Fe3O4@SiO2@TiO2@Pt Hierarchical Core–Shell Microspheres

Article pubs.acs.org/crystal

Fe3O4@SiO2@TiO2@Pt Hierarchical Core−Shell Microspheres: Controlled Synthesis, Enhanced Degradation System, and Rapid Magnetic Separation to Recycle Xiyan Li, Dapeng Liu, Shuyan Song,* and Hongjie Zhang* State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun, 130022, P. R. China S Supporting Information *

ABSTRACT: Magnetic composite microspheres consisting of a SiO2-coated Fe3O4 core, an ordered TiO2 hierarchically structured shell, and a Pt nanoparticle layer dispersed on the surface of the TiO2 nanoplatelets have been successfully synthesized using a facile and efficient method. The shells of TiO2 hierarchical microspheres were assembled from nanoplatelets, which exposed the high-energy {001} facets, and the Pt nanoparticles were evenly deposited on the surface of the TiO2 nanoplatelets, with a concentration of ∼1 wt %. The resulting composite microspheres exhibited flower-like hierarchical structures with a 202.42 m2 g−1 surface area and possessed superparamagnetic properties with a high saturation magnetization of 31.5 emu g−1. These features endow the obtained composite microspheres with a high adsorption capacity and strong magnetic responsivity that could be easily separated by an external magnetic field. The high photocatalytic activity toward Rhodamine B (RhB) degradation may be caused by the hierarchically structured TiO2 with exposed high-energy {001} facets and the Pt nanoparticle deposits on TiO2 surfaces, which would be efficient for the electron transfer reactions. In addition, the composite microspheres showed high recycling efficiency and stability over several separation cycles.

1. INTRODUCTION Photocatalytic degradation of organic contaminants in wastewater is one of the promising techniques in handling the increasing amount of river pollutants. Among the many investigated photocatalysts, TiO2 is considered to be the most suitable candidate for commercial scale-up because of its excellent photocatalytic activity, low-cost, and stability.1−6 Since the first report on the use of TiO2 nanostructures as photocatalysts by Fujishima and Honda,7 intensive studies have been encouraged to improve their photocatalytic efficiency.4,5,8 One attractive approach to enhancing the photocatalytic performance of TiO2 nanostructures is to tune their physical and chemical properties determined by crystal phase, particle size, and surface structures.9−11 The synthesis of {001} facet-terminated anatase TiO2 nanocrystals is most beneficial for enhancing their photocatalytic reactivity because of their high average surface energies: the average surface energies of {001} facets is higher (0.90 J m−2) than those of {100} (0.53 J m−2) and {101} (0.44 J m−2) facets.8,10−12 However, most of the available TiO2 nanostructures are terminated by thermodynamically less-reactive {101} surfaces.13,14 Since the synthesis of anatase TiO2 sheets with exposed {001} facets was successfully reported by Lu and Qiao et al.,15−17 attention has been paid to expose the reactive {001} facets to further improve their photocatalytic activity. Although anatase TiO2 nanocrystals completely enclosed with large specific surface area {001} facets have been synthesized and reported by several groups,5,8,18−20 only their electrochemical © 2014 American Chemical Society

energy storage abilities were investigated, with no mention of photocatalytic performance.21 Combining photocatalysts with magnetic nanoparticles will provide a way to separate and recycle catalysts.6,22 The functionalization of magnetic nanoparticles (such as Fe3O4) with semiconducting nanocrystals shells, such as TiO2, is the most accepted method for synthesizing recyclable photocatalysts. The assembled TiO2-based magnetic catalysts inherited the features of both magnetic components and TiO2 nanostructures, generating high photocatalytic activity and improving separation capability.23−25 However, assembling TiO2 with highly active exposed facets (particularly {001} facets) on the surface of magnetic nanoparticles remains a challenge and has not yet been reported. Herein, we demonstrate a facile and efficient synthetic method for the preparation of magnetic composite microspheres (Fe3O4@SiO2@TiO2@Pt, ∼420 nm), which are composed of a silica-coated magnetite core, an ordered hierarchical TiO2 shell structure, and a Pt nanoparticle layer dispersed on the surface of the TiO2 nanoplatelets. A SiO2 layer between the magnetic core and the TiO2 shell was introduced to prevent the electrons from being trapped by the magnetic core, which usually serves as an electron−hole recombination center.26,27 As anticipated, these final obtained composite Received: May 8, 2014 Revised: October 7, 2014 Published: October 9, 2014 5506

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source, situated 6 cm above the reaction solution. The reaction solution containing different catalysts (50 mg) and RhB (10 mg L−1) was first stirred in the dark for approximately 30 min to obtain the adsorption/desorption equilibrium of the RhB on the surface of the catalysts. The light density was estimated to be 1.55 mW cm−1. The concentration variation of RhB during degradation was monitored by UV−visible absorbance spectra. For the recycling experiments, the catalysts of Fe3O4@SiO2@TiO2@Pt microspheres were first separated from the reaction solution by an external magnet. When the clear upper solution was discarded and replaced with fresh RhB solution (50 mL, 10 mg L−1), the collected Fe3O4@SiO2@TiO2@Pt microspheres were redispersed into the solution by slight shaking. The irradiation time was 20 min for each experiment cycle to allow for the photodegradation of the RhB solution.

microspheres not only showed high photocatalytic activity toward the degradation of RhB but also could be efficiently separated from the reaction solution by using external magnetic fields without a significant loss of photocatalytic activity.

2. EXPERIMENTAL SECTION Synthesis of Fe 3 O 4 Microspheres. The magnetic Fe3 O 4 microspheres were prepared using a solvothermal method reported in previous studies.23,24 Typically, FeCl3 (0.65 g, 4 mmol) was first dissolved in ethylene glycol (20 mL) by stirring; trisodium citrate (0.25 g, 0.85 mmol) and sodium acetate (1.0 g, 12.2 mmol) were then added with magnetic stirring. The obtained yellow solution was transferred into a 50 mL Teflon-lined stainless steel autoclave and maintained at 200 °C for 10 h. Finally, the black products were washed with ethanol and deionized water three times after cooling to room temperature. Synthesis of Fe3O4@SiO2 Microspheres. The Fe3O4@SiO2 core−shell structures were prepared using the Stöber method. Briefly, 0.10 g of Fe3O4 microspheres was first added into a mixture of ethanol (80 mL) and deionized water (20 mL); 2 mL of a concentrated ammonia aqueous solution (28 wt %) was then added by stirring. After adding 100 μL of tetraethyl orthosilicate (TEOS), the mixture was stirred at room temperature for 6 h. Finally, the Fe3O4@SiO2 microspheres were obtained, separated by a magnet, and washed three times with ethanol and deionized water before drying at 60 °C for 6 h. Synthesis of Fe3O4@SiO2@TiO2 Microspheres. Specifically, 0.1 g of Fe3O4@SiO2 microspheres and 0.03 mL of diethylenetriamine (DETA) were first added in 41.7 mL of isopropanol. Then, titanium isopropoxide (1.8 mL) was added to the mixture solution while gently shaking by hand. The reaction mixture was transferred into a 60 mL Teflon-lined stainless steel autoclave and kept at 200 °C for 24 h. The obtained gray products were separated by a magnet and washed thoroughly with ethanol and deionized water before drying at 60 °C. Subsequently, the powder was calcined at 350 °C in air for 2 h to improve the crystallinity of the obtained Fe3O4@SiO2@TiO2 microspheres. Synthesis of Fe 3O4@SiO2@TiO2@Pt Microspheres. The Fe3O4@SiO2@TiO2@Pt microspheres were prepared by mixing Fe3O4@SiO2@TiO2 composites with preprepared Pt nanoparticles and stirring overnight before separating and thoroughly washing with ethanol and deionized water. The Pt nanoparticles were synthesized using a facile ultrasound-treated method. Typically, 0.1 mmol H2PtCl6 was mixed with 0.5 mmol of PVP in the presence of 15 mL of ethylene glycol and 5 mL of water, followed by ultrasound treatment for 10 min using an ultrasonic cell crusher. The as-obtained products were purified by acetone, washed three times with water, and then redissolved in 10 mL of water. Characterization. X-ray diffraction (XRD) patterns data were recorded on a Rigaku-D/max 2500 V X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å). The operating voltage and current were kept at 40 kV and 40 mA, respectively. Scanning electron microscope (SEM) images and energy-dispersive X-ray (EDX) spectra were acquired using a Hitachi S4800 microscope. The elemental compositions of the products were analyzed via EDX spectroscopy using the SEM instrument. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were collected using a TECNAI G2 high-resolution transmission electron microscope at a 200 kV operating voltage. N2 adsorption/desorption isotherms were measured using a Micromeritics ASAP 2020 Analyzer (USA). Pore sizes were calculated using the Barrett, Joyner, and Halenda (BJH) method. Magnetic measurement data of the as-prepared products were obtained using a Magnetic Property Measurement System (MPMS XL-7). UV−visible absorption spectra were acquired with a Cary 500 scan 117 UV/vis/NIR spectrophotometer (Varian, Harbor City, CA). Photocatalytic Degradation of RhB. The photocatalytic degradation of RhB was conducted in a 50 mL aqueous solution contained in a beaker agitated by a magnetic stirrer. A 15 W Xe lamp (254 nm, Spectroline XX-15G, USA) was used as the UV radiation

3. RESULTS AND DISCUSSION The entire growth process of the composite microspheres is shown in Scheme 1. Fe3O4 microspheres were first prepared Scheme 1. Schematic Illustration of the Synthesis of Fe3O4@ SiO2@TiO2@Pt Microspheres

using the solvothermal method and subsequently coated with a silica shell via hydrolysis of TEOS in accordance with the classical Stöber approach.28,29 As shown in Figure 1, the Fe3O4

Figure 1. SEM images of (A) Fe3O4 microspheres, (B) Fe3O4@SiO2, and (C, D) Fe3O4@SiO2@TiO2 microspheres.

and Fe3O4@SiO2 microparticles exhibited a spherical morphology, with average diameters of ∼250 nm (Figure 1A) and ∼260 nm (Figure 1B), respectively. After an in situ growth strategy, a shell of hierarchical TiO2 nanoplatelets was constructed on the surface of Fe3O4@SiO2 microspheres (Figure 1C,D). The TEM image of the as-prepared Fe3O4 shown in Figure 2A further supported the SEM results. Judging from the HRTEM image of Figure S1 (Supporting Information), the Fe3O4 microspheres were assembled by small nanocrystals, with sizes ranging from 5 to 10 nm. Using the classical Stöber method, the magnetite microspheres were coated with a silica layer with a thickness of ∼10 nm (inset of Figure 2B). After a further coating process, the Fe3O4@SiO2@TiO2 composites were formed and calcined at 350 °C in air for 2 h to obtain highly crystalline TiO2. The TEM image of the Fe3O4@SiO2@TiO2 microspheres (Figure 5507

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signal (∼1 wt %) was also observed in the spectrum, which indicated the optimum contents of the Pt nanoparticles in the photocatalytic reaction.30 ICP analysis of Fe3O4@SiO2@ TiO2@Pt further confirmed that the content of Pt in the sample was approximately 0.921 wt %. The XRD patterns of Fe3O4 microspheres and Fe3O4@ SiO2@TiO2@Pt composites are recorded in Figure 3A. All of

Figure 3. (A) XRD patterns of (a) Fe3O4 and (b) Fe3O4@SiO2@ TiO2@Pt microspheres. (B) N2 sorption isotherms and pore size distribution (inset) of Fe3O4@SiO2@TiO2@Pt microspheres.

the diffraction peaks were indexed to the face-centered cubic (fcc) phase of Fe3O4 (curve a, JCPDS card 19-629). In the XRD pattern of the Fe3O4@SiO2@TiO2@Pt microspheres (curve b), except for the diffraction peaks ascribed to the Fe3O4 component, the identified peaks located at 2θ = 25, 38, 54 and 48° were attributed to anatase (101), (004), (200), and (105) diffractions, respectively (JCPDS card 21-1272). Because of its amorphous structure, no characteristic SiO2 crystal peaks were detected. Pt nanoparticles were not detected by the XRD pattern due to both their small size and low numbers, which is less than the resolution of the instrument (∼1.5%). The specific surface area and porosity of the as-prepared Fe3O4@SiO2@ TiO 2 @Pt microspheres were measured using nitrogen adsorption/desorption instruments. Figure 3B exhibits the N2 adsorption/desorption isotherms and the corresponding pore size distribution of Fe3O4@SiO2@TiO2@Pt microspheres. The specific surface area was calculated to be 202.42 m2 g−1 using the Brunauer−Emmett−Teller (BET) method. Such a large surface area obtained is beneficial for adsorption of RhB dyes and photocatalytic reactions. In addition, a typical IV isotherm with a H3-type hysteresis loop (P/P0 > 0.4) in Figure 3B indicated the presence of mesoporous structures.31 The pore sizes are in the range of 5−15 nm, according to the narrow pore size distributions in the inset of Figure 3B. The magnetic properties of Fe3O4, Fe3O4@SiO2, Fe3O4@ SiO2@TiO2, and Fe3O4@SiO2@TiO2@Pt microspheres were investigated using a Magnetic Property Measurement System (MPMS XL-7). From the magnetic hysteresis curves of Figure 4A, it can be discerned that all four products showed

Figure 2. TEM images of (A) Fe3O4, (B) Fe3O4@SiO2, (C) Fe3O4@ SiO2@TiO2, and (D, E) Fe3O4@SiO2@TiO2@Pt microspheres. (F) EDX result of Fe3O4@SiO2@TiO2@Pt microspheres.

2C) illustrated the flower-like morphologies with compact cores (∼260 nm) that were fully covered by the interlaced TiO2 nanoplatelets shells (∼80 nm). All images recorded from regions 1, 2, and 3 of the edge in Figure S2A (Supporting Information) exhibited the lattice fringe, with a spacing of 0.19 nm. Region 2 also indicated two sets of perpendicular lattice fringes with an equal interplanar spacing of 0.19 nm corresponding to the (020) and (200) planes of anatase. The fast Fourier transform (FFT) pattern (inset of Figure S2A) measured from the same region can be indexed to the [001] zone. All of the information suggested that the shell of the TiO2 nanoplatelets was bound by {001} facets.29 The formation of this unique hierarchical structure may be ascribed to the presence of tridentate DETA in the sol−gel process, which can prohibit the growth of amorphous TiO2 along the [001] direction and form the ultrathin nanoplatelets. These flexible ultrathin nanoplatelets self-assembled into hierarchical structures in the subsequent solvothermal process. Furthermore, the highly crystallized hierarchical structures were finally formed via high-temperature calcination.21 The TEM images of Figure 2D show the Fe3O4@SiO2@ TiO2@Pt composites after direct deposition of Pt nanoparticles on the surface of the TiO2 shell. As indicated, the Pt nanoparticles were primarily distributed on the edge and junction sites of the interlaced TiO2 nanoplatelets, with a mean diameter of ∼3.0 nm, which was expected to contribute to the high catalytic performance of the catalysts. The lattice spacing of 0.23 nm in Figure 2E corresponded to the (111) facets of face-centered cubic Pt nanoparticles. The chemical composition was confirmed by EDX and inductively coupled plasma (ICP) analysis. The EDX spectrum of the final Fe3O4@SiO2@TiO2@ Pt microspheres (Figure 2F) showed the presence of strong Fe, Ti, and O signals with a Fe/Ti molar ratio of ∼1.0. A weak Pt

Figure 4. (A) Magnetic hysteresis loops of the products at room temperature (300 K). (B) Magnetic separation and redispersion process of Fe3O4@SiO2@TiO2@Pt microspheres. 5508

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superparamagnetic properties, with no obvious remanence or coercivity at 300 K. The superparamagnetic property may be ascribed to the small nanocrystals in a range of 5−10 nm, which behaved as superparamagnets in Fe3O4 microsphere cores. Additionally, according to previously reported results,32 the citrate stabilizer added in the formation process of Fe3O4 microspheres may help decrease the coercivity by screening the dipolar interactions between the nanocrystals. The magnetization saturation values (Ms) of the Fe3O4 microspheres were first measured, and the value reached to 75.7 emu g−1. Upon coating of a 10 nm thick amorphous SiO2 layer, the Ms value for the Fe3O4@SiO2 microspheres was reduced to 65.2 emu g−1. In addition, after coating TiO2 nanoplatelets and Pt nanoparticles, their Ms values were further decreased to 47.0 and 31.5 emu g−1, respectively. However, the final Ms value of Fe3O4@SiO2@TiO2@Pt microspheres was sufficiently high to allow for efficient separation and purification. As shown in Figure 4B, with an external magnet, the homogeneous dispersion of the Fe3O4@SiO2@TiO2@Pt microspheres could be separated quickly from the solution and form aggregates in only 20 s. While removing the magnet, the aggregates were redispersed into the solution quickly by slightly shaking the beaker. Thus, it can be concluded that the composite microspheres possess excellent magnetic responsivity and redispersibility, which is beneficial for their practical manipulation. To test the potential performance of the magnetic composites toward the removal of contaminants from wastewater, the degradation of RhB was chosen as a model reaction. The change in absorption spectra of the RhB solution over time using Fe3O4@SiO2@TiO2 composites without Pt nanoparticles as photocatalysts is shown in Figure 5A. As indicated, the

Figure 6. (A) Photocatalytic activity of various types of catalysts and (B) repeated photocatalytic degradation of RhB over Fe3O4@SiO2@ TiO2@Pt with five cycles.

P25 with an identical total amount of TiO2 were also used as photocatalysts. As expected, Fe3O4@SiO2 and Fe3O4@SiO2@ TiO2 before calcination showed almost no apparent photocatalytic activity under identical UV exposure times, whereas, for the Fe3O4@SiO2@TiO2 and Fe3O4@SiO2@TiO2@Pt composites, the RhB dyes were degraded over 50% in 10 min under the same UV irradiation. As shown in Figure 6A, these two composite catalysts showed much higher photocatalytic efficiency than did the well-known commercial Degussa P25 photocatalyst. Several factors for understanding the photocatalytic activity of the Fe3O4@SiO2@TiO2 and Fe3O4@SiO2@ TiO2@Pt composites have been considered. One major reason may be due to the formation of hierarchically structured TiO2 nanoplatelet exposed {001} facets. Normally, high-energy {001} facets are oxygen-deficient facets that are composed of unsaturated 5-fold Ti atoms and large Ti−O−Ti bond angles.15 The oxygen-deficient facets and their unique electronic structures could enhance photoinduced electron transfer and reduce the electrons/holes recombination rate.33,34 Additionally, the band structure of TiO2 and the Fermi level of Pt nanoparticles must be discussed to explain the higher photocatalytic activity of the Fe3O4@SiO2@TiO2@Pt composites relative to Fe3O4@SiO2@TiO2. It is well-known that Pt contacted with TiO2 surfaces would facilitate efficient electron transfer reactions.34 As a well-known n-type semiconductor, when TiO2 comes into contact with Pt nanoparticles, a band bending would be generated at the surface of the TiO2 due to electron transfer from TiO2 to Pt. This band bending induces Pt to arrest more transferred electrons from TiO2 to form a Schottky barrier.35 When irradiating Fe3O4@SiO2@TiO2@Pt composites with UV light, electrons in the valence band of TiO2 could be excited to the conductive band, thus leaving holes in the valence band to oxidize the organic reagents. Without the synergistic effect of Pt nanoparticles, the light would induce the recombination of excited electrons and holes and, therefore, reduce the photocatalytic activity of TiO2 hierarchical structures. In the presence of Pt with a lower Fermi level (−5.1 eV vs vacuum), the excited electrons in the conductive band of TiO2 (−4.3 eV vs vacuum) could overcome the Schottky barrier and then transfer to the Fermi level of Pt, leading to spatial charge separation (Scheme 2).36,37 Thus, Pt combined with TiO2 could suppress the recombination of electrons and holes effectively, consequently improving the photocatalytic activity. In addition, the SiO2 layer between the TiO2 outer layer and the Fe3O4 core could prohibit the transfer of excited electrons from TiO2 into the lower-lying conduction band of the Fe3O4 core, resulting in the elimination of possible photodissolution by Fe3O4 in the reaction process.38 As mentioned, photocatalysts with magnetic responsivity could endow them with advantages of recycling and stability

Figure 5. UV−visible absorption spectra of RhB over time with (A) Fe3O4@SiO2@TiO2 microspheres and (B) Fe3O4@SiO2@TiO2@Pt microspheres.

typical absorption peak at 553 nm gradually decreased upon increasing the UV exposure time and completely disappeared after 25 min, indicating the complete photodegradation of RhB by the Fe3O4@SiO2@TiO2 composite photocatalysts. However, when using Fe3O4@SiO2@TiO2@Pt microspheres as photocatalysts with an identical total amount of TiO2 with respect to the Fe3O4@SiO2@TiO2 microspheres, the absorption peak of RhB disappeared completely within 20 min (Figure 5B), which indicated that the introduction of Pt indeed increased the photocatalytic activity of Fe3O4@SiO2@TiO2 composites in the photolysis of RhB. The changes in the RhB concentration (C) during the photodegradation process in the presence of Fe3O4@SiO2@ TiO2 or Fe3O4@SiO2@TiO2@Pt were measured and are shown in Figure 6A. For comparison, Fe3O4@SiO2, Fe3O4@ SiO2@TiO2 before calcination, and the commercial Degussa 5509

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ACKNOWLEDGMENTS The authors are grateful for the financial aid from the National Natural Science Foundation of China (Grant Nos. 21371165, 51372242, 91122030, and 21210001) and the National Natural Science Foundation for Creative Research Group (Grant No. 21221061).

Scheme 2. Schematic Energy Structure between TiO2 and Pt of the Fe3O4@SiO2@TiO2@Pt Composites

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4. CONCLUSION In conclusion, we have demonstrated a facile and efficient method for the synthesis of magnetic composite microspheres containing a SiO2-coated Fe3O4 core, an ordered TiO2 hierarchically structured shell, and a Pt nanoparticle layer dispersed on the surface of the TiO2 nanoplatelets. The shells of TiO2 hierarchical microspheres were assembled from the nanoplatelets, which exposed the high-energy {001} facets, and the Pt nanoparticles were evenly deposited on the surface of the TiO2 nanoplatelets without any modification. The Fe3O4@ SiO2@TiO2 and Fe3O4@SiO2@TiO2@Pt composites showed higher photocatalytic activity in the degradation of RhB dyes than did the famous commercial Degussa P25. The higher photocatalytic activity may be due to the hierarchically structured TiO2 with exposed high-energy {001} facets and the beneficial charge separation due to the presence of Pt nanoparticles deposited on TiO2 surfaces. In addition, the composite microspheres showed high recycling efficiency and stability over several separation cycles. Therefore, this proposed innovative method opens new opportunities for the preparation of hierarchical composite structures combined with magnetic cores and functionalized TiO2/Pt shells, which have potential applications in the practical setting of photocatalysis. ASSOCIATED CONTENT

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AUTHOR INFORMATION

REFERENCES

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features in catalytic reactions by external magnetic fields. To study the recyclability and stability features of the Fe3O4@ SiO2@TiO2@Pt composites, a cyclic test was executed. After each recycling experiment, the Fe3 O4@SiO2@TiO2@Pt composites could be separated from the solution by a magnet. When the clear upper solution was discarded, the used Fe3O4@ SiO2@TiO2@Pt composites were redispersed in fresh RhB solution for another cycle. As shown in Figure 6B, it is obvious that, after five cycles, the Fe3O4@SiO2@TiO2@Pt catalysts maintain high photocatalytic efficiency without any loss of activity.

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S Supporting Information *

TEM images, XPS spectra, the zero field cooling (ZFC) and field cooling (FC) curves of the prepared samples. This material is available free of charge via the Internet at http:// pubs.acs.org. Corresponding Authors

* E-mail: [email protected]. Fax: +86-431-85698041 (S.S.). * E-mail: [email protected]. Fax: +86-431-85698041 (H.Z.). Notes

The authors declare no competing financial interest. 5510

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