Novel hybrid Sr-doped TiO 2 /magnetic Ni 0.6 Zn 0.4 Fe 2 O 4 for enhanced separation and photodegradation of organics under visible light

Titanium dioxide (TiO 2 ) has been intensively used as a photocatalyst for the degradation of organic pollutants in water, but is typically obstacle by a low e ﬃ ciency, costly separation, limited visible light response, and poor recyclability. Herein, we provided a reliable method to simultaneously tackle these four obstacles by developing an integrated and multifunctional hybrid photocatalyst/magnetic material, i.e. , Sr – TiO 2 /Ni 0.6 Zn 0.4 Fe 2 O 4 . This novel hybrid not only demonstrated a high e ﬃ ciency (90 – 100%) and a good cycling performance (90% maintenance) for photodegradation of bisphenol A (BPA) under both UV and visible light irradiation, but it can also e ﬃ ciently work at a wide pH range (4 – 10) and can be easily separated from water for reuse only by introducing an external magnetic ﬁ eld. The materials structure-to-activity correlation has also been investigated. It was found that doping Sr 2+ and a coupling magnetic material with TiO 2 could extend the visible light response and create active defects in TiO 2 , which were responsible for the nearly three times higher activity than that of commercial TiO 2 (P25) under visible light. On the other hand, doping excessive Sr 2+ lowered the surface area, enlarged the crystalline size and caused particle aggregation; thus, leading to a decrease in photocatalytic activity of the hybrid. These further modi ﬁ cations in the hybrid materials can provide a competitive alternative to control the organic pollutants in waste water.


Introduction
Photodegradation of organic pollutants using photocatalysts (e.g., ZnO, TiO 2 , WO 3 , CdS) holds a great promise to purify water and has been investigated intensively over the last decade. [1][2][3] Among the photocatalysts, TiO 2 has received great interest because of its chemical stability, high photocorrosion resistance, non-toxicity, and low-cost. [3][4][5][6][7][8] However, bare TiO 2 has a very low quantum efficiency primarily because of fast recombination rates of electron and hole pair (e À -h + ) and limited visible light responses (due to its wide band gap in the range of 3.0-3.2 eV). 1,8 The low efficiency of TiO 2 is a major barrier for commercializing this photocatalytic technology. Complicated separation of photocatalyst for regeneration and reuse may also impede its practical application, but this challenge is seldom addressed in the literature. [9][10][11] Generally, a TiO 2 -based power was suspended in a solution for the photodegradation of organic pollutants, and it oen required a ltration or centrifugation process to separate the photocatalyst from water. This process may increase the cost and brings a potential genotoxicity due to some residual organics adsorbed on the solids. 2,12 Therefore, it is urgent and necessary to develop high performance and easily recycled TiO 2 -based photocatalysts.
To overcome the obstacle of TiO 2 powder separation from solution, several efforts have been attempted to anchor TiO 2 on solid substrates (e.g., Ti mesh) 4,11,[29][30][31][32][33] or incorporate TiO 2 with magnetic materials (e.g., ferrite). [34][35][36][37][38][39][40] Immobilizing TiO 2 on solid substrates is restricted to laboratory or small-scale applications because of the complicated preparation process, weak attachment of TiO 2 on the foreign substrates, and decient immersion and dispersion of TiO 2 in the slurry solution. On the other hand, magnetic separation is a promising route to recover the used photocatalysts only by applying an external magnetic eld. A number of studies have been attempted to fabricate core-shell structured magnetic material/photocatalysts for the photodegradation of organics, including Fe/TiO 2 /Ag, Fe/V/TiO 2 , nickel ferrite/N-TiO 2 , and strontium ferrite/N-TiO 2 . 9,34,37,40 Unfortunately, the nanosized core magnetic materials are easily oxidized (e.g., Fe 3 O 4 oxidized to Fe 2 O 3 ) or rapidly transform in the crystal phase (e.g., g-Fe 2 O 3 ferromagnetic to a-Fe 2 O 3 paramagnetic) when the calcination temperature is over 400 C. 34,40 Obviously, it is still a challenge to produce the TiO 2 -coated magnetic nanoparticles with concurrent good stability, high visible light activity and good magnetic properties.
To simultaneously overcome the challenges in the catalytic efficiency and separation of TiO 2 -based photocatalysts, in this work, we designed a novel hybrid by integrating a photocatalyst with magnetic material, i.e., Sr-TiO 2 /Ni 0.6 Zn 0.4 Fe 2 O 4 . We hypothesize that (1) doping TiO 2 with Sr 2+ could create some active defects and promote visible light absorption, and thus enhancing the charge separation and extending visible light response, (2) introducing Zn into NiFe 2 O 4 could prevent the phase transition and assure good magnetic properties, 41,42 and using Ni 0.6 Zn 0.4 Fe 2 O 4 as magnetic core not only harvests visible light but also facilitates photocatalyst separation, and (3) an interface may be formed between Sr 2+ -doped TiO 2 and magnetic Ni 0.6 Zn 0.4 Fe 2 O 4 , and thus inducing a synergistic effect to remove organics with an exceptional performance. To the best of our knowledge, for the rst time this multifunctional material for water treatment has been developed. All the materials used (Ti, Sr, Ni, Zn, and Fe) to build the nanostructure are inexpensive and earth-abundant. The sol-gel method employed to prepare the hybrid is also simple and easy to scale-up. Another feature in this work is to evaluate the cycling performance of the hybrid photocatalyst/magnetic material under both UV and visible light irradiation, and attempt to establish the structure-to-activity relationships.  3 $9H 2 O) were dispersed into 100 ml deionized water with vigorous stirring. Citric acid (0.5 mol) was then added into the solution, using ammonium hydroxide to adjust the pH around 10.0. Next, the solution was heated in a water bath at 70 C by microwave irradiation until the sol was formed. The as-received sol was nally dried in an oven at 100 C for 24 h.

Synthesis of hybrid Sr-TiO 2 /magnetic material
The hybrid Sr-TiO 2 /magnetic materials were prepared by a solgel method. Typically, in a beaker A, 10 ml of tetrabutyl titanate (C 16 H 36 O 4 Ti), 1 g of Ni 0.6 Zn 0.4 Fe 2 O 4 nanoparticles, and 40 ml of absolute ethyl alcohol were uniformly mixed. In a beaker B, a certain amount of strontium nitrate (Sr(NO 3 ) 2 ) and starch (the mass ratio of Sr(NO 3 ) 2 to starch is 0.02 : 0.5, 0.05 : 1, 0.1 : 1.5, 0.2 : 2.5), 5 ml de-ionized water, and 15 ml acetic acid were mixed. The mixture was heated for 5 min by microwave irradiation to get a homogeneous solution. Next, the solution in the beaker B was added dropwise to beaker A with mechanical stirring for 2 h. The mixed solution was aged at 70 C for 24 h. The obtained gel was then dried and calcined at 550 C in air for 4 h. The nal hybrid materials were denoted as Sr x Ti/M (M ¼ Ni 0.6 Zn 0.4 Fe 2 O 4 ), where x is the nominal weight percentage of Sr 2+ (i.e., 0.1 wt%, 0.25 wt%, 0.5 wt%, and 1 wt%). For comparison, Ti/M and Sr 0.25 Ti were also prepared using a same procedure as SrTi/M.

Characterization
The crystal structure of Sr-TiO 2 /Ni 0.6 Zn 0.4 Fe 2 O 4 materials were characterized by X-ray diffraction (XRD) using a Bruker D8 Advance X-ray diffractometer with monochromated highintensity CuKa radiation (l ¼ 0.15418 A) in the 2q range of 10-80 . The morphology and particle size of the materials were identied by eld emission scanning electron microscopy (FESEM). Transmission electron microscopy (TEM) (JEOL TEM-3010) was used to approach the lattice fringes operating at an accelerating voltage of 300 keV. The surface area and porosity of the as-synthesized materials were examined by N 2 adsorption/ desorption at 77 K using the Brunauer-Emmett-Teller (BET) method (Micromeritics, ASAP 2020). The UV-vis diffuse reectance spectra were recorded using a UV-vis-NIR spectrometer (Cary 5000, Varian). The valence states of Ti and O were iden-tied by X-ray photoelectron spectroscopy (XPS), using a PHI 5000 versaprobe system using monochromatic Al KR radiation (1486.6 eV). All binding energies were referenced to the C 1s peak at 284.6 eV. The magnetic properties of the photocatalyst were evaluated at room temperature using a vibrating sample magnetometer (VSM, 9600-1 LDJ, USA) at a maximum applied eld of 10 kOe.

Photocatalytic activity measurement
The photocatalytic degradation of BPA was carried out in a hollow cylindrical photoreactor. The light source is a lowpressure mercury lamp (Beijing Ceaulight Co., Model CEL-LUV254, 10 W) that emits principally near 254 nm, or a longarc xenon lamp (Beijing Ceaulight Co., Model CEL-LAX500, 500 W) to simulate visible light (>400 nm). The photoreactor was cooled by circulating water through a quartz channel inside, and the temperature was maintained at around 25 AE 2 C. Prior to illumination, a 300 ml suspension with a certain amount of photocatalyst (0.5 g l À1 ) was stirred for 30 min to ensure the homogenous dispersion and full contact of BPA (10 ppm) with catalysts. The pH of the BPA solution was controlled using a NaOH solution (5 M, 1 M, 0.1 M) and an HNO 3 solution (2 M. 0.1 M) to prepare basic and acidic solutions, respectively. Aer that, the lamp was turned on to irradiate the solution. A sample of 0.5 ml of solution was withdrawn at every 30 min. Moreover, the pH of the BPA solution was recorded at different reaction times using a pH meter (Mettler Toledo). The BPA concentration in each batch was measured by highperformance liquid chromatography (HPLC) equipped with a C-18 column (LUNA 5u 100A, 4.6 mm Â 250 mm, Phenomenex) and a diode array detector (SPD-M20A, Shimadzu). The isocratic methanol-water mixture (70 : 30, v/v) as an eluent was employed at a ow rate of 1 ml min À1 . The residual BPA content in the aqueous solution was determined with a standard curve (R 2 ¼ 0.9999) using a standard BPA solution for the calibration (i.e., 0, 0.2, 0.5, 1, 2, 5, 10 and 15 ppm). The degradation efficiency (DE) of BPA was calculated by the following equation: DE ¼ C t /C o , where C o and C t are the initial and residual BPA concentration (ppm) at different reaction times, respectively.

Crystal structure, morphology and texture
The crystal structure of the hybrid was identied by XRD. As shown in Fig. 1, all the samples displayed the same diffraction peaks, which are indexed to a mixture of the anatase phase of TiO 2 (JCPDS no. 21-1272) and the spinel phase of Ni 0.6 Zn 0.4 Fe 2 O 4 . 43,44 No additional peaks for perovskite SrTiO 3 or SrO crystallites appeared because of the low calcination temperature and low dopant concentration of Sr 2+ . 23,27 Noticeably, Ti/M without Sr 2+ dopant exhibited sharp diffraction peaks. By contrast, Sr x Ti/M showed broadened and weakened diffraction peaks for both the TiO 2 (101) plane and magnetic Ni 0.6 Zn 0.4 Fe 2 O 4. This comparison indicated that doping Sr 2+ ions into the TiO 2 lattice could stabilize the crystal phase of TiO 2 anatase and inhibit the aggregation and growth of the particles, thus inducing a decrease in TiO 2 crystalline size. Among the samples, Sr 0.1 Ti/M showed the smallest crystalline size (see Table 1). Increasing the concentration of Sr 2+ ions resulted in an increase in the TiO 2 crystalline size from 15.7 nm to 18.8 nm, probably because the excessive Sr 2+ ions cannot be completely doped into the TiO 2 lattice and caused particle agglomeration. Moreover, the uniformly dispersed Sr-TiO 2 particles may fully attach on the surface of Ni 0.6 Zn 0.4 Fe 2 O 4 and result in a decrease in its peak intensity.
The morphology and particle size of the Sr x Ti/M samples were characterized by SEM (Fig. 2) and TEM (Fig. 3). As shown in Fig. 2, Sr 0.1 Ti/M and Sr 0.2 Ti/M have uniformly dispersed particles with an average size of 20 nm. By contrast, Sr 1.0 Ti/M has randomly mixed small particles and big grains (over 100 nm). The TEM image at a low magnication in Fig. 3a further conrmed that Sr 0.1 Ti/M was composed of relatively uniform spherical or rectangle particles with a diameter of $20 nm. The TEM image at a high magnication in Fig. 3b clearly shows wellfaceted TiO 2 nanocrystals with an interplanar spacing of 0.352 nm that matches the (101) plane of anatase phase. 5,45,46 Ni 0.6 Zn 0.4 Fe 2 O 4 nanoparticles ($15 nm), located beyond TiO 2 particles, was also observed with a lattice spacing of 0.187 nm. 47 Again, SEM and TEM images directly supported the XRD observations that a small amount of Sr 2+ (below 0.25 wt%) doped TiO 2 particles were well-patched onto the magnetic Ni 0.6 Zn 0.4 Fe 2 O 4 , while excessive Sr 2+ dopant easily induced particles aggregation into large grains. Table 1 also compared the measured BET surface area, pore size, pore volume of the Sr x Ti/M samples to explore the effect of Sr 2+ -dopant content on the textural property. The presence of mesopores in Sr x Ti/M could result from the space between particles, as evidenced by the SEM images in Fig. 2. At low loadings of Sr 2+ , Sr 0.1 Ti/M and Sr 0.25 Ti/M displayed similar surface areas, pore sizes, and pore volumes, which were  considerably higher than those of Sr 0.5 Ti/M and Sr 1.0 Ti/M at high loadings of Sr 2+ . The data in Table 1 agrees well with XRD and SEM observations that high contents of Sr 2+ results in the aggregation of the particles and the growth in crystalline size, which could block the micro/meso-pores within TiO 2 particles and decrease the BET surface area, pore size, and pore volume. XPS has been conducted to approach the chemical states of TiO 2 with and without doping Sr. As shown in Fig. 5a, Ti/M displayed Ti 2p 3/2 and the Ti 2P 1/2 binding energies at 458.6 and 464.2 eV, respectively, corresponding to a typical characteristic of the Ti 4+ oxidation state. 16,40 Interestingly, the Ti 2p binding energies of Sr 0.1 Ti/M slightly shied to a lower level (by 0.4 eV). Similarly, doping Sr 2+ into TiO 2 also led to a chemical shi in the O 1s binding energy at 529.9 eV that is associated with lattice oxygen (O l ) bonded to metal ions, while the O 1s related to surface oxygenated species (O a ) (e.g., OH groups, adsorbed H 2 O) remained at a same position at 531.5 eV. 45 Moreover, the relative amount of O l (by calculating the peak area ratio of O l /(O l + O a ) in Fig. 5b) in Sr 0.01 Ti/M (78.6%) was slightly higher than that in Ti/M (74.8%). Generally, the chemical shis in XPS spectra resulted from either the formation of a new oxidation state or the changes in the local chemical and physical environment. Because no shoulder peak for the new state of Ti 3+ was observed in Sr 0.1 Ti/M, the slight chemical shi is primarily related to a change in the local chemical environment of TiO 2 . In other words, Sr 2+ has been successfully doped into TiO 2 lattice. Because some Ti 4+ sites have been substituted by Sr 2+ ions, some defects (e.g., oxygen vacancy, V O ) were formed for charge compensation. 16,29 This is also supported by the XPS quantitative analysis that Ti/M has a higher ratio of O/(Ti + Zn + Ni + Fe) (2.61) than does Sr 0.1 Ti/M (2.02). Hence, the change in the chemical environment of TiO 2 was possibly caused by the partial replacement of Ti 4+ by Sr 2+ dopants and the formation of V O in TiO 2 .  Table 1, Fig. 1  and 2), the inferior activity at a high content of Sr 2+ is probably   because Sr 0.5 Ti/M and Sr 1.0 Ti/M have larger crystalline sizes, smaller specic surface areas and pore volumes, and less uniform particle dispersions. These drawbacks on Sr 0.5 Ti/M and Sr 1.0 Ti/M could hinder the adsorption and activation of BPA on the surface. Excessive Sr 2+ may also induce the formation of a new e À -h + recombination center that shortened the lifetime of photogenerated charge carriers. 28 Another important result in Fig. 6 is that Sr 0.1 Ti/M and Sr 0.25 Ti/M displayed a considerably higher efficiency compared to Ti/M, Sr 0.25 Ti, commercial TiO 2 (P25), and bare M. Sr 0.1 Ti/M is able to eliminate nearly 100% BPA at 4 h, while TiO 2 (P25) and bare M can only reach 40% and 25%, respectively. In addition, Sr 0.25 Ti and Ti/M was even more active than bare TiO 2 (P25) and M. In this regard, it is believed that there is a synergy between Sr 2+ dopant and magnetic M that remarkably prevents the charge recombination in TiO 2 , as a consequence of leading to a superior activity of Sr 0.1 Ti/M.

Photocatalytic performance measurement
We also performed photocatalytic degradation of BPA at different pH conditions using a Sr 0.25 Ti/M composite under UV irradiation because the waste water may be acidic or basic. The initial pH of the reactant solution was controlled by adding a desired amount of NaOH or HNO 3 . Fig. 7a compares the photocatalytic performance of Sr 0.25 Ti/M in a pH range of 4-10. It was found that either at basic (pH ¼ 10) or acidic conditions (pH ¼ 4), Sr 0.25 Ti/M displayed a higher efficiency that at near neutral conditions (pH ¼ 6-8) within the beginning 2 h, while the efficiency eventually remained almost the same at the later stage and reached as high as nearly 99% at 3.5 h in the entire pH range (4)(5)(6)(7)(8)(9)(10). This result strongly suggested that our novel hybrid Sr x Ti/M is able to efficiently work at a wide pH range. Moreover, the pH evolution during the photocatalytic reaction was also monitored, as shown in Fig. 7b. At a basic condition, the pH gradually decreased and reached a steady state to near neutral, while at an acidic condition, the initial pH gradually increased. If starting with a near neutral solution, the pH only slowly decreased. The evolution of solution pH revealed that both proton (H + ) and OHc radicals play a critical role in the photodegradation of BPA. A possible mechanism has been proposed to explain the pH impacts, as shown in Fig. 8. The starting solution with a high pH contains more OH À anions, which could act as hole (h + ) scavengers and react with photogenerated h + to form active OHc radicals (Fig. 8a). 16,27 Hence, the consumption of OH À led to the decrease in pH over the reaction time. The OHc radicals have a strong oxidation ability to degrade BPA to CO 2 and H 2 O, which could explain why the activity is higher at pH ¼ 10. On the other hand, the starting solution with a low pH contains more H + ions, which serves as electron scavengers and inhibits the recombination of e À -h + pairs (Fig. 8b). Moreover, the Hc radicals, generated from H + + e À / Hc, could react with O 2 to form hydrogen peroxide that decomposes into oxidative OHc under UV irradiation. 48 Thus, the suppressed recombination and the formation of OHc leads to an increase in pH over time and a higher efficiency than at neutral pH values.
The photocatalytic performance of the Sr x Ti/M samples was also evaluated under visible light (400-1000 nm) irradiation, as shown in Fig. 9. The order of activity of the hybrids was Sr 0.1 Ti/   15-20%). In addition to the larger surface area, smaller crystalline size, and more uniform particle dispersion, the superior performance of Sr 0.1 Ti/M under visible light is probably also due to another two reasons. First, incorporating an appropriate amount of Sr 2+ dopant and magnetic M with TiO 2 narrowed its band gap (see UV-vis spectra in Fig. 4); thus, enhancing the harvest and utilization of visible light. Second, as evidenced by XPS results, doping Sr 2+ into TiO 2 also created some active defect sites (oxygen vacancies), which may induce the formation of the new energy state located below the conduction band minimum of TiO 2 . The active defect sites could facilitate charge separation and trap electrons even under visible light. 16,28,49 The electrons accumulated at defect sites could easily attach on O 2 molecules to produce powerful superoxide radicals (cO 2À ) that promote the activation and oxidation of BPA molecules.

Material separation and cycling performance
The above photocatalytic activity results indicated that among all the tested samples, Sr 0.1 Ti/M has an outstanding performance under both UV and visible light irradiation. To further understand its superiority, we measured the cycling performance of Sr 0.1 Ti/M during three runs of photodegradation alternations. In between each cycle, the spent Sr 0.1 Ti/M was separated only by adding an external magnetic eld around the solution and washed with water without any high temperature treatment or centrifugation. Above all, we measured the magnetic property of Sr 0.1 Ti/M at room temperature using vibrating magnetometer. Fig. 10     encapsulated with nonmagnetic Sr 0.1 TiO 2 nanoparticles. Although Sr 0.1 Ti/M has a relatively low M s , it still can be easily magnetically separated for reuse. As evidenced by the inset photograph in Fig. 10, the suspended solution quickly became clear once placing a magnet near the bottle wall for 30 s, and the powder was accumulated and attached on the wall. Fig. 11 compares the photodegradation efficiency of Sr 0.1 Ti/ M in each cycle under both UV and visible light irradiation aer 4 h. Aer three cycles, the efficiency in the 3 rd run only slightly dropped at both conditions, but still maintained as high as 89% and 78% under UV and visible light irradiation, respectively. Obviously, Sr 0.1 Ti/M demonstrated a good stability and recyclability, where M could provide a magnetic eld for separation and harvest visible light concurrently. This good cycling performance also conrms our original hypothesis that integrating metal ion doped photocatalysts (e.g., Sr-TiO 2 ) and magnetic materials (e.g., Ni 0.6 Zn 0.4 Fe 2 O 4 ) is a reliable and convenient method to simultaneously enhance material separation and visible light activity for water purication.

Conclusions
In this work, we designed a novel hybrid photocatalyst/ magnetic material, i.e., Sr 2+ doped TiO 2 /Ni 0.6 Zn 0.4 Fe 2 O 4 , in order to enhance the efficiency of photo-degrading organic pollutants under visible light and to easily separate and reuse the material. The hybrid was synthesized by a simple sol-gel method. We found that doping a low concentration of Sr 2+ (below 0.25 wt%) induced a smaller crystalline size, larger surface area and pore volume, and more uniform particle dispersion than did loading a high concentration of Sr 2+ . Moreover, doping Sr 2+ ions and coupling a magnetic material with TiO 2 narrowed the band gap and induced the generation of defect sites in TiO 2 . As a result, the integrated hybrid with a low loading of Sr 2+ not only demonstrated a high efficiency (over 90%) and a good recycling performance (90% maintenance) under both UV and visible light irradiation, but also can efficiently work at a wide pH range (4-10) and be easily separated only by adding an external magnetic eld. Furthermore, the hybrid Sr-TiO 2 /Ni 0.6 Zn 0.4 Fe 2 O 4 showed an over two-times higher activity than those of TiO 2 /Ni 0.6 Zn 0.4 Fe 2 O 4 , commercial TiO 2 (P25) and bare Ni 0.6 Zn 0.4 Fe 2 O 4 , as well as 50% higher activity than that of Sr-doped TiO 2 , indicating that there is a synergy between the doped photocatalyst and magnetic material. The ndings in this work suggest a new direction to engineer a smart photocatalyst/magnetic material heterojunction and control the interface between them, and it sheds light on the material's application in other aqueous-solid phase photocatalytic reactions.