Magnetically recoverable and visible-light-driven nanocrystalline YFeO₃ photocatalysts

Abstract

The single nanocrystalline perovskite YFeO₃ was synthesized using Fe(NO₃)₃·9H₂O and Y(NO₃)₃·6H₂O by a facile microwave-assisted approach. Powder X-ray diffraction (XRD) and scanning electron microscopy (SEM) demonstrate the successful synthesis of single perovskite YFeO₃ and an average grain size of 60–70 nm in diameter. The prepared YFeO₃ shows a Brunauer–Emmett–Teller (BET) surface area of 11.7 m² g⁻¹. UV-visible diffuse reflectance spectroscopy (DRS) shows the optical absorption onset of 511 nm, indicating the optical band gap of 2.43 eV. Ferromagnetic character was demonstrated by the hysteresis loop in the as-prepared YFeO₃ nanoparticles. The photocatalytic experiment shows a high activity for decomposition of Rhodamine B under visible-light irradiation, which is attributed to the strong visible-light absorption. The recovery of YFeO₃ photocatalysts was performed after the photocatalytic experiment, and it is found that YFeO₃ nanocrystalline photocatalysts can be efficiently recovered by an external magnet without the need for a conventional filtration step.

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1. Introduction

Visible-light-driven photocatalysis is highly expected to be an ideal “green” technology for remediation of environment pollution,¹hydrogen energy production from water² and CO₂ fixation,^3,4etc. To date, most studies on photocatalysis are focused on TiO₂ because of its high physical/chemical stability, low-cost and high activity under ultraviolet (UV) light.^5–9 Unfortunately, the wide band gap (>3.0 eV) of TiO₂ limits its application in the harvesting of visible-light, which occupies a large fraction of the solar illumination onto the earth (>43%), and thus results in the low visible-light photocatalytic activity.¹⁰ Accordingly, a photoactive material under visible-light has been extremely expected from the viewpoint of efficient utilization of solar illumination. Recently, much effort has been made to develop visible-light driven photocatalysts, including cation- or anion-doping,^11,12 organic dye-sensitizing¹³ and compositing with a semiconductor of narrow band gap.¹⁴ However, the resulting photocatalysts still are deficient in either activity and/or stability.

Besides those strategies mentioned above, developing new materials of narrow band gap is another plausible strategy to realize visible-light photocatalysis. Zou et al.,^15,16 Kudo et al.,^17–19 and our group²⁰ recently reported several visible-light responsive photocatalysts. Meanwhile, iron(III)-based semiconductors have also drawn increasing attention in recent years, such as Fe₂O₃, BiFeO₃ and YFeO₃, and have been reported to bear narrow band gaps and visible-light photocatalytic activities.^10,21 Based on the phase diagram, perovskite YFeO₃ is thermodynamically unstable and can transform to thermodynamically stable Fe₃O₄ or Y₃Fe₅O₁₂ at high temperature, which makes the synthesis of pure perovskite YFeO₃ a difficult task.²² Although synthesis approaches such as sol–gel,²³coprecipitation^24,25 and solid state reaction have been reported for synthesis of perovskite YFeO₃, most of the current approaches for synthesis of single phase perovskite YFeO₃ require the calcination process at high temperature, which results in high energy-consumption and the growth of large size particles. In addition, YFeO₃ has been found to be ferromagnetic, which renders a possibility for recovery of YFeO₃ with an external magnet in practical applications.^22,23 Herein, we synthesized the single phase perovskite YFeO₃ by a microwave-assisted approach, and investigated its photocatalytic activity under visible-light as well as magnetic recovery in the photocatalysis experiment.

2. Experimental section

2.1. Synthesis of nanocrystalline YFeO₃

Nanocrystalline YFeO₃ particles were synthesized by a microwave-assisted approach. In a typical process, 2.6 mmol of Fe(NO₃)₃·9H₂O, 2.6 mmol of Y(NO₃)₃·6H₂O and 0.55 g of polyvinyl alcohol were dissolved into 150 mL deionized water under magnetic stirring, respectively, and then 0.31 g of urea was added. After stirring for 1 h at room temperature, the resulting solution was transferred to a microwave, and the microwave processing was carried out until the complete combustion. The resulting powders were centrifuged and washed 5 times for further use.

2.2. Photocatalytic activity testing and photocatalyst recovery

The photocatalytic activity of YFeO₃ nanoparticles was evaluated by decomposition of Rhodamine B (RhB) in a self-assembled apparatus in which a metal halogen lamp (150 W) equipped with a JB400 filter was used as the irradiation source. Typically, 20 mg YFeO₃ nanoparticles were added to a 10 mL RhB aqueous solution (10 mg L⁻¹), and then stirred for 30 min in the darkness to get the adsorption equilibrium followed by visible-light irradiation. The adsorption experiment was conducted in the darkness using the same procedure as the photocatalysis experiment. After a given time, the solution was centrifuged, and the upper transparent solution was subjected to the UV-visible absorption spectra measurement. The concentration of the residual RhB was evaluated by the absorbance at 554 nm. The commercial photocatalyst P25 (Degussa, Germany), which is a photocatalyst model, was used for the control experiment.

The photocatalyst was recovered after the photocatalysis experiment. Specifically, the container with YFeO₃ photocatalysts and residuals was placed on a bulky magnet, and kept for 5 min in the darkness. The container was inclined to remove the liquid and the YFeO₃ photocatalysts were attracted to the side of the bulky magnet. Thus, the recovered photocatalysts were weighed to calculate the recovery rate.

2.3. Characterization

Powder X-ray diffraction (XRD) was performed with a Hitachi Rigaku D/max-3B X-ray diffractometer using Cu Kα radiation (λ = 0.15406 nm) operated at 40 kV and 35 mA. The average crystalline size was evaluated by Scherrer's formula based on the crystal plane (112).²⁶ The morphology of the samples was observed using a Hitachi S-4800 field emission scanning electron microscope (FE-SEM) with an accelerating voltage of 5 kV. The Brunauer–Emmett–Teller (BET) surface area was measured by the nitrogen adsorption–desorption method at 77 K on a JW-K analyzer (Beijing Jingwei Gaobo Sci.-Tech. Ltd., Beijing, China). UV-visible diffuse reflectance spectra (DRS) and UV-visible absorption spectra were taken in a Hitachi UV-4100 spectrometer. The magnetization behavior was measured at 300 K in a 7407 Series Vibrating Sample Magnetometer (Lake Shore Cryotronics, Inc., USA).

3. Results and discussion

3.1. Synthesis of nanocrystalline YFeO₃ and its structure analysis

Fe₂O₃ and Y₂O₃ were used as starting materials to prepare YFeO₃ by hydrothermal method, sol–gel and solid state reaction in most previous work,^23–25 which requires the calcination step at a high temperature of above 700 °C. The reaction is heterogeneous due to the poor solubility of the used precursors in the reaction medium. In our present work, the use of water soluble Fe(NO₃)₃ and Y(NO₃)₃ precursors enables the adequate mixing and thus the homogenous reaction, benefiting the formation of monophasic YFeO₃ in our synthetic strategy although it is thermodynamically unstable.

Fig. 1a shows the XRD pattern of the as-prepared YFeO₃ sample. All peaks can be indexed to the characteristic XRD peaks of the perovskite YFeO₃ according to the JCPDS card No. 73-1345 and there are no additional peaks observed, which indicates the successful synthesis of monophasic perovskite YFeO₃ without external impurities like Y₃Fe₅O₁₂, Fe₃O₄ and Fe₂O₃. The average grain size was estimated to be 60 nm according to that from Scherrer's formula based on the crystal plane (112). In general, the metastable YFeO₃ is transformed into the thermodynamically stable Y₃Fe₅O₁₂ at high temperature. Fortunately, our present approach does not need the calcination step. Therefore, the present approach is a facile and superior one to prepare single phase perovskite YFeO₃. The SEM image in Fig. 1b demonstrates that the as-prepared YFeO₃ sample consists of nanospheres with diameters of 60–70 nm, which is consistent with the result from XRD. The BET result shows that the prepared YFeO₃ has a specific surface area of 11.7 m² g⁻¹. Results have demonstrated the successful synthesis of single phase nanocrystalline perovskite YFeO₃ by a microwave-assisted approach.

Fig. 1 (a) XRD pattern and (b) SEM image of nanocrystalline YFeO₃ synthesized in the present work.

3.2. UV-visible diffuse reflectance spectra and magnetic behavior

The optical properties of the as-prepared YFeO₃ nanoparticles were investigated by the UV-visible DRS spectroscopy, and the results are shown in Fig. 2a. The absorption onset is located in the visible region of above 511 nm, and the visible light absorption is remarkably stronger in comparison with that of the commercial P25. Therefore, the visible-light irradiation can excite the present nanocrystalline YFeO₃, which makes it the potential visible-light-driven photocatalyst.

Fig. 2 (a) UV-visible diffuse reflectance spectra of the as-prepared YFeO₃ sample and the commercial P25 photocatalyst. (b) Hysteresis loop at 300 K for the as-prepared YFeO₃ sample.

The hysteresis loop of YFeO₃ at 300 K is shown in Fig. 2b, and it is clearly indicated that YFeO₃ has a weak ferromagnetic behavior. The coercive field of the YFeO₃ nanoparticles is found to be around 1.2 kOe. The saturated M value is 1.7 emu g⁻¹, in agreement with the value reported earlier.²²

3.3. Photocatalytic activity and photocatalyst recovery

As well shown above, the nanocrystalline YFeO₃ has a small optical band gap and strong visible-light absorption. We evaluated the visible-light photocatalytic activity by decomposition of RhB under visible-light irradiation, and the commercial P25 photocatalyst was used as the control sample (Fig. 3). The adsorption curves of YFeO₃ and P25 are shown in Fig. 3. It was found that YFeO₃ has strong adsorption ability toward RhB in comparison with P25 in spite of the smaller BET surface (11.7 m² g⁻¹) than P25 (40.8 m² g⁻¹). Both YFeO₃ and P25 reach the sorption equilibrium after 30 min. The RhB is very stable under visible-light, and there is no observable decomposition of RhB even after 120 min exposure to the visible-light. The RhB is completely decomposed after 120 min visible-light exposure in the presence of YFeO₃ nanoparticles while only 25% of RhB is decomposed in the case of P25. This suggests that our prepared nanocrystalline YFeO₃ is much more visible-light active, which may be ascribed to the strong visible-light absorption and its unique electronic structure. Furthermore, the chemical oxygen demand (COD) experiment shows that RhB is completely decomposed into CO₂and H₂O, etc., after 120 min exposure to the visible-light. So far, the optical properties, electronic structural characteristics of YFeO₃ and their correlations with photocatalytic activity have been paid limited attentions. More concerns are needed to improve better understanding for correlation between the photoelectric properties and the visible-light activity.

Fig. 3 Adsorption and photocatalytic decomposition efficiency of Rhodamine B as a function of equilibrium time and irradiation time in the absence or presence of YFeO₃ and P25, respectively.

Recovery of nanomaterials from the reaction mixture is a tough issue because of their small dimension. The conventional techniques such as filtration are time-consuming and labor-intensive as well as not efficient. YFeO₃ as a magnetic material can be recovered with an external magnet. More importantly, it can be reused after recovery. Fig. 4 compares the recovery rate of YFeO₃ with that of P25. More than 70% of YFeO₃ photocatalysts can be recovered by using a bulky magnet while the recovery rate of P25 is very low, less than 13% because of its poor magnetization ability. Therefore, YFeO₃ is one kind of superior photocatalyst and can be efficiently recovered using an external magnet in the practical application to realize the green catalysis.

Fig. 4 Recovery rate of the as-prepared YFeO₃ and P25 TiO₂ photocatalysts in various cycles.

4. Conclusions

We synthesized single phase nanocrystalline YFeO₃ using Fe(NO₃)₃·9H₂O and Y(NO₃)₃·6H₂O as starting materials by a facile microwave-assisted approach. XRD and SEM demonstrate successful synthesis of single phase perovskite YFeO₃ and an average grain size of 60–70 nm. The homogenous reaction between soluble precursors is proposed to facilitate the formation of single phase perovskite YFeO₃ in the present work. The prepared nanocrystalline YFeO₃ shows strong visible-light absorption with the absorption onset of 511 nm. Further photocatalytic experiment for the decomposition of RhB shows the superior visible-light photocatalytic activity. More importantly, the ferromagnetic behavior enables the magnetic recovery of YFeO₃ in the practical application, and realizes the green catalysis.

Acknowledgements

This work was supported by the Zhejiang Province Natural Science Foundation of China (No. Y4100471), Zhejiang Province Education Department Project of China (No. Y200903866), and Science and Technology Planning Project of Zhejiang Province (No. 2008F70042)

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