Alkali metal ion induced cube shaped mesoporous

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Alkali metal ion induced cube shaped mesoporous hematite particles for improved magnetic properties and efficient degradation of water pollutants† Mouni Roy and Milan Kanti Naskar* Mesoporous cube shaped hematite (a-Fe2O3) particles were prepared using FeCl3 as an Fe3+ precursor and 1-butyl-3-methylimidazolium bromide (ionic liquid) as a soft template in the presence of different alkali metal (lithium, sodium and potassium) acetates, under hydrothermal conditions at 150 1C/4 h followed by calcination at 350 1C. The formation of the a-Fe2O3 phase in the synthesized samples was confirmed by XRD, FTIR and Raman spectroscopy. Unlike K+ ions, intercalation of Li+ and Na+ ions occurred in a-Fe2O3 crystal layers as evidenced by XRD and Raman spectroscopy. Electron microscopy (FESEM and TEM) images showed the formation of cube-like particles of different sizes in the presence of Li+, Na+ and K+ ions. The mesoporosity of the products was confirmed by N2 adsorption–desorption studies, while their optical properties were analyzed by UV-DRS. Na+ ion intercalated a-Fe2O3 microcubes showed improved coercivity (5.7 kOe) due to increased strain in crystals, and shape and magnetocrystalline anisotropy. Temperature dependent magnetization of the samples confirmed the

Received 12th April 2016, Accepted 30th June 2016

existence of Morin temperature in the range of 199–260 K. Catalytic degradation of methylene blue (MB),

DOI: 10.1039/c6cp02442d

process. The degradation products were traced by electrospray ionization-mass spectrometry (ESI-MS).

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The a-Fe2O3 microcubes obtained in the presence of Na+ ions exhibited a more efficient degradation of MB to non-toxic open chain products.

a toxic water pollutant, was studied using the synthesized products via a heterogeneous photo-Fenton

1. Introduction In recent years, various efforts have been devoted to the development of nano- or micro-scale magnetic materials due to their applications in different fields of research such as MRI contrast agents,1 ferrofluids,2 hyperthermia treatment,3 catalysis4 and so on. Magnetic materials such as a-Fe2O3 (hematite), in particular, find a wide range of applications in data storage devices,5 biosensors,6 catalysts,7 gas sensors,8 lithium ion batteries,9 solar cells,10 pigments,11 etc. due to their cost-effectiveness, environmental friendliness, and corrosion resistant properties. The magnetic behaviour of such materials depends strongly on their size, shape, strength, anisotropic character, etc.12 The size and surface anisotropy are dominant factors13 influencing the magnetic quantities (coercive field, remnant magnetization, etc.) for their effective application in recording devices, spin electronics, etc.14 The alloys of rare earth and transition metals15,16 (Nd2Fe14B, Y2Fe14B, SmCo5, etc.) are known to be permanent magnets with Sol–Gel Division, CSIR-Central Glass and Ceramic Research Institute, Kolkata 700 032, India. E-mail: [email protected]; Fax: +91 33 24730957 † Electronic supplementary information (ESI) available. See DOI: 10.1039/c6cp02442d

20528 | Phys. Chem. Chem. Phys., 2016, 18, 20528--20541

high coercive field. However, the applications of these rare earth alloys are limited due to their high cost, limited availability, and low corrosion stability and resistivity. Thus, research on the synthesis and study of cost-effective transition metal oxides with high coercive field has gained importance. Moreover, the selfassembly of magnetic nanoparticles into organized structures leads to interesting magnetic properties due to structural organisation.17 In this context the synthesis of a-Fe2O3 with an improved shape and magnetocrystalline anisotropy constant could be a promising method. To date, for the synthesis of a-Fe2O3, various synthesis processes such as sol–gel,18 solvothermal,19 microwave heating,20 thermal decomposition,21 and chemical vapour deposition22 have been reported. The different morphologies of a-Fe2O3 such as nanoparticles23 (0D); nanorods,24 nanotubes,25 nanowires26 and nanobelts27 (1D); nanoplatelets28 and nanorings29 (2D); and 3D structures like hollow spheres,30 hollow sea urchin-like,8 cube shaped,31 dodecahedral- and octadecahedral-shaped32 have been successfully prepared. For advanced application of a-Fe2O3, the morphology based synthesis is important to delve its unknown properties. Ionic liquids (ILs), green solvents possessing specific properties such as low volatility, high thermal stability,

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dissolvable ability, and well-organized structures with extended hydrogen bonding in the liquid state, are used as templates for the synthesis and stabilization of inorganic materials having different morphologies.33 Recently, cation exchange and/or incorporation of metal ions in iron oxides have led to the development of materials with improved properties for various applications in chemical sensors, catalysis,34 etc. Wu et al.35 synthesized different shaped single crystalline a-Fe2O3 in the presence of various transition metal (Zn2+, Cu2+, Ni2+, Co2+ and Mn2+ ions) acetates by a hydrothermal method. The synthesized materials possessed improved coercivity. Barik et al.36 prepared lithium doped flower shaped a-Fe2O3 for waste water purification by the adsorption and removal of F ions. The Li doped a-Fe2O3 samples possessed improved magnetic behaviour compared to undoped samples. In the last few decades, water pollution has emerged as the most challenging concern globally owing to the contamination of effluents from textile industries to water sources, causing diseases and even death.37 Generally, biodegradation by native microorganisms, physical treatments such as ion exchange and adsorption, and chemical methods are available for the treatment of waste water discharge.38–40 However, they fail to serve the purpose properly. To overcome the disadvantages of the aforementioned processes, an advanced oxidation process (AOP) has emerged as an appropriate alternative. The most intensely studied AOP technology is based on Fenton reaction (Fe2+/Fe3+ + H2O2) which degrades non-biodegradable water soluble organic pollutants by powerful oxidising hydroxyl radicals ( OH) generated during the degradation reaction.41 However, there are few drawbacks which limit its practical use: (i) removal of a huge amount of Fe3+ ions (secondary pollutant) in the treated water source makes the process complicated and expensive and (ii) the process works well under highly acidic conditions (pH 2–3). Thus, the homogeneous Fenton reagent is needed to be replaced by a heterogeneous catalyst where the active metal remains attached to the solid support. The mechanism of degradation is as follows: Catalyst-Fe3+ + dye - [dye(adsorbed)Catalyst-Fe3+]

(1)

Catalyst-Fe3+ + H2O2(aq) - [H2O2(adsorbed)Catalyst-Fe3+] (2) [H2O2(adsorbed)Catalyst-Fe3+] - Catalyst-Fe2+ + OOH + H+ (3) [H2O2(adsorbed)Catalyst-Fe2+] - Catalyst-Fe3+ + OH + OH (4)

OH + [dye(adsorbed)Catalyst-Fe3+] - degraded products + Catalyst-Fe3+

(5)

The catalyst should be cheap, stable and catalytically active.42 The most advanced AOP technology is the photo-Fenton oxidation process under UV light irradiation. The method possesses certain advantages over the ordinary heterogeneous Fenton process: (i) more OH radicals are produced compared to the


ordinary Fenton process due to UV light irradiation according to the following reactions. H2O2 + hn - 2 OH

(6)

[H2O(adsorbed)Catalyst-Fe3+] + hn - Catalyst-Fe2+ + OH + H+ (7) (ii) In the ordinary Fenton process, the reduction step (rate determining step) i.e. conversion of Fe3+ to Fe2+ occurs slowly in the absence of UV light irradiation which limits the efficiency of the process. However, under UV light irradiation the reduction process occurs at a much faster rate generating Fe2+ ions which can produce excess amounts of oxidative radicals and hence increase the rate of degradation.43,44 Thus, the heterogeneous photo-Fenton like system seems to be convenient for the purpose of waste water treatment. With a motivation to improve the magnetic and catalytic properties of a-Fe2O3, in the present study, we have prepared mesoporous cube shaped nano/micro-scaled a-Fe2O3 using FeCl3 as the Fe3+ precursor and 1-butyl-3-methylimidazolium bromide as an IL (the structure is shown in Fig. S1A, ESI†) in the presence of alkali metal (lithium, sodium and potassium) acetates under hydrothermal conditions at 150 1C for 4 h followed by calcination at 350 1C. Herein, to the best of our knowledge, we report for the first time the synthesis of ionic liquid mediated alkali metal ion induced hematite particles in a shorter hydrothermal time of 4 h. The role of preferential intercalation of alkali metal ions in the synthesized a-Fe2O3 crystals has been investigated to evaluate their crystal, spectroscopic, magnetic and catalytic properties. The degradation of a cationic industrial effluent, methylene blue (MB), belonging to the thionin class of dyes (the structure is shown in Fig. S1B, ESI†) was studied using the synthesized a-Fe2O3 and using H2O2 as an oxidant under UV light irradiation. The degradation products were traced by electrospray ionization-mass spectrometry (ESI-MS).

2. Experimental section 2.1.

Materials

Ferric chloride hexahydrate (FeCl36H2O), sodium acetate (NaAc), potassium acetate (KAc), lithium acetate (LiAc), 1-butyl-3-methylimidazolium bromide and terapthalic acid (TA) were purchased from Sigma Aldrich. Methylene blue and hydrogen peroxide (H2O2, 30%) were purchased from Merck, India. The reagents were used without any further purification. Deionized (DI) water was used for the experiments; however, catalytic experiments were performed using HPLC grade water. 2.2.

Synthesis of mesoporous cube shaped a-Fe2O3 particles

Mesoporous cube shaped a-Fe2O3 particles were synthesized using a facile hydrothermal method. In a typical synthesis process, 2 mmol FeCl36H2O, 6 mmol sodium acetate and 8 mmol 1-butyl-3-methylimidazolium bromide were dissolved in 20 mL of DI water under stirring. The deep brown color solution was then transferred to a 60 mL Teflon lined autoclave and heated

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at 150 1C for 4 h. After hydrothermal treatment, the precipitate was collected by centrifugation, washed with DI water thrice followed by ethanol washing and then dried in an oven overnight. The dried as-prepared samples were calcined at 350 1C with a heating rate of 1 1C min1 and a dwell time of 2 h each to obtain mesoporous a-Fe2O3 particles. The experimental procedure was repeated by replacing sodium acetate with lithium acetate and potassium acetate. The a-Fe2O3 particles obtained by using lithium, sodium and potassium acetates were designated as Fe2O3-Li, Fe2O3-Na and Fe2O3-K, respectively.

range of 0.05–0.20, and the pore size distribution was calculated using the Barrett–Joyner–Halenda (BJH) method. The nitrogen adsorption volume at the relative pressure (P/P0) of 0.99 was used to determine the pore volume. The solid state UV visible spectra in the wavelength range of 200–800 nm were obtained using a UV-Vis-NIR spectrophotometer (Shimadzu UV-PC-3100). Magnetic property measurements were performed using a Vibrating Sample Magnetometer (VSM), Laka-shore-7400-S instrument. Zeta potential was measured using a nano-series Zetasizer (ZEN 3600), Malvern instrument.

2.3.

2.4. Heterogeneous photo-Fenton degradation of methylene blue (MB)

Characterization

The X-ray diffraction (XRD) studies of the obtained powders were performed using a Philips X’Pert Pro PW 3050/60 powder diffractometer under Ni-filtered Cu-Ka radiation (l = 0.15418 nm) operated at 40 kV and 30 mA. The crystallite size of the prepared samples was determined using the Scherrer equation. D¼

Kl b cos y

(8)

where D is the average crystallite size, b is the full-width half maximum (FWHM) of the diffraction peaks, y is the Bragg angle, l is the wavelength of an incident X-ray source. The constant K is related to the crystallite shape normally taken as 0.9 assuming spherical crystals.45 In the rhombohedral (hexagonal) structure of a-Fe2O3, plane spacing (d) is related to the lattice parameters (‘a’ and ‘c’) and Miller indices (hkl) as follows: 4 h2 þ hk þ k2 1 l2 ¼ þ 2 (9) 2 2 dhkl c 3a The cell volume of the rhombohedral (hexagonal) phase a-Fe2O3 can be calculated using the equation pffiffiffi 2 3a c (10) V¼ 2 The lattice parameters were calculated considering (104) and (110) planes obtained from the XRD pattern. The thermal behaviours of the uncalcined particles were studied by thermogravimetry (TG) (Netzsch STA 449C, Germany) from room temperature to 600 1C in an air atmosphere at the heating rate of 5 1C min1. The characteristic vibration bands of the products were confirmed by FTIR (Perkin-Elmer Spectrometer) with KBr pellets at a resolution of 4 cm1. The Raman spectra were recorded using a RENISHAW spectrometer under 514 nm radiation from an argon laser at room temperature in the range of 100–1500 cm1. The morphology of the particles was examined by field emission scanning electron microscopy, FESEM, using a Zeiss, Suprat 35VP instrument operating with an accelerating voltage of 10 kV, and transmission electron microscopy, TEM, using a Tecnai G2 30ST (FEI) instrument operating at 300 kV. Nitrogen adsorption–desorption measurements were conducted at 77 K using a Quantachrome (ASIQ MP) instrument. The samples were outgassed under vacuum at 250 1C for 4 h prior to the measurement. The surface area was obtained using the Brunauer– Emmett–Teller (BET) method within the relative pressure (P/P0)


The water pollutant MB was degraded via a heterogeneous Fenton process under UV A light irradiation (l = 365 nm, average light intensity is 0.008 W cm2) at room temperature in a photoreactor. In a typical test, 15 mg (0.375 g L1) of the calcined a-Fe2O3 sample was suspended in 40 mL of 1.5 105 M MB dye solution followed by stirring for about 30 minutes in the dark for homogenization. Before irradiation, 0.2 mL of 30% H2O2 aqueous solution was added to the above reaction mixture. Aliquots were drawn from the reaction mixture after a time interval of 10 minutes and were immediately monitored using a UV-visible spectrophotometer (Jasco V-730 spectrophotometer). The decrease in absorption intensity at lmax = 663 nm was observed. The degradation efficiency of the catalyst was calculated as (1 Ct/C0) 100. The degradation follows pseudo first order reaction kinetics and can be expressed as ln(Ct/C0) = kt, where k is the observed rate constant of the reaction, C0 and Ct are the dye concentration initially and at time t, respectively. To detect the amount of OH radicals formed in the above heterogeneous photo-Fenton test, terephthalic acid (TA) was used which traps OH radicals. Actually, OH radicals rapidly react with TA to produce highly fluorescent 2-hydroxyterephthalic acid (2-HTA). 2-HTA shows fluorescence emission at 425 nm upon excitation at 315 nm. The fluorescence intensity was measured using an Agilent Technology, Cary Eclipse fluorescence spectrophotometer. The photoluminescence (PL) intensity was proportional to the produced OH radicals. Thus, the method is simple, rapid, sensitive and specific. In this experiment instead of using MB dye, a solution (40 mL) comprising of 1.5 105 M TA and 6 105 M NaOH was used following the same procedure as mentioned in the photo-Fenton process. 2.5. Electrospray ionization (ESI)-mass spectrometry (MS) study A Micromass Q-Tof micro (Waters) Mass Spectrometer was used for the detection of degradation fragments. An electrospray ionization (ESI) source in positive mode helps in the separation of the degraded fragments on the basis of their mass to charge ratio. The sample solutions were analyzed by introducing aliquots into the ESI source using a syringe pump with a flow rate of 15 mL min1. The spectral data obtained were averaged over 10 scans at 0.2 s each. The capillary was heated at a temperature of 275 1C in the presence of N2 gas with a flow rate of 4 L min1.


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3. Results and discussion


3.1.

Mesoporous cube shaped hematite (a-Fe2O3) particles

TG analysis of the as-prepared samples obtained from (a) LiAc (b) NaAc and (c) KAc is shown in Fig. S2 (ESI†). It is clear that the weight loss for the samples occurred in two steps i.e., up to 200 1C with the weight losses of about 1.2%, 5.2% and 0.5% for the samples obtained from LiAc, NaAc and KAc, respectively; and in the second step weight losses from 200 to 340 1C of the corresponding samples as 1.3%, 7.8% and 3.5%. The first step weight loss could be attributed to the removal of adsorbed water, while the release of organic templates (1-butyl-3-methylimidazolium) and acetates of the alkali metals was caused by second step weight loss.46 On the basis of the above analysis the calcination temperature of the as-prepared samples was chosen as 350 1C. Fig. 1A shows the XRD pattern of (i) uncalcined and (ii) calcined a-Fe2O3 particles obtained in the presence of (a) lithium acetate, (b) sodium acetate and (c) potassium acetate in each case. All the diffraction peaks matched well with the standard JCPDS file 33-0664 of the rhombohedral (hexagonal) phase of a-Fe2O3 with a R3% c space group and cell parameters: a = 5.035 Å, c = 13.74 Å. No characteristic peaks for other impurities were observed in spite of the presence of alkali metal ions. The % relative intensities (RI) of various diffraction peaks were compared with the standard rhombohedral (hexagonal) phase of a-Fe2O3 (Table S1, ESI†). It was found that % RI of various planes changed significantly. Considering the intensity ratio of the diffraction planes (110) : (104) of the uncalcined samples we found that % RI for the (110) plane of both Fe2O3-Li and Fe2O3-Na samples was close to 84% which seems to be

Fig. 1 (A) XRD patterns of (i) uncalcined, (ii) calcined samples, (iii) comparison of the shift of (104) and (110) peaks in calcined samples and (B) Raman spectra of (i) calcined samples, (ii) magnified view of A1g(1) peak prepared using (a) LiAc (b) NaAc and (c) KAc in each case.


higher compared to the standard JCPDS data of a-Fe2O3 (for (110) plane % RI is 70%), indicating the presence of more (110) facets in Fe2O3-Li and Fe2O3-Na samples. However, for the Fe2O3-K sample, % RI of the (110) plane almost matched with the standard data. The above phenomenon occurred probably due to strong interaction of the IL (1-butyl-3-methylimidazolium bromide) with the closely packed, polar (110) plane47 in the case of Fe2O3-Li and Fe2O3-Na samples. It is discussed shortly in the reaction mechanism. To understand the peak shifting of (104) and (110) planes in the presence of different alkali metal ions, the XRD peak of calcined samples in the 2y range of 32–371 is shown separately in Fig. 1A(iii). Unlike K+ ions, the 2y values for the (110) plane were incrementally shifted by 0.151 and 0.181 for Na+ and Li+ ions, respectively, and correspondingly by 0.111 and 0.121 for the (104) plane with respect to the standard a-Fe2O3 sample. Table 1 shows that crystallite sizes of the samples increased after heat treatment. Interestingly, the increase was more significant for the Fe2O3-K sample compared to that of Fe2O3-Na and Fe2O3-Li samples. Table 1 also reveals that the lattice parameter of the Fe2O3-K sample obtained after calcination matched well with that of standard a-Fe2O3. However, the lattice parameter and cell volume decrease continually in the presence of Na+ and Li+ ions. It suggests that unlike K+ ions in Fe2O3-K, Li+ and Na+ ions were intercalated in Fe2O3-Li and Fe2O3-Na samples, respectively in their crystal layers resulting in a small increase in the crystallite size after calcination. Moreover, after calcination, the right shift of (104) and (110) diffraction peaks of Fe2O3-Li and Fe2O3-Na samples compared to that of standard a-Fe2O3 resulted in a decrease in lattice parameters (‘a’ and ‘c’) and cell volume.31,48 Thus, XRD data demonstrate that after calcination, Na+ and Li+ ion incorporation in a-Fe2O3 crystal layers results in the lattice deformation of the resulting a-Fe2O3 particles. Fig. S3 (ESI†) reveals the FTIR spectra of (A) uncalcined and (B) calcined samples obtained from (a) LiAc (b) NaAc and (c) KAc. All the curves show absorption bands at 472, 546, 1390, 1630 and 3426 cm1. The appearance of bands at 3426 and 1630 cm1 was assigned to O–H stretching and bending vibrations, respectively, of the physisorbed water molecules. The bands at 472 and 546 cm1 correspond to Fe–O stretching of hematite particles. Interestingly, it was reported that a weak band at around 1390 cm1 usually emerges when FTIR of the samples was performed in an air atmosphere.6 Apart from the above bands the as-prepared samples show absorption bands at 1102, 2854 and 2930 cm1. The bands at 2854 and 2930 cm1 were assigned to the C–H symmetric and asymmetric stretching frequency while the band at 1102 cm1 was due to the symmetric ring stretching frequency of the imidazolium moiety.49 Importantly, apart from these bands no other significant bands were observed in the FTIR spectra which supported the formation of pure a-Fe2O3 in the synthesized samples. Raman spectroscopy was performed to study the surface conditions and homogeneity of the synthesized materials. According to the literature report, a-Fe2O3 belongs to the R3% c space group with a corundum type structure i.e. the D63d crystal space group which possesses even phonon lines in Raman scattering.31 The Raman spectra of calcined


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Paper Crystallite size, lattice parameters, unit cell volume of cube shaped a-Fe2O3


Lattice parameter (Å) Crystallite size (nm)

a=b

Sample ID

Uncalcined

Calcined

Uncalcined

Calcined

Uncalcined

Calcined

Uncalcined

Calcined

Fe2O3-Li Fe2O3-Na Fe2O3-K

30.40 47.62 27.60

32.11 55.17 46.53

5.054 5.052 5.040

5.016 5.022 5.036

13.830 13.810 13.785

13.655 13.719 13.749

305.922 305.238 303.962

297.523 299.636 301.968

c

samples show two peaks at around 225 and 498 cm1 which are ascribed to the A1g mode of vibration (Fig. 1B(i)); the magnified part of the former peak is shown in Fig. 1B(ii) which is related to the active optical phonon. However, the appearance of peaks around 247, 293, 411 and 613 cm1 was due to the Eg mode of vibration. An intense peak at around 1314 cm1 was attributed to two-magnon scattering arising from the interaction of two magnons.50 The well-defined seven Raman peaks concord with that of commercial hematite samples.51 Bersani et al. pointed out that the Raman peak at about 660 cm1 of the a-Fe2O3 samples is related to disorder effects and/or the presence of a-Fe2O3 nanocrystals, and does not indicate the presence of traces of magnetite in the sample.52 It is worth noting that no other peaks related to other forms of iron oxide as well as alkali metal oxides were observed in the samples. Interestingly, it was noticed that for the samples Fe2O3-Li and Fe2O3-Na, shifting of the Raman peaks to lower wavenumbers 222 cm1 and 215 cm1, respectively occurred. This shifting can be attributed to the phonon confinement and stress effects in the nanocrystals. It demonstrated that unlike K+ ions, Li+ and Na+ ions were intercalated into the a-Fe2O3 crystal layers.53 Fig. 2 shows the FESEM microstructures of the calcined a-Fe2O3 samples prepared using (a) LiAc, (c) NaAc and (e) KAc; the corresponding higher magnified images are shown in Fig. 2b, d and f. In the presence of LiAc, nanocube-like (20–50 nm) a-Fe2O3 particles were obtained (Fig. 2a and b). The particles were aggregated due to a higher surface charge of the smaller particles. However, NaAc rendered monodisperse a-Fe2O3 microcubes of around 2 mm in size (Fig. 2c and d). With the close observation of the microcubes, it is clear that the smaller particles in the nanometer range self-assembled together to form microcubes via the Ostwald ripening process. In the presence of KAc, highly aggregated nanocube-like (25–50 nm) particles were observed (Fig. 2e and f). It is interesting to note that the nanocube-like particles self-assembled in a chain-like fashion. Fig. 3 shows the TEM images of the calcined a-Fe2O3 samples prepared using (a) LiAc, (c) NaAc and (e) KAc; the corresponding higher magnified images are shown in Fig. 3b, d and f. TEM images also reveal that the nanocube-like a-Fe2O3 particles were obtained from LiAc (Fig. 3a and b) and KAc (Fig. 3e and f) while NaAc rendered a-Fe2O3 microcubes (Fig. 3c and d). Interestingly, the mesopores are visible in nanocube-like microstructures (Fig. 3a, b, e and f). However, the microcubes (Fig. 3c and d) appeared to be dense solids without revealing any pores, which may be due to the high thickness of the microcubes, as observed from FESEM images. Fig. S4 (ESI†) represents the HR-TEM images (a, d, g), selected area electron diffraction (SAED)


Unit cell volume

Fig. 2 FESEM micrographs and their corresponding magnified view of the calcined samples synthesized using (a) and (b) LiAc; (c) and (d) NaAc and (e) and (f) KAc.

patterns (b, e, h), and energy dispersive X-Ray spectra (EDS) (c, f, i) of calcined a-Fe2O3 samples prepared using (a–c) LiAc (d–f) NaAc and (g–i) KAc. The HR-TEM images of the calcined samples show d-spacing values of 0.27 nm for all the samples corresponding to the lattice plane of (104) of the rhombohedral (hexagonal) a-Fe2O3 phase. The concentric rings in the SAED patterns of the samples demonstrate the polycrystalline nature of a-Fe2O3, and the identified planes support the XRD results. The EDS of the calcined samples indicates the presence of Fe and O in their atomic ratio close to the stoichiometry of Fe2O3 (the C and Cu atoms appeared from the grid). It is worth noting that though the incorporation of Li ions was evidenced from XRD results it was not observed in the EDS pattern of the Fe2O3-Li sample (Fig. S4c, ESI†) because K X-rays of Li have too low energy to be detected by EDS. The presence of Na in Fe2O3-Na (Fig. S4f, ESI†) and the absence of K in Fe2O3-K indicate the incorporation of Na atoms in the former sample while K atoms could not be incorporated in the latter sample which was supported by the XRD results. Fig. 4A shows the nitrogen adsorption–desorption isotherms of the calcined hematite (a-Fe2O3) samples obtained from (a) LiAc,


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Scheme 1 Schematic diagram of the proposed formation mechanism of cube shaped mesoporous a-Fe2O3 samples.

Fig. 3 TEM images and their corresponding magnified view of the calcined samples synthesized using (a) and (b) LiAc; (c) and (d) NaAc and (e) and (f) KAc.

pore volume (0.653 cc g1) and pore diameter (34.3 nm) compared to those of the Fe2O3-Li and Fe2O3-K samples where the values are found to be comparable. It was reported that alkali metal ions behave as templating agents.55 Herein, Na+ ions lead to a more facile templating effect via coordination with a 5-membered imidazole ring of ionic liquid 1-butyl-3-methylimidazolium bromide56 compared to Li+ and K+ ions. It led to the highest surface area, pore volume and pore diameter of the Fe2O3-Na sample. Scheme 1 shows the probable mechanism for the formation of a-Fe2O3 samples. The Fe3+ precursor, alkali metal acetates, and 1-butyl-3-methylimidazolium bromide (IL) were dissolved in water and were subjected to hydrothermal treatment. The alkali metal acetates in aqueous medium hydrolyzed to render hydroxide (OH) ions. The Fe3+ ions in FeCl3 interact with the OH ions to form a-Fe2O3 under hydrothermal conditions. The probable reaction mechanism is provided below: CH3COO + H2O 2 CH3COOH + OH

(11)

hydrothermal condition

2Fe3þ þ 6OH þ 2H2 O ! a-Fe2 O3 þ 5H2 O (12)

Fig. 4 (A) N2 adsorption–desorption isotherms and (B) pore size distribution curves of the calcined samples prepared using (a) LiAc (b) NaAc and (c) KAc.

(b) NaAc and (c) KAc, while the BJH pore size distributions of the corresponding samples are depicted in Fig. 4B. All the curves show type IV isotherms according to the IUPAC classification, indicating mesoporosity in the samples. The H3 type hysteresis loop in the isotherms indicated the presence of asymmetric, interconnected slit-like pores in the samples.54 The pore size distributions are in the mesopore region with a relatively wide distribution for the Fe2O3-Na sample. Table S2 (ESI†) shows the textural properties (surface area, pore volume and average pore diameter) of the calcined a-Fe2O3 samples. Interestingly, the sample Fe2O3-Na possesses the highest surface area (88 m2 g1),


It was noticed that after hydrothermal treatment, pH of the medium decreased indicating the consumption of the produced OH ions. During hydrothermal treatment, at first nuclei of a-Fe2O3 were produced, which grow by the adsorption of Fe3+ cations on the preferred site by surface hydroxyl groups.35 Jin et al.57 reported that adsorption of alkaline earth metal ions (Ba2+) at the preferred crystal site resulted in the formation of rod like e-Fe2O3 during the sintering process. The presence of Ba2+ ions enhanced the growth of e-Fe2O3. We have studied the growth mechanism of the Fe2O3-Na sample in detail. As seen from Fig. S5 (ESI†) the gradual evolution of shapes of a-Fe2O3 particles from small nanoparticles to microcubes occurred with reaction time. With the increase in hydrothermal reaction time from 1.5 h to 3 h the a-Fe2O3 microcubes were generated (Fig. S5b, ESI†) with the expense of a-Fe2O3 nanoparticles via the Ostwald ripening mechanism followed by the formation of


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uniform bulky microcubes after 4 h reaction time. During heat treatment the adsorbed alkali metal ions on the surface of a-Fe2O3 particles gradually diffused into the crystal lattice via anionic coordination, followed by their intercalation (topotactic incorporation) at the defect site of hematite during the crystal growth process.36 The size of the alkali metal ions played a significant role in such intercalation. Due to the larger size of the K+ ion (1.38 Å) compared to Li+ (0.70 Å) and Na+ (1.02 Å), the incorporation of former ions into a-Fe2O3 crystals was restricted, which was reflected in XRD, Raman and EDS studies of the calcined a-Fe2O3 samples. To understand the role of 1-butyl-3-methylimidazolium bromide (IL) in the formation of shaped hematite particles, a blank experiment was performed in the absence of the IL keeping all the other experimental parameters the same. In this case irregular shaped aggregated microparticles were obtained (Fig. S6, ESI†) instead of regular microcubes as obtained in Fe2O3-Na. In the case of a-Fe2O3 crystals O and Fe atoms were closely packed along the (110) plane. During the synthesis process, a strong interaction between the IL and the (110) plane of a-Fe2O3 particles could be generated to stabilize and neutralize the ionic charges towards the growth along the preferred orientation.47 To study the optical absorption characteristics and band gaps of the synthesized a-Fe2O3 samples, the UV-visible spectroscopy study was performed. The optical band gap Eg of the synthesized materials can be evaluated using Tauc’s equation. (ahn)n = K(hn Eg)

(13)

where a is the absorption coefficient, hn is the energy of a photon, the value of n depends on the synthesized material (n = 2 for direct allowed transition and n = 1/2 for indirect allowed transition), Eg is the band gap, and K is a constant. a-Fe2O3 can absorb a broad range of ultraviolet light but the absorption properties are strongly dependent on the shape and size of the samples.58 Fig. S7 (ESI†) shows the optical absorption spectra of calcined a-Fe2O3 samples. The intense bands at 260 nm and 380 nm for the Fe2O3-K sample (Fig. S7(c), ESI†) were consistent with those of the reported values for hematite nanoparticles.33 For Fe2O3-Li nanocubes, the absorption bands were noticed in three regions: two broad bands at around 260 and 385 nm, and a hump-like shoulder at around 520 nm (Fig. S7(a), ESI†). Interestingly, for Fe2O3-Na microcubes (Fig. S7(b), ESI†), the broad bands at around 577 and 674 nm appeared along with a hump-like shoulder at around 265 nm. It is worth mentioning that for a bigger size of the particles in Fe2O3-Na microcubes, the scattering of visible light superimposes on the absorption of the as-prepared nanoparticle assembly in the microcubes resulting in the absorption band at a higher wavelength, i.e., near 674 nm.59 It is known that three electronic transitions occur in the optical absorption spectra of Fe3+ substances: (i) Fe3+ ligand field transition (the d–d transitions), (ii) the ligand to metal charge-transfer transitions, and (iii) the pair excitations resulting from the simultaneous excitations of two neighbouring Fe3+ cations that are magnetically coupled.60 The band occurring in the far-UV region in the range of 260 nm to 290 nm was ascribed


to Fe–O electron transmission. The ligand field transition of Fe3+ occurred in three regions: (i) the absorption at 360–380 nm corresponds to 6A1 - 4E(4D) and 6A1 - 4T2(4D) transitions, (ii) the absorption at 390 nm relates to 6A1 - 4E(4G) transition and (iii) the absorption at 550–600 nm corroborates to 6 A1 - 4T2(4G) transition.33,60 Indirect and direct band gaps of hematite have been reported in the ranges of 1.38–2.05 eV61,62 and 1.95–2.35 eV,63 respectively. Fig. S8A and B (ESI†) show the plots of (ahn)2 versus hn (for direct band gap calculation) and (ahn)1/2 versus hn (for indirect band gap calculation), respectively, of the calcined a-Fe2O3 samples. The band gaps were calculated by extrapolating a tangent to the x-axis. The values obtained in each case with different morphologies matched with the literature report. The direct band gaps of Fe2O3-Li nanocube-like particles (Fig. S8A(a), ESI†), Fe2O3-Na microcubes (Fig. S8A(b), ESI†), and Fe2O3-K nanocube-like particles (Fig. S8A(c), ESI†) were found to be 2.16, 1.88 and 2.12 eV, respectively, while the corresponding indirect band gaps were obtained at 1.79, 1.46 and 1.87 eV. It is interesting to note that the direct and indirect band gaps of Fe2O3-Na microcubes were found to be less than those of Fe2O3-Li and Fe2O3-K nanocube-like particles. The lower values of band gaps of Fe2O3-Na microcubes could be due to the larger size of the particles. It is reported that light scattering and the absorption coefficient are influenced by the shape of the particles and crystallographic orientation.61 3.2.

Magnetic properties

To investigate the effect of the observed morphology and microstructure of the samples on their magnetic properties, magnetization measurement experiments were performed. Fig. 5a–c shows the isofield temperature (T) dependence of the magnetization (M) of the calcined a-Fe2O3 samples from 80 to 400 K, in the presence of a 500 Oe applied magnetic field. The zero field cooled (ZFC) measurement (the sample was cooled at the zero magnetic field to 80 K followed by recording magnetization data with heating) and the field cooled (FC) measurement (the same experiment was performed at the applied magnetic field of 500 Oe) were performed. The FC and ZFC curves almost overlap in the entire temperature range. The Morin transition temperatures (TM) were obtained from the ZFC curve derived by the maximum value of the first derivative of magnetization (M) with respect to temperature (T) (dM/dT) (shown in Table 2). Bulk a-Fe2O3 is antiferromagnetically ordered below TM = 263 K. It shows weak ferromagnetic properties in between TM and TN (Neel temperature). The reason for the conversion of an antiferromagnetic to a ferromagnetic material may be due to: (i) the change of the a-Fe2O3 spin axis from the c axis to the c plane of the crystal with the increase in thermal fluctuations and/or (ii) self-grown oxygen vacancies in a-Fe2O3 destroy antiferromagnetic interactions between sublattices.64 It is also known that shifting of TM is strongly dependent on the particle size of the samples and seems to be insensitive when the particle diameter is close or larger than 60 nm.65 The TM values of around 216 and 166 K were found in hematite samples for the particle sizes of around 50 and 30 nm,66,67 respectively. Zhou et al. showed a


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Fig. 5 (a–c) ZFC–FC curves under applied field of 500 Oe and (d–f) M–H curves at 80 K, 300 K and 400 K for calcined samples synthesized using (a) and (b) LiAc; (c) and (d) NaAc and (e) and (f) KAc.

Table 2

Sample ID

Magnetic parameters for calcined a-Fe2O3 particles

HC (Oe)

MR (emu g1)

M14kOe (emu g1)

TM 80 K 300 K 400 K 80 K 300 K 400 K 80 K 300 K 400 K (K)

Fe2O3-Li 489 2492 1584 0.001 0.077 0.063 0.273 0.482 0.500 199 Fe2O3-Na 597 5681 4896 0.005 0.203 0.191 0.217 0.475 0.467 260 Fe2O3-K 379 1469 1284 0.009 0.074 0.058 0.200 0.400 0.393 201

higher TM value (236 K) for the nanoparticle assembled 1D nanochains (1–3 mm) of a-Fe2O3.68 They pointed out that not only the size factor, but TM also depends on the other factors like morphology, crystallinity and/or surface properties. In our present study, the TM values decreased to 199 K and 201 K for Fe2O3-Li and Fe2O3-K samples, respectively (Table 2) as compared to that of 260 K for the Fe2O3-Na sample, which could be due to the smaller size (20–50 nm) of the former samples. Fig. 5d–f shows the field dependent magnetization (M–H) plots of the calcined a-Fe2O3 samples recorded at three temperatures viz. 80, 300 and 400 K in the magnetic field range of 14 kOe to +14 kOe. It is to be noted that magnetization plots were recorded below (at 80 K) and above (at 300 and 400 K) the TM temperatures. The values of magnetization at the highest applied magnetic field of 14 kOe (M14kOe), remnant magnetization (MR), and coercivity (HC) are presented in Table 2. It is seen that M–H curves for all the samples could not reach saturation even at a maximum applied field of 14 kOe (Fig. 5d–f). For all the samples, M14kOe values were significantly decreased at 80 K as compared to those recorded at 300 and 400 K. This decrease in M14kOe values at 80 K is a signature of an antiferromagnetic


material69 with a low range (0.4–0.6 kOe) of HC values along with MR values approaching zero (Table 2). The M–H curves show a significant increase in hysteresis loops above TM, which signifies weak ferromagnetic character of the samples. The HC and MR values decreased a little upon increasing the temperatures from 300 K to 400 K. This decrease in values signifies that with approaching the recording temperature to Neel temperature (TN = 948 K), the thermal energy becomes significant to destroy the macroscopic magnetic ordering within the material. The coercivity of hematite is sensitive to local magnetic anisotropy where the magnetic spins are preferentially aligned along the axis and their reversal to the opposite direction requires higher energies.70 In the case of microcubes of the Fe2O3-Na sample, the numerous nanosized subparticles were organized as 3D structures to promote shape anisotropy in the sample resulting in a high HC value.71 Furthermore, inducing internal strain, and increasing crystal size and impurity content increased the coercivity. When an external magnetic field was applied, the nanoparticles became magnetized, and due to their close contact the interfaces were deformed. The strain originated to resist the deformation produced within nanoparticles. The other possibility of the development of strain was due to lattice mismatch originated during the synthesis process or due to the presence of any other impurity.72 In our case, the Fe2O3-Na microcubes rendered enhanced HC (5.7 kOe) and MR (0.203 emu g1) values above TM (particularly at room temperature) due to increased strain in crystals, shape and magnetocrystalline anisotropy in the presence Na+ ions within the crystal layers. 3.3.

Photo-Fenton degradation of methylene blue (MB)

Fig. 6A shows the % removal of MB dye in the presence of the synthesized mesoporous a-Fe2O3 catalyst under UV light irradiation for 120 min in the presence of H2O2. It shows that the % of dye removal for Fe2O3-Li, Fe2O3-Na and Fe2O3-K catalysts was 48%, 90% and 78%, respectively. For determination of the

Fig. 6 Plot of (A) degradation efficiency, (B) ln(Ct/C0) vs. time, and (C) bar plot representing rate constant of photo-Fenton like degradation of methylene blue (MB) dye for calcined Fe2O3 catalysts: (a) Fe2O3-Li, (b) Fe2O3-Na and (c) Fe2O3-K.


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rate constant, the decolorization profile of MB in the presence of different a-Fe2O3 was plotted (Fig. S9, ESI†). The rate constant of dye decolorization using the synthesized mesoporous a-Fe2O3 catalysts is shown in Fig. 6B, and Fig. 6C depicts a bar plot of the rate constant values. The rate constant values for different samples are follows as: Fe2O3-Na 4 Fe2O3-K 4 Fe2O3-Li. The maximum rate constant value of Fe2O3-Na could be due its highest BET surface area, maximum pore volume and pore diameter (Table S2, ESI†), which rendered its highest rate of decolorization of MB. Interestingly, instead of the comparable textural properties (BET surface area, pore volume and pore diameter) of Fe2O3-Li and Fe2O3-K, the latter reveals higher catalytic efficiency for the degradation of MB, which could be explained based on surface charges of respective samples. We have measured the zeta potential of three samples Fe2O3-Li, Fe2O3-Na and Fe2O3-K, and the values were found to be +28.8, 17.1 and 28.1 mV, respectively (Fig. S10, ESI†). Due to a higher negative zeta potential of Fe2O3-K, the adsorption of cationic dye MB onto the negative catalytic surface of Fe2O3-K became more facile due to electrostatic attraction. The catalytic degradation mechanism of methylene blue is governed by the adsorption of MB on the catalytic surface followed by degradation of MB. Both the Fe2O3-Na and Fe2O3-K samples have negative surface charge facilitating better adsorption of MB. It is inferred that both the textural properties and negative surface charge are important for the catalytic degradation of MB dye. Therefore, faster catalytic efficiency of Fe2O3-Na for MB degradation is explained based on its negative surface charge, highest BET surface area and maximum pore volume. To study the synergetic effect of the catalyst (e.g. Fe2O3-Na), H2O2 and the presence of UV light for the removal of MB dye, 5 batch experiments of dye degradation were performed: (a) in the presence of H2O2 only without any catalyst in the dark, (b) in the presence of UV light without H2O2 and a catalyst, (c) in the presence of the Fe2O3-Na catalyst without H2O2 in the dark, (d) in the presence of the Fe2O3Na catalyst and UV light without H2O2 and (e) the Fe2O3-Na catalyst and H2O2 in the dark. The absorption spectra (Fig. S11, ESI†), % degradation (Fig. S12 A, ESI†) and rate constant (Fig. S12B, ESI†) for the above 5 batch experiments show the degradation efficiency in the order of a o b o c o d o e. The decolorization rate of MB in the absence of the catalyst was noticeably low in the presence of H2O2 in the dark (Fig. S12B(a), ESI†) or under UV irradiation (Fig. S12B(b), ESI†). This could be due to the lower oxidation potential of H2O2 than hydroxyl ( OH) and perhydroxyl radicals ( OOH),73 which are responsible for dye degradation. It is worth noting that the formation of OH and OOH radicals is enhanced in the presence of a catalyst. The sole presence of a catalyst could decolorize the dye via adsorption in the absence of OH and OOH radicals which are generated in the presence of H2O2 and/or UV light. These experiments demonstrate that the synergetic effect of the catalyst, H2O2 and UV light irradiation was essential for the effective decolorization of MB dye.74 The photo-Fenton like reaction of MB is controlled by several parameters like catalyst dosage, the amount of H2O2 and pH of the reaction mixture. In the following part the influence of the above parameters on the decolorization of MB dye was studied.


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Based on the UV-vis absorption spectra (Fig. S13A and B (ESI†)), the effect of the catalyst dosage on the degradation activity in the photo-Fenton like process is illustrated in Fig. S14A and B (ESI†). It can be concluded that the rate of decolorization was accelerated as the amount of catalyst increased from 0.125 to 0.375 g L1, but dropped with excessive dosage (1.0 g L1). With the increase in the catalyst dosage, the increase in the decolorization rate could be attributed to the increase in catalytic sites which enhanced the production of OH radicals.75,76 However, excess of catalyst dosage results in the increase of Fe3+ as well as Fe2+ (formed by the photo-reduction of Fe3+ ions). The excess formation of Fe2+, acting as the OH radical scavenger, competes with dye molecules for OH radicals.77,78

OH + Catalyst-Fe2+ - Catalyst-Fe3+ + OH

(14)

Moreover, excess Fe3+ ions can also react with H2O2 to form more hydroperoxyl radicals ( OOH), a relatively less oxidizing agent than OH radicals.79 Therefore, an optimum concentration of the catalyst is required for effective catalytic degradation of MB. It is well known that H2O2 plays an important role in the photo-Fenton like reaction as it is the source of the OH radical. To investigate the effect of H2O2 on the rate of decolorization of dye, different amounts of H2O2 were added to the solution keeping other parameters constant. Based on the UV-vis absorption spectra (Fig. S13C and D (ESI†)), the % decolorizations and rate constants are illustrated in Fig. S14C and D (ESI†). The results show that the % decolorization of dye increases with the amount of H2O2 from 0.05 mL to 0.2 mL, which is attributed to the increase in the rate of generation of OH radicals. However, with the increase in the amount of H2O2 up to 0.8 mL, the rate of decolorization was found to be decreased. The generated OH radicals react with excess H2O2 to form a higher amount of OOH radicals, which are less reactive with lower oxidation abilities than OH radicals. The OOH radicals can further consume OH radicals to form oxygen and water molecules.80,81 H2O2 + HO - H2O + HOO

(15)

HOO + HO - H2O + O2

(16)

Thus, an optimum H2O2 amount (0.2 mL) is required for the best catalytic performance for the degradation of MB. The pH of the reaction solution affects the surface properties of the semiconductor and also controls the generation of OH radicals,82 leading to dye degradation. The pH of homogeneous photo-Fenton like degradation is narrowly critical. In the present case of heterogeneous catalysis, the reaction successfully occurs over a range of pH between 4 and 9. Based on the UV-vis absorption spectra (Fig. S13E and F (ESI†)), the effect of pH on the % decolorizations and rate constants is illustrated in Fig. S14E and F (ESI†). The reactions were performed at pH = 4, 6.5 (natural pH) and 9, with remaining other parameters unchanged. Fig. S14E and F (ESI†) show that the decolorization of MB occurred faster at natural pH. At lower pH (pH 4), H+ ions stabilize H2O2 and restricts the formation of OH radicals. H+ ions acts as a


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OH radical scavenger.77 At a higher pH (pH 9), the concentration of hydroperoxy anions (OOH) increases resulting in the decrease of H2O2 and OH radicals. Furthermore, the oxidation potential of hydroxyl radicals decreases with increasing pH resulting in a significant decrease of decolorization rates.81 Therefore, an optimum pH (pH 6.5) is necessary for effective catalytic decolorization of MB. It is well known that the efficiency of the heterogeneous photo-Fenton like process solely depends on the amount of hydroxyl ( OH) radical produced during the dye degradation process. So the OH trapping experiment was performed in the most active catalytic system i.e. the Fe2O3-Na catalyst in the presence of the H2O2 oxidant under UV A light irradiation (Fig. S15, ESI†). The increase in the PL intensity with time at 425 nm signifies the appearance of highly fluorescent 2-HTA molecules via the generation and addition of OH radicals to TA. Thus, the active OH radicals were photogenerated in the above photo-Fenton like system, which effectively decolorizes the organic water pollutant MB. ESI-MS analysis was performed to detect the products obtained after degradation of MB dye in the presence of the catalyst, H2O2, and UV light. It is worth mentioning that blue color of MB could convert to colorless products of leuco MB (a reduced form of MB) without any fragmentation of the MB moiety. Fig. 7 reveals ESI-MS analysis of (A) pure MB solution before catalysis, and catalytic degradation of MB using (B) Fe2O3-Li, (C) Fe2O3-Na, (D) Fe2O3-K catalysts in the presence of H2O2 under UV A light irradiation for 30 min, 60 min, 120 min each. The fragmented molecular ion was detected in the positive mode. Several studies reveal that fragmentation of MB occurred in numerous steps83,84 which include (i) N-demethylation, (ii) deamination, (iii) cleavage of chromophore or destruction of conjugation, (iv) rupture of ring structure and (v) open chain structures. Further it is reported85 that OH radicals furnished by H2O2 degrade MB via the following reaction pathway. Initially the produced OH radicals attack the C–S–C functional group of MB (C16H18N3S) forming sulfoxide (RS(QO)R 0 ) (eqn (17)). Sulfoxide in the presence of OH radicals is converted into sulfone (RSO2R 0 ), causing the dissociation of two benzenic rings (eqn (18) and (19)). Sulfone is converted into sulfonic acid (RSO3H) (eqn (20) and (21)) followed by its conversion to sulphate ions and aromatic amines (eqn (22) and (23)) in the presence of OH radicals.


C16H18N3S + OH - C16H18N3SQO

(17)

C16H18N3SQO + OH + 3/2H2O - C8H11N2SO2 + C8H11N + 3/4O2

(18)

C16H18N3SQO + OH + 2H2O - C8H10NSO2 + C8H11N–NH2 + O2

(19)

C8H10NSO2 + OH - C8H10N–SO3H

(20)

C8H11N2SO2 + OH - C8H11N2–SO3H

(21)

C8H10N–SO3H + OH - [C8H10N] + SO42 + 2H+


(22)

C8H11N2–SO3H + OH - [C8H11N2] + SO42 + 2H+

(23)

The plausible degraded products of MB after photo-Fenton like degradation are shown schematically (Scheme 2): (i) fragmented products of MB without any damage of the ring skeleton, (ii) fragmented products of MB after the cleavage of the chromophore moiety, and (iii) fragmented products of MB after its (A) ring rupture and (B) ring opening. Analysis of pure MB (Fig. 7A) by ESI-MS shows a major peak at m/z = 284 corresponding to the molecular ion peak of MB, and a second small peak at m/z = 270 due to contamination with azure B, a monodemethylated product of MB. There were no peaks at a higher m/z value, indicating that the ESI-MS technique did not cause any further degradation or polymerization of MB. For a photo-Fenton like catalytic system in the presence of Fe2O3-Li as the catalyst (Fig. 7B), the peak at m/z = 284 (for MB) was found to be the most intense after 30 min of irradiation along with the other major peaks for N-demethylated (at m/z = 270, 256) products. It suggests that the parent compound (MB) along with its N-demethylated forms were present in major amounts. For intermediate time of irradiation (after 60 min), it was found that the relative intensity of a molecular ion peak of MB (m/z = 284) decreased with the increase of the relative peak intensity for N-demethylated and hydroxylated products having m/z values of 275 and 291. In addition to the above peaks, the low intensity peaks at m/z = 204, 195, 157, 135, 117 and 102 indicated the onset of ring cleavage and ring opening processes (Scheme 2(iii)). Interestingly, irradiation after 120 min resulted in the disappearance of the molecular ion peak of MB; however, the other peaks corresponding to N-demethylation and hydroxylation (m/z = 291, 275, 261, 231), cleavage of the chromophore moiety (m/z = 248), ring cleavage (m/z = 207, 157) and ring opening (m/z = 195, 135, 131, 102) became prominent. For the Fe2O3-Na catalyst, the relative peak intensity corresponding to m/z = 291, 275, 261, 247, 231, 204, 207, 195, 178, 157, 135, 117, 110, 102 values indicated the presence of N-demethylation and hydroxylation, chromophore moiety cleavage, ring ruptured and open chain intermediates of MB upon 30 min of irradiation (Fig. 7C), in the absence of a peak at m/z = 284 for pure MB. The relative intensity of ring rupturing and ring opening intermediates of MB was found to increase with increase in irradiation time to 60 min. After 120 min of irradiation the peaks of the m/z value above 250 disappear, and the relative intensities for ring ruptured and open chain intermediates with m/z values of 247, 231, 217, 203, 189, 179, 163, 149, 135, 117, 110 were noticed significantly. Thus, the enhanced degradation rate of MB by the Fe2O3-Na catalyst was also confirmed by the ESI-MS study. Fig. 7D demonstrates the ESI-MS of the aliquots after 30 min, 60 min and 120 min of irradiation in the presence of Fe2O3-K as a catalyst. After 30 min of irradiation, the molecular ion peak of MB vanished, and the relative intensity of the peak at m/z = 275 due to N-demethylation and hydroxylation was found to be maximum. Apart from the above peak, significant relative intensities of peaks at m/z = 291, 261, 248, 220, 195, 157, 135, 102 for N-demethylation and hydroxylation, chromophore moiety


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Fig. 7 ESI-MS in the positive mode for of (A) pure methylene blue (MB) dye solution before catalysis, and catalytic degradation of MB for (B) Fe2O3-Li, (C) Fe2O3-Na, (D) Fe2O3-K catalysts in the presence of H2O2 under UV A light irradiation for 30 min, 60 min, 120 min.

cleavage, ring rupturing, and ring opening intermediates of MB were observed. Upon increasing the irradiation time to 60 min the relative intensity of m/z = 275 for N-demethylation and hydroxylation decreased, while the peaks of m/z = 204 and 110 for ring rupturing intermediates of MB were


becoming more intense. The other peaks at m/z = 291, 261, 248, 231, 220, 207, 195, 178, 157, 135, 117, 102 were also observed. With a further increase in irradiation time to 120 min, the relative intensity of the intermediates corresponding to the N-demethylated and hydroxylated conjugated


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Scheme 2 Plausible primary fragmented products of MB (i) without any damage of ring skeleton, (ii) after cleavage of chromophore moiety, and (iii) after its (A) ring rupture and (B) ring opening.

system and the ring ruptured system (m/z = 291, 275, 261, 231, 195, 157) diminished whereas the relative intensity of open chain intermediates (corresponding to m/z = 148, 135, 120, 117, 102) became prominent. Thus, from ESI-MS analysis it is confirmed that decolorization of MB in the presence of the


a-Fe2O3 catalyst renders the non-toxic open chain compounds and not the leuco MB. From the above observation it is clear that Fe2O3-Na exhibits better catalytic performance for the degradation of MB to nontoxic open chain products. Therefore, recycling and stability


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performance of Fe2O3-Na were studied. Recycling performance of the Fe2O3-Na catalyst was checked up to 3 cycles (Fig. S16, ESI†). It is clear that the catalyst was almost stable up to 3 cycles. To check the stability of the catalyst, Fe2O3-Na after photo-Fenton like degradation of methylene blue, the catalyst after degradation was characterized by XRD and Raman spectroscopy for its crystal structure analysis, and FESEM for morphological investigation (Fig. S17, ESI†). It was observed that the crystal structure (from XRD and Raman spectroscopy) and morphology of the Fe2O3-Na catalyst remained unchanged before and after MB degradation.

4. Conclusion Mesoporous cube shaped a-Fe2O3 particles were prepared using FeCl3 and 1-butyl-3-methylimidazolium bromide (IL) in the presence of different alkali metal (lithium, sodium and potassium) acetates under hydrothermal conditions at 150 1C/4 h followed by calcination at 350 1C. XRD and Raman studies confirmed the intercalation of Li+ and Na+ ions into the crystal layers of a-Fe2O3 particles. FESEM and TEM studies revealed the formation of cubelike particles of different sizes in the presence of lithium acetate, sodium acetate and potassium acetate, respectively. The a-Fe2O3-Na microcubes rendered enhanced HC (5.7 kOe) and MR (0.203 emu g1) values at room temperature (300 K) due to increased strain in crystals, shape anisotropy and magnetocrystalline anisotropy in the presence Na+ ions within the crystal layers. The synthesized mesoporous a-Fe2O3 samples were employed as catalysts for heterogeneous photo-Fenton like degradation of the toxic water pollutant MB via photogenerated active OH radicals. Both the textural properties and negative surface charge are important for the catalytic degradation of MB dye. The degradation of MB to non-toxic open chain products occurred faster using the Fe2O3-Na catalyst as confirmed by ESI-MS. Alkali metal ion induced a-Fe2O3 provides a new strategy for the design and synthesis of other transition metal oxides with improved magnetic and catalytic properties.

Acknowledgements The authors would like to thank the Director of this Institute for his kind permission to publish this paper. M. Roy is thankful to CSIR for her fellowship. The authors would like to thank Mr Mritunjoy Maity, SRF of IICB for some experimental help. The Nano-structured Materials Division is gratefully acknowledged for Raman spectra. The financial support from the Department of Science and Technology under the DST-SERB sponsored project, GAP 0616 (Grant No. SR/S3/ME/0035/2012), Government of India, is gratefully acknowledged.

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