Effect of the magnetic core of (MnFe)₂O₃@Ta₂O₅ nanoparticles on photocatalytic hydrogen production

Abstract

The combination of the pre-formed magnetic (Mn_0.983Fe_0.017)₂O₃ particles with Ta₂O₅ prepared from ionic liquids affords (MnFe)₂O₃@Ta₂O₅ core–shell composites. The photocatalytic activities of these core–shell materials were evaluated for hydrogen production, for which they appeared to be superior catalysts to the individual (Mn_0.983Fe_0.017)₂O₃ and Ta₂O₅ components.

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Introduction

Nanomaterials, having one or more dimensions on the nanometer scale (≤100 nm), exhibit different properties from those of the bulk materials. Initially researchers showed much interest in homogeneous (made up of only one type of material) nanoparticles (NPs) but later in the 1980s, heterogeneous semiconductor composite materials were studied extensively.¹ During the early 90s, multi-layered semiconductor nanomaterials (called “core–shells”) were fabricated.² Moreover, researchers devoted their attention to developing advanced materials and the advancement of technology also greatly supported the synthesis of specific nanomaterials. Amongst them, special interest is given to those containing different shapes, such as: cubes, prisms, rods, tubes, disks, wires, octahedra and hexagons, among others, and also those with different combinations of core–shells including inorganic–inorganic, inorganic–organic, organic–inorganic and organic–organic composite materials.³ Amongst these advanced core–shell materials, semiconductors with wide band gaps play a significant role due to their potential applications in electronics, optics, magnetism and photocatalysis.^4–8 Multi-component core–shells have been designed to incorporate many functions into one system.

Indeed, magnetic TiO₂ core–shells have been in the limelight due to their high efficiency in photocatalysis as well as recovery from treated water.^9,10 Nevertheless, UV-light active TiO₂ coated core–shell composites are difficult to synthesize without sacrificing their magnetic properties. Some research has been conducted on the use of a magnetic material as the core.^11,12 More recently, super magnetic materials have been employed as core materials with the incorporation of transition metals into ferrite (MFe₂O₄, M = Ni, Co, Sr) and have displayed good results for the photocatalytic degradation of organic pollutants.^13–17

As per literature reports, Misra et al.¹⁸ have synthesized a NiFe₂O₄@TiO₂ core–shell composite with nickel ferrite as the inner magnetic core and titania as the outer photocatalytic shell via reverse micelle and chemical hydrolysis techniques. H. S. Kim et al.¹⁹ have synthesized a NiFe₂O₄@TiO₂ core–shell nanomaterial photocatalyst for the production of hydrogen from an aqueous system using a methanol/water solution. Guo et al. have synthesized various mesoporous metal oxides such as Ta₂O₅, Nb₂O₅ and TiO₂ which have been used to measure photocatalytic HER and OER.²⁰ Punnoose et al. have prepared single and double coated barrier layers on TiO₂ films using MgO and Al₂O₃, which enhances charge separation and also electron lifetime. In addition, layers of PbS with CdS significantly enhance the photo-current.²¹ Hamad et al. have synthesized CoFe₂O₄–SiO₂–TiO₂ magnetic core–shells in which SiO₂ acts as an insulator between the CoFe₂O₄ core and the TiO₂ shell and assists in the separation of the core and shell during photocatalysis. The CoFe₂O₄–SiO₂–TiO₂ composite has been tested for degradation of DCPIP dye.²² Li et al. have fabricated a stable ZnO–Ta₂O₅ photo-electrode for photoelectrochemical water splitting in which an ALD grown Ta₂O₅ layer protects ZnO NRs for water oxidation over a period of 5 h with no photocurrent decay.²³ Misra et al. have prepared Au–Ag–ZnO core–shell nanoparticles to reduce charge recombination and enhance the visible light photocatalytic activity.²⁴ Chang et al. have made Fe₃O₄@ZnS and NiCoO₄@ZnS core–shell photocatalysts. The electronic structure and morphology of the materials affect the photocatalytic hydrogen production and better H₂ production was observed.²⁵ Chang et al. have prepared photoactive and magnetically recyclable CoFe₂O₄@ZnS core–shells by adding various ZnS concentrations and they observed better H₂ production for the CoFe₂O₄@ZnS 0.5 h photocatalyst calcined at 500 °C.²⁶ It was reported that CoFe₂O₄ or NiCo₂O₄ based core–shell nanomaterials have enhanced photocatalytic H₂ production activity which can be attributed to the formation of a hetero-junction at the interface.

Alternatively, here, we have chosen ionic liquids (ILs) for the synthesis of core–shells because these fluids combine their unique properties (negligible vapor pressure, low melting point, high thermal stability, ionic conductivity, tailorable structures and wide electrochemical window) with the ability to provide suitable media to generate active nanocatalysts for several applications.^27–29 The successful use of ILs for the synthesis of photocatalysts has also been previously reported.^30–32 In particular, tantalum oxide nanoparticles were prepared from imidazolium hexachlorotantalate ILs and showed high activity for hydrogen production from water/ethanol solutions.³² In this case, it was suggested that the remaining IL at the surface of the Ta₂O₅ NPs plays a fundamental role in affording hydrophilic regions, which facilitate the approach of polar molecules.

Herein, we report the synthesis of a novel, advanced and useful bi-layered (MnFe)₂O₃@Ta₂O₅–IL core–shell material (IL: BMICl = 1-n-butyl-3-methylimidazolium chloride; DMICl = 1-n-decyl-3-methylimidazolium chloride). In order to cover the pre-formed (Mn_0.983Fe_0.017)₂O₃ core with Ta₂O₅ (shell) a similar method to that reported for the synthesis of Ta₂O₅ NPs has been employed.²⁵ These core–shell materials were demonstrated to be efficient photocatalysts for hydrogen production in aqueous systems. Here, ILs containing different alkyl side chains were used in order to test the stabilizing effect of the groups on the synthesis of the core–shell materials as well as to observe the performance in hydrogen production reactions.

Results and discussion

The addition of a magnetic core (Mn_0.983Fe_0.017)₂O₃ (MF) to the Ta₂O₅ shell assists in charge separation, which results in better H₂ production. The (MnFe)₂O₃@Ta₂O₅ core–shell has been prepared using ILs and studies have been performed. Powder XRD patterns of the individual bare compounds and core–shells are shown in Fig. 1. In Fig. 1a, all the diffraction peaks are indexed to the cubic phase of manganese iron oxide (Mn_0.983Fe_0.017)₂O₃ (MF) (JCPDS No. 24-507, a = 9.4146 Å), with the space group la3̄d (no. 206). All of the diffraction peaks in Fig. 1b can be indexed to the orthorhombic phase of tantalum oxide (JCPDS No. 25-922, a = 6.198 Å, b = 40.29 Å, c = 3.888 Å) with the space group P2₁2₁2 (no. 18). In the XRD patterns in Fig. 1c and d, we have observed broad diffraction peaks of the core–shells due to the presence of the ionic liquid. However, a few broad and weak diffraction peaks appeared that resembled the tetragonal phase of manganotapiolite (MnFe)₂O₃–Ta₂O₅ with JCPDS No. 49-1870, lattice parameters of a = 4.7660 Å and c = 9.2772 Å and the space group P42/mnm. This clearly explains the weak diffraction patterns of tantalum oxide and manganese iron oxide as they are bound by the outer tantalum oxide layer. The broad peaks that appear in the XRD patterns (Fig. 1c and d) suggest that the size of the core–shell is in the nanometer range.¹⁸

Fig. 1 XRD patterns of (a) MF, (b) bare Ta₂O₅, (c) MFT–BMICl and (d) MFT–DMICl.

The FTIR spectra of core–shells prepared using two different ionic liquids are illustrated in Fig. 2a and b. It is possible to observe the different bands corresponding to metal oxides and organic moieties. The strong and broad bands between 3200 and 2900 cm⁻¹ indicate the presence of water molecules. C–H stretching and in-plane vibrations of the imidazolium ring were observed in the range of 1600–1500 cm⁻¹.^33,34 The band at 1170 cm⁻¹ is assigned to aliphatic in-plane vibrations. All of these bands confirm the existence of the ionic liquid even after repeated washing. The absorption bands of Ta–O (∼640–650 nm) were slightly shifted to higher frequencies due to the presence of more ‘Ta’ forming a strong and broad absorption band at ∼820 nm, which is attributed to the Ta–O–Ta stretching mode.³⁵ The sharp band between 565 and 575 nm is due to the Fe–O bond vibration from MF, which is also observed in Fig. 2a.³⁶ The strong band at 500 cm⁻¹ is due to the presence of pure Ta₂O₅.³⁷

Fig. 2 FTIR spectra of (a) MF, (b) bare Ta₂O₅, (c) MFT–BMICl and (d) MFT–DMICl.

Fig. 3 shows the UV-vis spectra of MF, Ta₂O₅, and MFT–BMICl/DMICl core–shells. Fig. 3a clearly shows a continuous absorption band in the range of 200–600 nm, which concurs with its brick red color. The band gap is around 2.06 eV. Fig. 3b displays a strong absorption band in the UV region, 230–330 nm. The main characteristics of this absorption are due to Ta₂O₅ with a crystalline structure. The band gap is estimated to be 3.75 eV.^38,39Fig. 3c and d of MFT–BMICl/DMICl clearly indicates that the edge of the absorption peak of Ta₂O₅ was red-shifted compared to the standard bare MF⁴⁰ due to the narrowing of the band gap of Ta₂O₅ (outer shell), and thereby proving that it absorbs in the UV as well as in the visible region. We suggest that MF has changed the band gap of Ta₂O₅.

Fig. 3 UV-vis spectra of (a) MF, (b) MFT–BMICl, (c) MFT–DMICl and (d) bare Ta₂O₅.

The SEM micrographs of MF (Fig. 4a) display a large number of particles that have aggregated to form a cluster. In Fig. 4b, the SEM image shows that bare Ta₂O₅ looks like a cluster of particles. SEM images of the core–shells clearly show that the sample contains pockmarked, spherical-shaped globules. A large number of controllable, crystalline monolith beads are clearly formed in the presence of BMICl (Fig. 4c and e) and are less controllable in the case of DMICl (Fig. 4d and f).

Fig. 4 SEM images of (a) MF, (b) bare Ta₂O₅, (c and e) MFT–BMICl and (d and f) MFT–DMICl.

Fig. 5a shows a TEM image of MF that consists of roughly spherical particles with average diameters in the range of 10–40 nm. Fig. 5b depicts a TEM image of bare Ta₂O₅ and it clearly shows that the particles are slightly agglomerated with average diameters of 15–50 nm. Fig. 5c and e displays a high magnification view of the double-layered core–shells prepared using different ionic liquids, MFT–BMICl and MFT–DMICl, respectively. Although the different layers are indistinguishable in Fig. 5c, two distinct core and shell layers are clearly visible in Fig. 5e. As per the experimental procedure, the primary inner MF core was bound by the secondary Ta₂O₅ shell, which was confirmed using TEM images. The average diameter of the primary core is about 15–20 nm and the thickness of the secondary shell is 5–10 nm in the case of MFT–BMICl. A diameter of 15–25 nm of the primary core and a thickness of ∼5 nm of the shell were observed for MFT–DMICl. Fig. 5d and f shows HRTEM images of MFT–BMICl/DMICl in which the plane spacing of 3.37 Å corresponds to the d-spacing of MF(222) and 2.59 Å corresponds to the Ta₂O₅(1 11 0) plane, respectively. The TEM images clearly confirm the presence of double-layered core–shell nanomaterials.

Fig. 5 TEM images of (a) MF and (b) bare Ta₂O₅; TEM and HRTEM images of (c and d) MFT–BMICl and (e and f) MFT–DMICl.

Generally, the photochemical hydrogen evolution process involves three important steps: (i) suitable light absorption by the photocatalyst, (ii) photo-generated charge formation, as well as charge separation and (iii) redox reactions on the surface of the photocatalyst or co-catalyst.⁴¹ Here, Ta₂O₅ is used for light absorption and as a redox site while the magnetic core MF is used to enhance charge separation. The detailed photochemical mechanism is predicted in Scheme 1, and is supported by earlier studies.¹⁹ The position of the conduction band (CB) of MF is at a lower energy than that of Ta₂O₅, and hence, the core acts as a storage site for photo-generated electrons. It is suggested that when light illuminates the photocatalyst, photo-generated electrons are transferred from the valence band (VB) to the CB of Ta₂O₅ and then they move into the CB of MF. Holes in the VB of Ta₂O₅ would assist in the production of OH˙, which is involved in the photocatalytic process.¹⁹ Progressively with the increase of the number of electrons in the CB of MF, they move into the VB of MF and then into the VB of the shell, which causes slow recombination. This kind of special redox cycle occurs in magnetic core–shell structures and helps with charge separation, resulting in a remarkable enhancement in H₂ production. As we all know, in MFT–BMICl/DMICl core–shells the Fe ions exist in a +3 oxidation state (electronic configuration 3d⁶ = t₂g⁶ eg⁰). Hence, during photochemical reactions Fe³⁺ ions can accept electrons from the Ta₂O₅ shell and become saturated to a stable oxidation state of Fe²⁺ and this conversion process reduces the recombination rate. On the other hand, the formation of photogenerated holes in the valence band of Ta₂O₅ converts OH⁻ ions to OH radicals, which then oxidize water/ethanol to H₂, O₂ and CO₂. The conversion of Fe³⁺ to Fe²⁺ and the formation of holes assist in the enhancement of photocatalytic hydrogen evolution. This process of the core–shell mechanism is strongly supported in the reported literature.¹⁹

Scheme 1 Proposed mechanism for photochemical hydrogen production, promoted by the core–shell particles.

Hydrogen production was performed using the following photocatalysts: MF, bare Ta₂O₅ and MFT–BMICl/DMICl in the presence of different sacrificial agents such as ethanol, methanol and acetic acid. The variation of the amounts of the catalyst and pH was also studied. Initially, hydrogen production was studied using various photocatalysts such as MF, bare Ta₂O₅ and MFT–BMICl/DMICl and the results are shown in Fig. 6. In this reaction, the catalyst concentration was 1 mg mL⁻¹ (8 mg of catalyst and 8 mL of solution mixture). Fig. 6 shows that MFT–BMICl has the highest H₂ production activity (6373 μmol g⁻¹), followed by MFT–DMICl (5582 μmol g⁻¹), bare Ta₂O₅ (1224 μmol g⁻¹) and MF (1037.56 μmol g⁻¹) after 2.5 h of photocatalysis. The two materials synthesized using ILs, MFT–BMICl and DMICl, showed different H₂ production rates. The rate of H₂ production of MFT–BMICl is 2549.2 μmol g⁻¹ h⁻¹, while for MFT–DMICl it is 2232 μmol g⁻¹ h⁻¹. The activities of bare Ta₂O₅ and MF are 489.6 μmol g⁻¹ h⁻¹ and 415 μmol g⁻¹ h⁻¹, respectively.

Fig. 6 Hydrogen production of (a) MF, (b) bare Ta₂O₅, (c) MFT–BMICl and (d) MFT–DMICl.

We continued the experiments to observe H₂ production in the photocatalytic reaction system by varying the amount of the catalyst (MFT–BMICl) and the results are depicted in Fig. 7. This clearly indicates that increasing the amount of the photocatalyst increases the H₂ production. Moreover, the system containing 8 mg of the catalyst showed better results with respect to H₂ production and stability, when compared to 2 mg and 4 mg of the catalyst.

Fig. 7 Hydrogen production from core–shell MFT–BMICl using various amounts of the catalyst.

In addition, the role of pH in H₂ production under the optimized conditions was investigated (Fig. 8). The results indicate that the production of H₂ is higher under acidic conditions when compared to basic and neutral conditions. In fact, under acidic pH conditions more H⁺ ions could be adsorbed onto the surface of the photocatalyst, increasing the rate of H⁺ reduction to H₂ molecules.⁴⁶

Fig. 8 Hydrogen production from core–shell MFT–BMICl under different pH conditions.

Fig. 9 illustrates the effect on H₂ production of varying the sacrificial agents using the catalyst MFT–BMICl. As seen in Fig. 9, ethanol proved to be the best sacrificial agent (6373 μmol of H₂), followed by methanol (2098 μmol of H₂) and acetic acid (642 μmol of H₂). In the presence of electron donors (ethanol and methanol) these are oxidized by the positive valence band (VB) holes, avoiding the electron/hole recombination process. Then, the excited electrons in the conduction band (CB) can reduce the protons (from water) to hydrogen molecules. In the case of acetic acid, it undergoes oxidation during the photocatalytic reaction, producing H₂ and CO₂ as by-products. Details of the photocatalytic reaction mechanism can be found below.^42–45

H₂O−2e → O + 2H⁺ (1)

C₂H₅OH−2e → CH₃CHO + 2H⁺ (2)

CH₃OH−2e → HCHO + 2H⁺ (3)

At the CB, with the help of the catalyst, electrons would react with H⁺, leading to the production of H₂.

2H⁺ + 2e → H₂ (4)

In the case of an oxidizing agent, aqueous acetic acid can form H₂ and CO₂ as follows:

CH₃COOH_(aq) + O₂ → 2CO₂ + 2H₂. (5)

All of the catalytic results are summarized in Table 1. It is clear that the double layered core–shell composites have produced 5 times more H₂ than their individual components. Indeed, this behaviour can be confirmed by the superior TOF values that were estimated for the MFT–IL samples (Table 2).

Fig. 9 Hydrogen production from core–shell MFT–BMICl using various organic solvents.

Table 1 Photocatalytic H₂ production (μmol g⁻¹) after 2.5 h under different conditions

Catalyst	Sacrificial agent			pH			Amount of catalyst (mg)
	Ethanol^b	Methanol^b	Acetic acid^b	Neutral (only water)^b	Acidic^b	Basic^b	2^c	4^c	8^c
a In parentheses is shown the rate of H2 production (μmol g^−1 h^−1); in all cases 8 mL of solution was used. b 8 mg of the catalyst was used. c Ethanol was used as the sacrificial agent.
MF–Ta2O5–BMICl	6373.22 (2549.28)^a	2098.84 (839.53)^a	729.91 (256.92)^a	621.77 (248.7)^a	820.27 (328.1)^a	785.6 (314.2)^a	1999 (799.74)^a	2774.36 (1109.8)^a	6373.22 (2549.28)^a
MF–Ta2O5–DMICl	5582.5 (2232)^a	—	—	—	—	—	—	—	—
Bare MF	1037.56 (415)^a	—	—	—	—	—	—	—	—
Bare Ta2O5	1224 (489.6)^a	—	—	—	—	—	—	—	—

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Table 2 Summary of the photocatalytic hydrogen production

Catalyst	H2 (μmol g^−1)	TON^a	TOF^a (h^−1)
a Turnover number: TON = the rate of moles of H2 evolved/moles of the catalyst; turnover frequency: TOF = TON/time.
MF–Ta2O5–BMICl	6373.22	2.1	0.84
MF–Ta2O5–DMICl	5582.5	1.87	0.749
MF	1037.56	0.35	0.14
Bare Ta2O5	1224	0.41	0.16

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Conclusions

In conclusion, (MnFe)₂O₃@Ta₂O₅–IL core–shell materials prepared using two different ILs showed better photocatalytic results compared to their individual core and shell. Core–shells with clear boundaries have been formed without agglomeration. The H₂ production values recorded for MFT–BMICl, MFT–DMICl, MF (core) and Ta₂O₅ (shell) were 2549.28, 2232, 415 and 489.6 μmol g⁻¹ h⁻¹, respectively. Core–shell MFT–BMICl recorded the highest hydrogen production compared to the core and shell in the presence of ethanol as sacrificial agent. From these studies we found that the inner magnetic core successfully helps in charge separation and gives better hydrogen production. The material also can be recycled by separating the magnetic core from the shell using a bar magnet. Hence the material is reusable.

Experimental section

Materials

TaCl₅ was obtained from Alfa Aesar (99.99% purity). Manganese(II) acetylacetonate (95% pure) and FeCl₃ (98% pure) were purchased from Strem Chemicals and used without further purification.

Synthesis

1-n-Butyl-3-methyl imidazolium chloride (BMICl). 1-Methylimidazole and 1-chlorobutane (1 : 1.1 molar ratio) were mixed directly and stirred at 60 °C for 2 days. The heating was continued until a homogeneous solution was formed. Then, 100 mL of acetone and 1–2 mL of methanol were added under warm conditions. After this, a pinch of previously formed BMICl was added as a seeding agent and the reaction mixture was kept aside for the crystals to be formed in the freezer. The solution was separated and kept again in the freezer for about 24 h to form crystals. The crystals were washed with ethyl acetate and dried under vacuum (Fig. 10).

Fig. 10 Structure of 1-n-butyl-3-methylimidazolium chloride.

1-n-Decyl-3-methylimidazolium chloride (DMICl). 1-n-decyl-3-methylimidazolium chloride was synthesized using 1-methylimidazole (103 g, 1.25 mol) and 1-chlorodecane (265 g, 1.5 mol). The reagents were added to a 1 L flask equipped with a magnetic stirrer at 120 °C and the system was refluxed for 48 h. The product was cooled to room temperature, then 200 mL of ethyl acetate was added and the solution was magnetically agitated for 5 minutes. After this, the flask was kept in the freezer for about 24 h to precipitate the DMICl. The solution was filtered, washed with ethyl acetate (2 × 100 mL) and then dried under reduced pressure.

(Mn_0.983Fe_0.017)₂O₃ (MF) core. We used a previously reported procedure for the synthesis of the (Mn_0.983Fe_0.017)₂O₃ (MF) magnetic core.¹⁹ In a typical procedure, 2 M ferric chloride and 1 M manganese acetyl acetate were added slowly in increments to 250 mL of ethanol in a beaker under constant stirring until it formed a homogeneous solution. The system was maintained at pH 9 using ammonium hydroxide for rapid hydrolysis. Hence, the manganese and iron precursors were partially hydrolyzed to form their respective hydroxides in ethanol. The final solution was filtered and dried to get the amorphous metal oxides and then calcined at 500 °C for 3 h to afford the pure crystalline product.

Bare Ta₂O₅. Commercial Ta₂O₅ (0.34 mmol) was added slowly to 50 mL of water under slow stirring to obtain a homogeneous dispersion. The solution was transferred to a Teflon tube and treated hydrothermally at 120 °C for 1 day under slow stirring. After the reaction was complete, the reactor was cooled to room temperature naturally. The obtained product was washed with water and filtered. Then, the sample was dried and calcined at 400 °C for further characterization.

(MnFe)₂O₃–Ta₂O₅ (MFT) core–shell using BMICl/DMICl. TaCl₅ (2.26 mmol) was dissolved in 1 mL of 1-n-butyl-3-methylimidazolium chloride (BMICl) and stirred, similar to the reported procedure.²⁵ To the above mixture, 0.135 g of MF (previously prepared) and 0.14 mL of water were added under constant stirring. Hydrolysis took place, resulting in the formation of the MFT composite, which was indicated by the release of HCl fumes. The obtained product was dried in an oil bath under vacuum at 120 °C for 24 h to get the amorphous MFT composite in powder form. This was used for further characterization. The above procedure was repeated to synthesize hetero-structured MFT core–shells in the presence of DMICl as an ionic liquid medium.

Characterization

The morphology of the prepared materials was characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) using a JEOL 6060 and a JEOL JEM1200 EXII operating at 15 kV and 80 kV, respectively. For TEM analysis, the samples were prepared by dispersing a few milligrams of freestanding NTs in acetone at room temperature followed by ultrasonication. One or two drops were further deposited on a 400 mesh carbon-coated Cu grid. Chemical analyses using line scans and mapping were performed using an electron probe energy-dispersive X-ray spectrometer (EDX) in an SEM microscope. X-ray powder diffraction (XRD) experiments were conducted using a D/max-3B diffractometer with Cu Kα radiation. The scans were made in the 2θ range 10–80° with a scan rate of 0.5° min⁻¹ (low angle diffraction). Fourier transform infrared spectra of the samples were collected using a Bruker Alpha-P spectrometer. The absorption spectra of the samples were measured using a Perkin-Elmer Lambda-750 UV-VIS spectrometer. For the photocatalytic experiments, a Xe lamp (300 W, Cermax) operating at 240 W was used.

Photocatalytic experiments

The photocatalytic reactions were carried out in a 20 mL quartz glass reactor with a closed gas-circulating system. 8 mg of the core–shell sample and 6 mL of water were placed in the reactor and sonicated for 20 min. After sonication, 2 mL of a sacrificial agent was added (25% volume of an organic compound was used). Before irradiation, the system was de-aerated by bubbling argon for about 10–15 min to remove the oxygen content. During the entire experiment, the reaction temperature was kept at 25 °C by eliminating the IR radiation with the circulation of water in the outer jacket of the reactor. Analyses were conducted using an Agilent 6820 GC chromatograph equipped with a thermal conductivity detector (TCD) and 5 Å molecular sieves packing the column, with argon as the carrier gas. The amount of hydrogen produced was measured using a gas-tight syringe with a maximum volume of 50 μL at 0.5 h intervals. H₂ experiments were repeated by varying the sacrificial agents, catalytic load and pH.

Separation of the magnetic core

After photocatalysis, separation of the magnetic core (Mn_0.983Fe_0.017)₂O₃ particles from the (MnFe)₂O₃@Ta₂O₅ core–shell composite is possible, as shown in Fig. 11. Here, we have dispersed a certain quantity of (MnFe)₂O₃@Ta₂O₅ into aqueous solution in a glass container and sonicated it for a few minutes. Then a magnetic bar was moved along the glass container, and most of the larger magnetic core particles started being attracted to the magnetic bar, which extracted them.

Fig. 11 Separation of the magnetic particles from the photocatalyst.

Acknowledgements

The authors thank CNPq-TWAS (K. Manjunath) and CAPES for research support.

Notes and references

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