Facile method to synthesize magnetic iron oxides/TiO2 hybrid nanoparticles and their photodegradation application of methylene blue

Many methods have been reported to improving the photocatalytic efficiency of organic pollutant and their reliable applications. In this work, we propose a facile pathway to prepare three different types of magnetic iron oxides/TiO2 hybrid nanoparticles (NPs) by seed-mediated method. The hybrid NPs are composed of spindle, hollow, and ultrafine iron oxide NPs as seeds and 3-aminopropyltriethyloxysilane as linker between the magnetic cores and TiO2 layers, respectively. The composite structure and the presence of the iron oxide and titania phase have been confirmed by transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectra. The hybrid NPs show good magnetic response, which can get together under an external applied magnetic field and hence they should become promising magnetic recovery catalysts (MRCs). Photocatalytic ability examination of the magnetic hybrid NPs was carried out in methylene blue (MB) solutions illuminated under Hg light in a photochemical reactor. About 50% to 60% of MB was decomposed in 90 min in the presence of magnetic hybrid NPs. The synthesized magnetic hybrid NPs display high photocatalytic efficiency and will find recoverable potential applications in cleaning polluted water with the help of magnetic separation.


Introduction
Extended and oriented nanostructures are desirable for many applications, but facile fabrication of complex nanostructures with controlled crystalline morphology, orientation, and surface architectures remains a significant challenge [1]. Among their various nanostructured materials, magnetic NPs-based hybrid nanomaterials have attracted growing interests due to their unique magnetic properties. These functional composite NPs have been widely used in various fields, such as magnetic fluids, data storage, catalysis, target drug delivery, magnetic resonance imaging contrast agents, hyperthermia, magnetic separation of biomolecules, biosensor, and especially the isolation and recycling of expensive catalysts [2][3][4][5][6][7][8][9][10][11][12]. To this end, magnetic iron oxide NPs became the strong candidates, and the application of small iron oxide NPs has been practiced for nearly semicentury owing to its simple preparation methods and low cost approaches [13].
Currently, semiconductor NPs have been extensively used as photocatalyst. TiO 2 NPs have been used as aphotocatalytic purification of polluted air or wastewater, will become a promising environmental remediation technology because of their high surface area, low cost, nontoxicity, high chemical stability, and excellent degradation for organic pollutants [14][15][16][17]. Moreover, TiO 2 also bears tremendous hope in helping to ease the energy crisis through effective utilization of solar energy based on photovoltaic and water-splitting devices [18][19][20][21]. As comparing with heterogeneous catalysts, many homogenerous catalytic systems have not been commericalized because of one major disadvantage: the difficulty of separation the reaction product from the catalyst and from any reaction solvent for a long and sustained environment protection [22]. In addition, there are two bottleneck drawbacks associated with TiO 2 photocatalysis currently, namely, high charge recombination rate inherently and low efficiency for utilizing solar light, which would greatly hinder the commercialization of this technology [23]. Currently, the common methods are metals/non-metals-doping or its oxides-doping to increasing the utilization of visible light and enhancing the separation situation of charge carriers [24][25][26][27]. More importantly, the abuse and overuse of photocatalyst will also pollute the enviroment.
In this point, magnetic separation provides a convenient method to removing pollutants and recycling magnetized species by applying an appropriate external magnetic field. Therefore, immobilization of TiO 2 on magnetic iron oxide NPs has been investigated intensely due to its magnetic separation properties [28][29][30][31][32]. Indeed, the study of core-shell magnetic NPs has a wide range of applications because of the unique combination of the nanoscale magnetic iron oxide core and the functional titania shell. Although some publications reported the synthesis of iron oxide-TiO 2 core-shell nanostructure, these reported synthesis generally employed solid thick SiO 2 interlayer. For instance, Chen et al. reported using TiO 2 -coated Fe 3 O 4 (with a silica layer) core-shell structure NPs as affinity probes for the analysis of phosphopeptides and as a photokilling agent for pathogenic bacteria [33,34]. Recently, Wang et al. reported the synthesis of (γ-Fe 2 O 3 @SiO 2 ) n @TiO 2 functional hybrid NPs with high photocatalytic efficiency [35]. Generally, immobilization of homogeneous catalysts usually decreases the catalytic activity due to the problem of diffusion of reactants to the surface-anchored catalysts [36]. In order to increase the active surface area, hollow and ultrafine iron oxide NPs are employed in this paper. Moreover, we proposed a new utilization of magnetic NPs as a catalyst support by modifying the surface on three different-shaped amino-functionalized iron oxide NPs with an active TiO 2 photocatalytic layer via a seedmediate method, as shown in Figure 1. The surface amines on the magnetic iron oxide NPs can serve as functional groups for further modification of titania. We discuss the formation mechanism of iron oxide/TiO 2 hybrid NPs. The results maybe provide some new insights into the growth mechanism of iron oxide-TiO 2 composite NPs. It is shown that the as-synthesized iron oxide/TiO 2 hybrid NPs display good magnetic response and photocatalytic activity. The magnetic NPs can be used as a MRCs vehicle for simply and easily recycled separation by external magnetic field application.

Preparation of iron oxide seeds A. Spindle hematite NPs
According to Ishikava's report [37], we take a modified method to prepare the monodisperse spindle hematite NPs, in a typical synthesis, 1.8 ml of a 3.7 M FeCl 3 ·6H 2 O solution was added dropwise into 4.5 × 10 -4 M NaH 2 PO 4 solution at 95°C and the mixture was aged at 100°C for 12 h. The resulting precipitates were washed with a 1 M ammonia solution and doubly distilled water and finally dried under vacuum.

B. Hollow magnetite NPs
According to our previous report [38], in a typical synthesis, solution A was prepared by dissolving 2.02 g KNO 3 and 0.28 g KOH in 50 mL double distilled water, solution B was prepared by dissolving 0.070 g FeS-O 4 ·7H 2 O in 50 mL double distilled water. Then the two solution were mixed together under magnetic stirring at a rate of ca. 400 rpm. Two minutes later, solution C (0.18 g Gla in 25 mL double distilled water) was added dropwise into the mixed solution. The reaction temperature was raised increasingly to 90°C and kept 3 h under argon (Ar) atmosphere. Meanwhile, the brown solution was observed to change black. After the mixture was cooled to room temperature, the precipitate products were magnetically separated by MSS, washed with ethanol and water two times, respectively, and then redispersed in ethanol.

C. Ultrafine magnetite NPs
The ultrafine magnetite NPs were prepared through the chemical co-precipitation of Fe(II) and Fe(III) chlorides (Fe II /Fe III ratio = 0.5) with 0.5 M NaOH [39]. The black precipitate was collected on a magnet, followed by rinsing with water several times until the pH reached 6 to 7.

Preparation of amino-functionalized iron oxide NPs
A solution of APTES was added into the above seed suspensions, stirred under Ar atmosphere at 25°C for 4 h. The prepared APTES-modified seeds were collected with a magnet, and washed with 50 mL of ethanol, followed by double distilled water for three times [40].

Preparation of iron oxides/TiO 2 hybrid NPs
In a typical synthesis, 0.2 g amino-functionalized seeds, 0.2 g CTAB, and 0.056 g HMTA were dissolved in 25 ml ethanol solution under ultrasonic condition at room temperature. The mixture solution was then transferred into a Teflon-lined tube reactor. Then, 1 ml Ti(Bu) 4 dropwise added in the tube, and was kept at 150°C for 8 h.

Photodegradation of MB
The prepared samples were weighed and added into 80 mL of methylene blue solutions (12 mg/L). The mixed solutions were illuminated under mercury lamp (OSRAM, 250 W with characteristic wavelength at 365 nm), and the MB solutions were illuminated under UV light in the photochemical reactor. The solutions were fetched at 10-min intervals by pipette for each solution and centrifuged. Then, the time-dependent absorbance changes of the transparent solution after centrifugation were measured at the wavelength between 500 and 750 nm.

Characterization
TEM images were performed with a JEOL JEM-2010 (HT) (JEOL, Tokyo, Japan) transmission electron microscope operating at 200 kV, and the samples were dissolved in ethanol and dropped on super-thin cabon coated copper grids. SEM studies were carried out using a FEI Sirion FEG operating at 25 keV, samples were sprinkled onto the conductive substrate, respectively. Powder X-ray diffraction (XRD) patterns of the samples were recorded on a D8 Advance X-ray diffractometer (Germany) using Cu Kα radiation (l = 0.1542 nm) operating at 40 kV and 40 mA and with a scan rate of 0.05°2θ s -1 . X-ray photoelectron spectroscopy (XPS) measurements were made using a VG Multilab2000X. This system uses a focused Al exciting source for excitation and a spherical section analyzer. The percentages of individual elements detection were determined from the relative composition analysis of the peak areas of the bands. Magnetic measurements were performed using a Quantum Design MPMS XL-7 SQUID magnetometer. The powder sample was filled in a diamagnetic plastic capsule, and then the packed sample was put in a diamagnetic plastic straw and impacted into a minimal volume for magnetic measurements. Background magnetic measurements were checked for the packing material. The diffuse reflectance, absorbance and transmittance spectra, and photodegradation examination of the microspheres was carried out in a PGeneral TU-1901 spectrophotometer.

Formation mechanism and morphology
For the synthesis of the functional hybrid nanomaterials, we synthesized the colloidal solutions of iron oxides NPs with different shapes in ethanol at the first. These iron oxide NPs exhibit long sedimentation time, and are stable against agglomeration for several days. Then, iron oxides NPs were modified with amino group by APTES because silane can render highly stability and water-dispersibility, and it also forms a protective layer against mild acid and alkaline environment. As shown in Figure 2, hydroxyl groups (-OH) on the magnetite surface reacted with the -OH of the APTES molecules leading to the formation of Si-O bonds and leaving the terminal -NH 2 groups available for immobilization of TiO 2 [41]. The immobilization of TiO 2 can be explained by HSAB (hard and soft acids and bases) formula [42]. As a typical hard acid, Ti ions can be combined to the terminal -NH 2 groups (hard bases) easily, owing to there is small amount water in ethanol (95%), and then TiO 2 will be coated on the surface of amino-functionalized iron oxide NPs by hydrolysis and poly-condensation as follows: We prepared the monodisperse spindle-like iron oxide NPs by ferric hydroxide precipitate method for evaluating and verifying our experimental mechanism and functional strategies. The electron micrograph of the starting weak-magnetic spindle-like hematite NPs are shown in Figure 3a, which have longitudinal diameter in the range from 120 to 150 nm and transverse diameter (short axis) around 40 nm. After TiO 2 coating (FT-1), the transverse diameter increased to around 50 nm, and the representative image is shown in Figure 3b. Moreover, the obvious contrast differences between the pale edges and dark centers further clearly confirms the composite structure. Therefore, the results reveal that this functional strategy for fabricating the TiO 2 -functionalized iron oxide NPs is a feasible approach. Then, two strong magnetic iron oxide NPs with different shape and diameter as seeds were employed to fabricate the magnetic TiO 2 hybrid materials. As shown in Figure 3c, Fe 3 O 4 NPs with an obviously hollow structure have diameters around 100 nm, and the insert field-emission SEM image illustrates the hollow NPs present sphere-like shape. In our previous report, we have confirmed that the hollow Fe 3 O 4 NPs were formed by oriented aggregation of small Fe 3 O 4 NPs [38]. Figure 3d shows bright field TEM image of the corresponding iron oxide NPs after the same TiO 2 coating process (FT-2). However, the hybrid NPs present a shagginess sphere-like shape and cannot observe the hollow structure. Additionally, the diameters of hybrid NPs increased about 5 to 10 nm. The results reveal that the hollow Fe 3 O 4 NPs have been covered by TiO 2 . Owing to the loose struture of Fe 3 O 4 seeds, TiO 2 will fill to its internal and surface, and finally cause the hybrid products present a solid nature. The diameter of above two different iron oxide

Structure and composition
XRD and XPS surface analysis was used to further confirm the structure and composition of iron oxides/TiO 2 hybrid NPs. Figure 4a shows the XRD patterns of the as-synthesized α-Fe 2 O 3 seeds and α-Fe 2 O 3 /TiO 2 (FT-1). From the XRD patterns of α-Fe 2 O 3 seeds, it can be seen that the diffraction peaks conformity with that of rhombohedral α-Fe 2 O 3 (JCPDS no. 33-0664, show in the   It is noteworthy that many studies demonstrated that if particles possessed a real core and shell structure, the core would be screened by the shell and the compositions in the shell layer became gradually more dominant, the intensity ratio of the shell/core spectra would gradually increase [43][44][45][46][47]. The gradually subdued XPS signals of Fe after TiO 2 coating are discerned in Figure 5b. APTES coating increases the intensity of carbon and oxygen, and decreases the concentration of Fe; further TiO 2 coating decreases the intensity of silicon and Fe (as shown in Figure 5b, c). Therefore, after TiO 2 coating, corresponding XPS signals of Fe, and Si rule also are decreased, C and O do not match with this rule due to the formation of TiO 2 and surfactant impurities (as shown in Figure 5d, e). Additionally, interactions should exist among APTEScoated Fe 3 O 4 NPs and titania which cause the shift of binding energy of Fe. Usually, XPS measures the elemental composition of the substance surface up to 1 to 10 nm depth. Therefore, XPS could be regarded as a bulk technique due to the ultrafine particles size of the FT-3 (less than 10 nm). The XPS result indicates that the amino-functionalized Fe 3 O 4 seeds have been coated by a TiO 2 layer, thus greatly reducing the intensity signals of the element inside. Table 1 lists the binding energy values of Fe, Si, O, N, and Ti resolved from XPS spectra of the above three different NPs. In three cases, the value of binding energy of Fe 2p and other elements are very close to the standard binding energy values. Relative to the standard values [48], the binding energy values in FT-3 have decreased and this result is in agreement with the previous discussions.
Furthermore, XPS surface analysis is also used to quantify the amount of titanium and iron present in the near surface region of the three different hybrid NPs. Figure 6 is the typical XPS spectra of the FT-1, FT-2, and FT-3, where part (a) is the survey spectrum and parts (b)-(d) are the high-resolution binding energy spectrum for Fe, Si, O, C, N, and Ti species, respectively. According to the survey spectrum, all hybrid NPs exhibited typical binding energies at the characteristic peaks of Ti 2p, Fe 2p, Si 2p, N 1s and O1s in the region of 458, 710, 103, 400, and 530 eV, respectively. Details of the XPS surface elemental composition results of asobtained products are shown in Table 2. The XPS data of the titanium-to-iron ratio of hybrid NPs is calculated in which the elemental composition ratio of FT-1, FT-2, and FT-3 (titanium/iron) are about 2:1, 3.5:1, and 5.5:1. The results reveal that the quantity of Ti element is higher than that of Fe element on the surface of samples. That is, it may deduce that iron oxide NPs have been coated by TiO 2 . In all hybrid NPs, the amount of oxygen to titanium or iron calculated from XPS data is about 5:1, this results is in agreement with the other reports [49]. Nevertheless, the combined results from TEM and XPS suggest that the synthesized hybrid NPs are composed of amino-functionalized iron oxide NPs and TiO 2 .

Magnetic and magnetic response properties
Magnetic measurements of the hybrid NPs were performed on a SQUID magnetometer. As shown in Figure  7, hysteresis loops demonstrate that FT-2 and FT-3 have no hysteresis, the forward and backward magnetization curves overlap completely and are almost negligible. Moreover, the NPs have zero magnetization at zero applied field, indicating that they are superparamagnetic at room temperature, no remnant magnetism was observed when the magnetic field was removed [50]. Superparamagnetism occurs when the size of the crystals is smaller than the ferromagnetic domain (the size of iron oxide NPs should less than 30 nm), the size of the ultrafine Fe 3 O 4 component in our product is less than 10 nm, and the hollow Fe 3 O 4 is consist of small magnetite NPs, there are reasonable to suppose that the hybrid NPs showed superparamagnetic behavior. The results reveal that the products have been inherit the superparamagnetic property from the Fe 3 O 4 NPs, and the saturation magnetization value (M s ) of naked hollow Fe 3 O 4 and ultrafine Fe 3 O 4 is 89.2 and 72.1 emu/g, respectively. After TiO 2 coating, the corresponding value of M s decreases to 16.2 and 5.0 emu/g, respectively. The M s decreased significantly after coating with TiO 2 due to the surface effect arising from the non-collinearity of magnetic moments, which may be due to the coated TiO 2 is impregnated at the interface of iron oxide matrix and pinning of the surface spins [51]. Moreover, this decrease in magnetic behavior is very close to other reports [52,53]. As the most stable iron oxide NPs in the ambient conditions, the magnetic properties of hematite are not well understood [54][55][56]. We checked the magnetic properties of FT-1 hybrid NPs, the M s is about 2 × 10 -4 emu/g, and the composite NPs exhibit a typical ferromagnetism. Thereby, as a weak magnetic hybrid NPs, FT-1 cannot be separate by common magnet.
We checked the magnetic responsibility of FT-2 and FT-3 hybrid NPs under the external applied magnetic field by a common magnet. As shown in Figure 8, both hybrid NPs gather quickly without residues left in the solid and solution state when the magnet presence. The gathered hybrid NPs can be redispersed in the solution easily by a slight shake. The results illustrate that the hybrid NPs display a good magnetic response, and this is also important for the industrial application in water cleaning as MRCs for preventing loss of materials and save cost.

Optical adsorption and photocatalytic properties
The three different hybrid NPs were further characterized by UV-vis absorption spectra to compare their optical adsorption properties and the results are shown in Figure 9a. The spectra highlight a strong adsorption in the UV region, the results are in agreement with the other reports [57,58]. It is noteworthy that the hybrid NPs with different morphology (at same concentration) will cause the difference of adsorption intensity and peak location. Due to the small dimensions of semiconductor NPs, a discretization of the bandgap occurs with decreasing particle size, leading to smaller excitation frequencies. A blue shift of FT-3 is observed in the extinction behavior, and the absorption edge is positioned at smaller wavelengths [59]. The result confirms that the diameter of FT-1 hybrid NPs is large than the other two different types hybrid NPs. Additionally, a concomitant tail can be clearly observed in the visible region of the absorption curve owing to scattering losses induced by the large number of inorganic NPs in the composite nanostructure [60].
In order to calculate the bandgap of hybrid NPs, the relationship between the absorption coefficient (a) and the photon energy (hν) have been given by equation as follows: ahv = A(hv-E E ) m , where A is a constant, E g is the bandgap energy, hν is the incident photon energy and the exponent m depends on the nature of optical transition. The value of m is 1/2 for direct allowed, 2 for indirect allowed, 3/2 for direct forbidden, and 3 for indirect forbidden transitions [61]. The main mechanism of light absorption in pure semiconductors is direct interband electron transitions. The absorption coefficient α has been calculated from the Lamberts formula [62], α = 1 t ln 1 T , where T and t are the transmittance (can be directly measured by UV-vis spectra) and path length of the colloids solution (same concentration), respectively. A typical plot of (ahν) 2 versus photon energy (hν) for the samples are shown in Figure 9b. The value of FT-1, FT-2, and FT-3 is 2.85, 2.89, and 2.73 eV, respectively.TiO 2 is important for its application in energy transport, storage, and for the environmental cleanup due to its well known photocatalytic effect with a bandgap of 3.2 eV [63]. Comparing with the pure TiO 2 NPs, the bandgap of hybrid NPs is obviously decreased, and the absorption edge generates obvious red shift. This red shift is attributed to the charge-transfer transition between the electrons of the iron oxide NPs and the conduction band (or valence band) of TiO 2 [64]. Iron oxide NPs can increase energy spacing of the conduction band in TiO 2 and finally lead to the quantization of energy levels and causes the absorption in the visible region. The other is that amino groups can act as a substitutional dopant for the place of titanium and change metal coordination of TiO 2 and the electronic environment around them [65]. Similar phenomenon of red shift in the bandgap for iron oxide/TiO 2 hybrid NPs were also found by other reports [53,[65][66][67].
The photocatalytic activity was examined by a colorant decomposition test using MB, which is very stable chemical dye under normal conditions. In general, absorption spectra can be used to measure the concentration changes of MB in extremely dilute aqueous solution. The MB displays an absorption peak at the wavelength of about 664 nm. Time-dependent photodegradation of MB is shown in Figure 10. It is illustrated that MB decomposes in the presence of magnetic TiO 2 hybrid materials. Generally, the pure TiO 2 NPs can decompose 40% MB in 90 min [68][69][70]. In our previous report, the pure TiO 2 NPs with a average diameter of 5 nm can be decomposed 53% MB in 90 min [71]. However, in our system, 49.0%, 56.5%, and 49.6% MB decomposed by FT-1, FT-2, and FT-3 in 90 min, respectively. The result reveals that the introduction of iron oxide NPs not only improve the photocatalytic activity but also employ the corresponding magnetic properties from itself. Thus, the Langmuir-Hinshelwood mechanism, which could be simplified as a pseudo-first order reaction as follows [72,73]: r = − dC t dt = kCt, where r is the degradation rate    of reactant, C is the concentration of reactant, k is the apparent reaction rate constant. The k for FT-1, FT-2, and FT-3 was 1.066% min -1 , 1.331% min -1 , 1.054% min -1 , respectively. It was surprising that the FT-2 exhibited such higher activity. This may be explained by light absorption capability of the FT-2 due to their rough shell contributes to the good photocatalytic activity. Compared to smooth surface, the rough surface layers can absorb more light because the UV-vis light can have multiple-reflections among the shagginess surface structure [74].

Conclusions
In summary, MRCs have been fabricated via a facile seed-mediate technology. These iron oxide/TiO 2 hybrid NPs were synthesized in a stepwise process.
First, three different shapes of naked iron oxide NPs were prepared. Next, amino groups encapsulated iron oxide NPs are synthesized by APTES modification. Finally, the iron oxide/TiO 2 hybrid NPs can be obtained after the TiO 2 coating. The FT-2 and FT-3 hybrid NPs show superparamagnetic and both display good photocatalytic properties. This MRCs combination of the photocatalysis properties of TiO 2 and the superparamagnetic property of Fe 3 O 4 NPs endows this material with a bright perspective in purification of polluted wastewater. Additionally, this work also discusses the formation mechanism and potentially provided a general method for synthesizing nanocomposites of magnetic iron oxide NPs and other functional NPs, which may find wider applications besides in photocatalysis.