on TiO2 was negligible when oxalic acid was used as the hole scavenger, whereas the same was found to be in the range of 25 to 35%, when ammonium oxalate and ethanol were used as the hole scavengers. As seen in Figure 6a, the rate of photoreduction of Cr(VI) under nitrogen atmosphere was comparatively



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faster with oxalic acid, and ammonium oxalate, than that with ethanol. Complete photoreduction of Cr(VI) with oxalic acid, and ammonium oxalate was achieved in 75 min. On the other hand, the photoreduction of Cr(VI) with ethanol as the hole scavenger resulted in only ~70% of conversion in 120 min. The trend was slightly different when the photoreduction experiments were carried out under aerobic atmosphere (Figure 6b). In this case, the complete photoreduction of Cr(VI) was accomplished in 75 min with oxalic acid, and 120 min with ammonium oxalate. With ethanol as the hole scavenger, the conversion after 120 min was found to be ~ 55%. TiO2 generates electrons in its CB and holes in its valence band (VB) when irradiated with suitable wavelength of light. The hole scavengers scavenge the holes from the VB, and thereby making the electrons in the CB available for photoreduction. When oxygen is present, it scavenges the electrons to get converted into superoxide radicals, and thereby minimizing the availability of electrons for the photoreduction, which slows down the Cr(VI) reduction. To prove this further, we performed an additional experiment in aerobic atmosphere by taking an additional electron scavenger, IO4-. The presence of iodate has resulted in further slowing down the reaction rate (see Figure 6b), which has proved that the availability of photogenerated electrons in the CB of the semiconducting catalytic material is crucial to reduce the Cr(VI). Total chromium concentration was quantified before and after photoreduction using AAS (data not shown). The analysis revealed that the total chromium concentration did not change after the photoreduction, which confirms that the reduced species remained in the solution. To identify the oxidation state of chromium after photoreduction, we treated the photoreduced product sample solution with 2,6-pyridine dicarboxylic acid (PDCA) that is known to specifically form a characteristic complex with Cr(III).51 A positive control sample was prepared wherein K2Cr2O7 was reduced by using NaHSO3, followed by heating with PDCA for 30 min. Both the solutions exhibiting a light violet color were subjected to UV-visible analysis (Figure S1). It was observed that both photoreduced as well as NaHSO3 reduced samples exhibited light absorption in the range of 550555 nm. However, a negative control of non-reduced K2Cr2O7 did not show any absorption in this wavelength range after treatment with PDCA. These results confirmed the successful conversion of Cr(VI) to Cr(III) under the photoreduction conditions. As the photoreduction under aerobic conditions has practical significance and also there was not much difference between aerobic and inert atmosphere, further photoreduction of Cr(VI) studies was carried out under aerobic conditions. To understand the chemistry of dispersed TiO2


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over a semiconducting magnetic support, TiO2/Fe3O4 was employed for aerobic photoreduction of Cr(VI). Since oxalic acid possessed the maximum hole scavenging ability, it was used for further photoreduction studies. Figure 7 shows the Cr(VI) photoreduction with 10-50% TiO2/Fe3O4 catalysts under UV light irradiation. Calcined Fe3O4, and 30% TiO2-calcined Fe3O4 physical blend were used as controls. As seen in the figure, the 10, 20, 30, and 50% TiO2/Fe3O4 catalysts reduced the Cr(VI) completely in 50, 40, 30, and 60 min, respectively. Whereas both the controls, calcined Fe3O4, and 30% TiO2-Fe3O4 physical blend, reduced the Cr(VI) in 75 min. For pristine TiO2, and calcined Fe3O4 the rate of Cr(VI) photoreduction was found to be 0.43, and 0.24 ppm g-1 min-1, respectively. When TiO2 was anchored onto Fe3O4, the reaction rates have been found to be 0.5, 0.59, 0.91, and 0.24 ppm g-1 min-1 for 10, 20, 30, and 50% TiO2/Fe3O4, respectively. The dispersion of the active sites clearly shows significant acceleration of the reaction rates up to 30% of TiO2 loading, beyond which the reaction rate has decreased. Although 50% TiO2/Fe3O4 has possessed roughly twice the surface area of 30% TiO2/Fe3O4, the decrease in the photocatalytic activity indicates the role of effective heterojunction between TiO2 and Fe3O4 support. In addition to the high dispersion, there is a significant synergistic role of the CBs of TiO2 and Fe2O3 towards the photocatalysis. In Figure 8, the Eg between VB and CB of TiO2 as well as Fe2O3 obtained from the diffuse reflectance spectra is merged with the standard reduction potential of Cr(VI)/Cr(III). Upon irradiation of UV light, the electrons move from the VB of TiO2 to its CB leaving the holes behind. The CB of Fe2O3 is slightly energetically lower than that of TiO2. The electron, therefore, may move downhill from the CB of TiO2 to CB of Fe2O3, which in turn could significantly reduce the electron-hole recombination in TiO2. The available electron in the CB of Fe2O3 can reduce the Cr(VI) to Cr(III) efficiently. Therefore, we observe a high rate of photoreduction with the sol–gel synthesized composite catalyst. However, when TiO2 was physically mixed with calcined Fe3O4, the TiO2-Fe2O3 interfaces were not so conducive for the effective hopping of electron from the CB of TiO2 to CB of Fe2O3. The optimum concentration for this effective band overlap was observed with 30% loading of TiO2 over Fe3O4, which could be due to the reason that the effective heterojunction between TiO2 and Fe3O4 is formed at this loading. In order to successfully recycle the composite catalysts, it is important to understand the magnetic recoverability. The saturation magnetization (Ms) was measured on Fe3O4, calcined Fe3O4 and 30% TiO2/Fe3O4 using a vibrating sample magnetometer (VSM). The corresponding



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hysteresis loops are shown in Figure 9. The commercial Fe3O4 exhibited a Ms value of 174 emu/g, which came down to 95 emu/g after calcination. This is due to the surface oxidation of Fe3O4 to α-Fe2O3 during the calcination. The 30% TiO2/Fe3O4 exhibited a normalized Ms value of 37.6 emu/g, which additionally confirmed the coating of TiO2 over Fe3O4. It is noteworthy that despite the decrease in value, the Ms of the final catalyst is acceptable for easy magnetic recovery of the catalyst, and the value is comparable to those Fe3O4-based catalysts found in the literature.39,52 Recyclability of the catalyst for photoreduction of Cr(VI) was carried out with 30% TiO2/Fe3O4 up to 4 cycles (Figure 10a). There was no decrease in the photocatalytic efficiency up to 4 cycles and ~84% of the catalyst was recovered at the end of the fourth cycle. The catalyst was recovered using a magnet at the end of each cycle (Figure 10b). To further probe the stability of the photocatalyst, we performed XRD, Raman spectroscopy, XPS, and SEM-EDS analyses of the 30% TiO2/Fe3O4 recovered catalyst after 4 cycles of photoreduction (Figure 11). The XRD pattern was found to be the same as that of assynthesized catalyst and thus confirmed that there is no change in the crystal structure before and after photoreduction. The core level XPS of Ti(2p) and Fe(2p) of the recovered catalyst did not show any change in the binding energy compared to that of as-synthesized catalyst. The SEM image as well as EDS analysis also revealed that the morphology and elemental composition of the catalyst remain unchanged even after 4 cycles of photoreduction. The same trend was observed in case of Raman spectral analysis as well. These results have proved the high stability of the catalyst under the reaction conditions and also the magnetic recoverability.

Conclusions: TiO2/Fe3O4 catalysts with various loading of TiO2 were synthesized by the polymerizable sol–gel route. The XRD and SEM characterization revealed high dispersion of TiO2 over Fe3O4. BET surface area measurements revealed increase in surface area with increase in TiO2 loading. The surface of the Fe3O4 was found to be partially oxidized to Fe2O3 during the calcination step. The diffuse reflectance spectra showed the semiconducting band gaps of TiO2 and calcined Fe3O4. The TiO2/Fe3O4 composite catalysts showed higher rate of photoreduction of Cr(VI) than the non-supported bulk TiO2 as well as calcined Fe3O4. The photoreduction mechanism was


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probed by using hole as well as electron scavengers. The photocatalytic studies show that the anchoring of TiO2 over Fe3O4 results in three-fold advantage. First, when the TiO2 is anchored over Fe3O4, the dispersion of the active site was higher. This is a crucial one for achieving higher reaction rate. Second, the significant band overlap of TiO2 and Fe3O4 helped in decreasing the electron-hole recombination reaction, thus enhancing the photoreduction rate. Third, Fe3O4 being a magnetic nanoparticle, the magnetic recyclability of the catalyst was convenient.

Acknowledgement: SR and RG thank the Department of Science & Technology (SERB/F/825/2014-15 and SERB/F/4864/2013-14) for the financial aid. The authors thank from Department of Science and Technology – fund for improvement of science and technology infrastructure (DST FIST; SR/FST/CSI-240/2012) to procure Raman microscopy. The authors also thank Prof. N. Rajesh, Dr. Balaji Gopalan, and Ms. T. Sathvika for helpful discussions. Thanks are due to Dr. Neha Y. Hebalkar of ARCI, Hyderabad for carrying out the surface area investigation. Table 1. Nitrogen adsorption isotherm (BET method) analysis of the photocatalysts.

#

Sample

Specific Surface area (m2/g)#

Pristine TiO2

35.1

Calcined Fe3O4

7.3

10% TiO2/Fe3O4

10.9

20% TiO2/Fe3O4

19.3

30% TiO2/Fe3O4

32.6

50% TiO2/Fe3O4

59.3

30% TiO2-Fe3O4 physical blend

11.9

Data obtained with 95 % confidence level.



Figure 1

Fe O (As-procured) 3 4

10 % TiO /Fe O 2 3 4

Intensity (Count)

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20 % TiO /Fe O 2 3 4

30 % TiO /Fe O 2 3 4

+ #

50 % TiO /Fe O 2 3 4

+ +*

10

20

#

+

30

40

#

+

#+

#

50

60

70

80

Angle (2θ)

Figure 1: XRD of Fe3O4, 10, 20, 30, and 50% polymerizable sol–gel synthesized TiO2/Fe3O4

catalysts. In 50% TiO2/Fe3O4, the anatase TiO2 (*), α-Fe2O3 (+), and Fe3O4 (#) phases are visible.


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Figure 2

Figure 2: Composite FESEM images of TiO2 (a, b), as-procured Fe3O4 (c, d), 10% TiO2/Fe3O4 (e, f), 20% TiO2/Fe3O4 (g, h), 30% TiO2/Fe3O4 (i, j), and 50% TiO2/Fe3O4 (k, l) photocatalysts. EDS analysis of 30% TiO2/Fe3O4 (n), and elemental mapping of Ti and Fe (o, p) from the selected area of (m).



Figure 3

134

TiO2 618 382 430 500 593

Counts (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Calcined Fe3O4

281 215 396

235

596

642

483

30 % TiO2/Fe3O4

284 215

397

236

486

136

0

200

645

400

596

600

800

1000

-1

Wavenumber (cm )

Figure 3: Raman spectra of pristine TiO2, calcined Fe3O4 and 30% TiO2/Fe3O4 synthesized by polymerizable sol–gel approach.


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Figure 4

Figure 4: Survey scan and core level photoelectron spectra of Fe(2p) and Ti(2p) of assynthesized 30% TiO2/Fe3O4 catalyst.



Figure 5

TiO2 Calcined Fe3O4

9

As-procured Fe3O4 10 % TiO2/Fe3O4

8

20 % TiO2/Fe3O4 30 % TiO2/Fe3O4

7

50 % TiO2/Fe3O4

1/2

(k*E)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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6 E = 1.9 5

E = 3.2 E = 2.2

4 3 2 1

2

3

4

5

6

E (eV) Figure 5: Plot of (Kubelka Munk factor (K) * Energy)1/2 vs Energy obtained from diffuse reflectance spectra of the catalysts.


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Figure 6

1.0

(a) 0.8

Oxalic Acid Ammonium Oxalate Ethanol

C/C0

0.6

0.4

Dark

0.2

0.0 0

20

40

60

80

100

120

Time (min)

1.0

(b)

0.8

Oxalic acid Ammonium Oxalate Ethanol KIO4

0.6

C/C0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Dark 0.4

0.2

0.0 0

20

40

60

80

100

120

Time (min)

Figure 6: Photocatalytic reduction of Cr(VI) with various hole and electron scavenger(s) under (a) inert and (b) aerobic conditions with polymerizable sol–gel synthesized TiO2.



Figure 7

50 % TiO2/Fe3O4

1.0

30 % TiO2/Fe3O4 20 % TiO2/Fe3O4 0.8

10 % TiO2/Fe3O4 30 % TiO2-Fe3O4 Physical blend Calcined Fe3O4

0.6

C/C0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Dark 0.4

0.2

0.0 0

20

40

60

80

Time (min)

Figure 7: Photoreduction of Cr(VI) with 10-50% TiO2/Fe3O4 , and calcined Fe3O4 catalysts under UV light irradiation in aerobic condition.


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Figure 8

Figure 8: Schematic illustration of the proposed mechanism for the Cr(VI) photoreduction using TiO2/Fe3O4 catalyst under UV light irradiation.



Figure 9

200 150

Magnetic Moment (emu/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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As-procured Fe3O4 Calcined Fe3O4

100

30 % TiO2/Fe3O4

50 0 -50 -100 -150 -200

-15

-10

-5

0

5 3 Magnetic field (x10 )(Oe)

10

15

Figure 9: Magnetic hysteresis plot of as-procured Fe3O4, calcined Fe3O4 and 30% TiO2/Fe3O4 catalysts.


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Figure 10

1.0

(a) 0.8

Dark

Dark

Dark

Dark

0.6

C/C0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.4

0.2

0.0 0

20

Cycle-1

40

0

20

40

0

20

Time (min) Cycle-2 Cycle-3

40

0

20

40

Cycle-4

(b)

Figure 10: (a) Recyclability of 30% TiO2/Fe3O4 for Cr(VI) photoreduction up to 4 cycles, and (b) photograph showing the magnetic recoverability of the catalyst.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 11

Figure 11: XRD (a), Raman spectrum (b), core level XPS of Ti(2p) (c) and Fe(2p) (d), SEM (e, f), and EDS (g) data of 30% TiO2/Fe3O4 recovered catalyst after 4 cycles of photoreduction.


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Supporting Information. UV-visible spectra of PDCA treated solutions of K2Cr2O7, and TGA thermogram of poly(methacrylic acid) and polymerized precursor of 30% TiO2/Fe3O4.

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Synopsis For Table of Contents Use Only Acrylate–based polymerizable sol–gel synthesis of magnetically recoverable TiO2 supported Fe3O4 for Cr(VI) photoreduction in aerobic atmosphere Swapna Challagulla‡, Ravikiran Nagarjuna‡, Ramakrishnan Ganesan* and Sounak Roy* Department of Chemistry, Birla Institute of Technology and Science (BITS) Pilani, Hyderabad Campus, Jawahar Nagar, Shameerpet Mandal, Hyderabad-500078, India The tailored polymerizable sol–gel synthesized TiO2/Fe3O4 catalyst exhibited an effective heterojunction of TiO2-Fe2O3 for optimum semiconducting band overlap. The effective heterojunction thus obtained, enhanced the efficiency of the catalyst towards Cr(VI) photoreduction in aerobic condition.


Acrylate-based Polymerizable Sol–Gel Synthesis ... - ACS Publications

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