Magnetically separable Fe₃O₄–Ag₃PO₄ sub-micrometre composite: facile synthesis, high visible light-driven photocatalytic efficiency, and good recyclability

Abstract

A magnetically separable Fe₃O₄–Ag₃PO₄ sub-micrometre composite was synthesized in large quantities by a fast and simple route, and was demonstrated to have a high photocatalytic efficiency toward the decomposition of methylene blue dye under visible light irradiation with a good recyclability.

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Introduction

Photocatalysis has been attracting a growing interest because it provides a new promising way to meet the challenges of the environment, energy and sustainability.^1,2 In the development of efficient photocatalytic systems, the synthesis of high-performance photocatalysts with good recyclabilities remains most important. Among a wide spectrum of the photocatalysts, titanium dioxide (TiO₂) has attracted much attention over the past decades because of its excellent properties such as high photocatalytic efficiency, good stability, low cost, nontoxicity and so on.^2,3 However, the wide band gap of TiO₂ makes it only responsive to high-energy UV light, resulting in a low-efficiency in the utilization of solar energy. Such a limitation activated a great interest in developing new visible light-driven photocatalysts with high catalytic performances, which remain promising for practical applications.^4–7

Very recently, silver orthophosphate (Ag₃PO₄) has been put forward as a novel photocatalyst that exhibits extremely high photooxidative capabilities for the oxidation of water and the photodecomposition of organic dyes under visible-light irradiation.^8–10 This excellent photocatalytic performance has been attributed partly to the highly dispersive Ag s-Ag s bands without localized d states, realizing a small effective mass of the electron, thus it can rapidly transfer to the surface prohibiting the carrier recombination.¹¹ Current research along this line mainly lies in decreasing the size of Ag₃PO₄ down to the nanometre level¹² since it is well known that the catalytic reaction mostly occurs on the surface of the catalysts, and thus the higher surface area of nanoparticles would be beneficial to the enhanced photocatalytic activity.^13,14 Unfortunately, when the photocatalytic reaction is completed, it is often difficult to isolate and recover the nano-sized Ag₃PO₄ catalysts from the mixed system in a simple way, resulting in the second contamination.

Herein, we report a facile and fast approach for the synthesis of a magnetically separable composite based on Ag₃PO₄ and Fe₃O₄ in large quantities. The Ag₃PO₄–Fe₃O₄ sub-micrometre composites (denoted as AF MCs) are simply synthesized by the precipitation of Ag₃PO₄ on Fe₃O₄ nanoparticles under sonication in the absence of any capping agents. The resulting AF MCs have an excellent photocatalytic activity toward the decomposition of methylene blue (MB) dye in solution under visible light and natural sunlight irradiation. More importantly, they are easily isolated from the solution by an external magnet and are subsequently reused in competitive photocatalysis with a good recyclability.

Results and discussion

Porous magnetite (Fe₃O₄) nanospheres were synthesized by a solvothermal method, as reported previously¹⁵ (ESI†). The mean diameter of these Fe₃O₄ nanospheres was about 160 nm (Fig. S1 A†). Each magnetic microsphere was constructed with many small magnetic grains and the surface was rough (Fig. S1 B†). X-Ray diffraction (XRD) patterns of these Fe₃O₄ nanospheres confirm their face-centered cubic (fcc) structure (Fig. S1 C†). Additionally, the Fe₃O₄ nanospheres were well dispersed in water to form a black-brown dispersion and could be easily drawn to the sidewall by an external magnet (Fig. S1 D†), suggesting a good magnet-controlled property.

Ag₃PO₄ was prepared by a simple precipitation process (see ESI† for experimental details), and a yellow milky solution was obtained (Fig. S2 A†). The particle size of the as-synthesized Ag₃PO₄ was about 100–450 nm with a smooth surface. The AF MCs were prepared in a similar precipitation process in the presence of Fe₃O₄ nanospheres. In order to determine the effect of the content of Ag₃PO₄ in the as-prepared AF MCs on the photocatalytic activity, three sets of samples with different quantities of Ag₃PO₄ were prepared by altering the amounts of Na₂HPO₄ and AgNO₃ added into the aqueous dispersion of Fe₃O₄, where the molar ratio of AgNO₃ to Na₂HPO₄ was kept to 3 (the stoichiometric ratio of Ag₃PO₄). It was found that, the resulting mixture became more and more turbid and some precipitation appeared when increasing the volume of the aqueous solutions of Na₂HPO₄ and AgNO₃ added to the aqueous dispersion of Fe₃O₄. As expected, the as-prepared AF MCs exhibited a magnetic response (Fig. 1 A), confirming the successful attachment of Ag₃PO₄ to the Fe₃O₄ nanospheres. From the SEM (scanning electron microscope) images (Fig. 1 C), we found that, upon formation of the sub-micrometre composites, the Fe₃O₄ nanospheres and Ag₃PO₄ particles entangle each other to form structurally uniform hybrids (Fig. 1 B).

Fig. 1 Photographs (A) of the AF MCs in sunlight, which were easily dispersed in water (left) and also could be drawn from the solution to the sidewall of the vial by an external magnet (right). SEM images (B and C) of the AF MCs at different magnifications. The white and black arrows in (C) indicate the Ag₃PO₄ and Fe₃O₄ nanospheres, respectively. XRD patterns (D) of the AF MCs and the standard Ag₃PO₄. The arrows indicate the Fe₃O₄ peaks. Ultraviolet-visible diffusive reflectance spectra (E) of the Ag₃PO₄ sub-micrometre particles (a), Fe₃O₄ nanospheres (b) and AF MCs (c). (F) The results on the adsorption ability (solid column) and photocatalytic activity (sparse column) of different samples under identical conditions.

The photocatalytic activities of these three sets of samples with different Ag₃PO₄ content were evaluated by degrading the MB dye under visible light illumination without any sacrificial reagents. Prior to irradiation, the mixtures containing the MB dye and the samples were kept in the dark for 30 min to achieve an equilibrium adsorption of the MB dye on the surface of the particles. From the results depicted in Fig S2 F†, we could see that sample 2 adsorbs more MB dye (over 30%) and exhibits the best photocatalytic efficiency. Therefore, we chose sample 2 for the subsequent photocatalytic investigations.

The magnetic properties of pure Fe₃O₄ nanospheres and the AF MCs were studied using a vibrating sample magnetometer, and the hysteresis loops of the samples are listed in Fig. S3.† It can be seen that both samples exhibit negligible coercivity and remanence. The saturated magnetization (SM) values were 79.5 and 11.1 emu g⁻¹ for the Fe₃O₄ nanospheres and the AF MCs, respectively. The decrease in the SM for the AF MCs could be explained by taking into account the diamagnetic contribution of the Ag₃PO₄ in the composites^16,17 (the weight percentage of the Fe₃O₄ nanospheres in the composites was about 15 wt%).

Fig. 1B shows the SEM image of the AF MCs with the optimized content of Ag₃PO₄ for photocatalytic activity. It was possible to distinguish these two kinds of particles in the hybrid materials by their different surfaces as marked in Fig. 1 C. Fig. 1 D depicts the XRD pattern of the as-synthesized AF MCs. Most of the diffraction peaks match well with the body-centered cubic phase of Ag₃PO₄, and three smaller diffraction peaks were indexed to the fcc phase of Fe₃O₄, again confirming that both Fe₃O₄ and Ag₃PO₄ were present in the hybrid materials. Fig. 1 E displays the ultraviolet-visible diffuse reflectance spectra of pure Fe₃O₄ nanospheres, Ag₃PO₄ sub-micrometre particles and AF MCs. Consistent with the previous report,⁸ Ag₃PO₄ shows a strong absorption in the visible region with a wavelength shorter than 530 nm because of its smaller band gap. There was an obvious red-shift in the absorption edge of the AF MCs, which exhibit an enhanced absorption in the visible region compared to the Ag₃PO₄ sample. This property might have a positive contribution to the photocatalytic reactions¹⁸ because a more efficient utilization of the solar energy could be achieved.

In addition, the photocatalytic activities of pure Fe₃O₄ nanospheres, Ag₃PO₄ sub-micrometre particles, and AF MCs were also evaluated by decomposing MB dye under visible light illumination. As shown in Fig. 1 F, the pure Fe₃O₄ nanospheres could adsorb about 10% of the MB dye, whereas no obvious photocatalytic activity was detected. However, the AF MCs exhibit a high photocatalytic performance, which is slightly higher than that of the pure Ag₃PO₄ sample, suggesting that the incorporation of magnetic Fe₃O₄ into AF MCs might improve the photocatalytic activity presumably by enhancing the adsorption ability of MB and/or extending the absorption into the visible region.^18–20 Therefore, the hybrid materials are beneficial for photocatalytic applications with good photocatalytic activity as well as magnetic recoverability.

Fig. 2 A shows a series of absorption spectra of the MB solution before and after mixing AF MCs into the aqueous solution of MB and placing the mixture under visible-light irradiation for various time periods without using any sacrificial reagents. As the irradiation time increases, the decomposition of the MB dye progressed gradually, and the decomposition was completed within 20 min. This result suggests that the MB dye was effectively decomposed under visible light with the assistance of the as-synthesized AF MCs. For comparison, the decomposition of MB with commercial TiO₂ P25 and Ag–AgCl nanoparticles (NPs) (prepared according to Ref. 6) as reference photocatalysts, and without catalyst, was also investigated under identical conditions. As displayed in Fig. 2 B, the as-prepared AF MCs display an obviously superior catalytic performance over TiO₂ P25, which could be explained by the weak adsorption of MB (2%) onto TiO₂ P25 and near no absorption in the visible region of TiO₂ P25. The as-prepared AF MCs also show a higher photocatalytic property than the visible light-driven photocatalysts Ag–AgCl NPs, which might be due to the higher charged anions (PO₄³⁻), leading to the stronger photocatalytic ability, as reported previously.²¹ When there was no catalyst in solution, no obvious change was observed in the UV-visible spectrum of the MB solution. Therefore, we may conclude that the as-prepared AF MCs possess a highly efficient photocatalytic activity under visible light.

Fig. 2 (A) UV-Visible spectra of the MB solution illuminated by visible light at different times with the assistance of AF MCs. (B) Photodecomposition of MB dye in solution over AF MCs, TiO₂ P25, Ag–AgCl NPs and without catalyst under visible-light irradiation. (C) Irradiation-time dependence of the relative concentration C/C₀ of MB in solution over AF MCs during repeated photooxidation experiments under visible light.

We found that the photocatalysts turned dark when the photocatalytic reaction was completed, indicating the formation of Ag⁰ species through the reduction of Ag₃PO₄ by photo-induced electrons. A similar phenomenon was previously observed in a AgBr/SiO₂ photolysis system, where the Ag metal was generated in the initial reaction stage and AgBr was not destroyed increasingly under successive UV irradiation.²² It has been reported that the metallic Ag could trap photo-induced electrons, and thus promote charge separation in contact with a photoexcited semiconductor,²³ which consequently inhibited the reduction of Ag salts and made the hybrid composites stable under illumination.^{5–7,24–26} To evaluate whether the AF MCs were active and stable after partial Ag₃PO₄ was converted to Ag⁰ species, we further examined the recyclability of the AF MCs, which was also very important from a practical application point of view. Fig. 2 C plots the kinetic curves for the degradation of different MB dyes by using the same batch of AF MCs. In this case, after MB was completely decomposed, the AF MCs were collected by an external magnet and used for the next photocatalytic reaction (Fig. S4†). The catalytic efficiency of the AF MCs retains over 95% of its initial activity after at least four consecutive uses, suggesting the highly recyclable ability of the AF MCs. The XRD patterns of the AF MCs after each of the consecutive photocatalytic experiments are listed in Fig. S5.† It can be seen that a new peak corresponding to Ag⁰ appeared together with the XRD peaks of Ag₃PO₄ and Fe₃O₄ after the first cycle of the reaction, whereas no obvious change was observed after the reaction was repeated for another three times. This implies that the Ag⁰ formation initially occurred on the AF MCs, which might not continuously occur in the consecutive recycling reactions. In addition, no significant loss of photocatalytic efficiency was observed after four cycles of the reaction (Fig. 2 C). These results further demonstrate that the prepared AF MCs were efficient photocatalysts and could be repeatedly used in the photocatalytic process with the advantage of an easy separation with an external magnet.

We consider that when the as-prepared Fe₃O₄–Ag₃PO₄ photocatalyst is irradiated by visible light, a number of electron–hole pairs are generated. The photogenerated electrons are preferably transferred to the Ag⁺,²⁶ and then the Ag metals that have been formed on the surface, which can trap the photoexcited electrons and thus inhibit the further decomposition of Ag₃PO₄. The self-stability mechanism makes the hybrid composites stable under visible light illumination (Fig. S6†). In general, the photogenerated holes can oxidize the MB dye directly. At the same time, the photogenerated electrons are expected to be trapped by O₂ dissolved in the solution to form the reactive oxygen species^1,5,27 that could also chemically oxidize the MB dye.

We further investigated the photocatalytic performance of the prepared AF MCs under natural sunlight. The absorption spectra of the solution at different illumination times are displayed in Fig. 3. The MB dye was completely decomposed within 10 min, indicating that the AF MCs could efficiently harvest sunlight to drive the catalytic reaction. Interestingly, we found that the photocatalytic activity of the AF MCs was higher under sunlight than under a lamp, based on the fact that it required a shorter time for MB decomposition under the former conditions. The enhanced activity might be elucidated by the broad band of the sunlight, including high-energy UV light.⁶

Fig. 3 UV-Visible spectra of the MB solution before and after exposure to sunlight for different lengths of time with the assistance of the AF MCs.

Conclusions

A simple and fast method has been demonstrated for the synthesis of magnetically separable AF MCs that can be well dispersed in water and easily recovered by an external magnet. The as-synthesized AF MCs exhibit a high adsorption ability toward MB dye, a strong absorption in the visible region, and a high efficiency and good recyclability for catalyzing the decomposition of MB dye under visible light and sunlight irradiation without any sacrificial reagents in the solution. It is believed that the easy preparation and efficient photocatalytic activity, in combination with the convenient recovery by an external magnet and high recyclability, of this type of nanophotocatalyst makes them substantially promising for practical applications in the degradation of organic pollutants for environmental remediation by the direct use of solar energy.

Acknowledgements

This work was financially supported by the NSF of China (grant no.s 20975104, 21127901, and 20935005), the National Basic Research Program of China (973 Program, 2010CB933502), and the Chinese Academy of Sciences (KJCX2-YW-W25 and Y2010015).

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Footnote

† Electronic Supplementary Information (ESI) available. See DOI: 10.1039/c2ra20504a/

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