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Enhanced Photocatalytic Performance through Magnetic Field Boosting Carrier Transport Jun Li, Qi Pei, Ruyi Wang, Yong Zhou, Zhengming ZHANG, Qingqi Cao, Dunhui Wang, Wenbo Mi, and Youwei Du ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b08770 • Publication Date (Web): 03 Apr 2018 Downloaded from http://pubs.acs.org on April 3, 2018

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Enhanced Photocatalytic Performance through Magnetic Field Boosting Carrier Transport Jun Li,†,‡ Qi Pei,§ Ruyi Wang,†,‡ Yong Zhou,†,‡ Zhengming Zhang,†,‡ Qingqi Cao,†,‡ Dunhui Wang,*,†,‡ Wenbo Mi,*,§ and Youwei Du†,‡ †

National Laboratory of Solid State Microstructures and Jiangsu Provincial Key Laboratory for

Nanotechnology, Nanjing University, Nanjing 210093, China ‡

Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing

210093, China §

Tianjin Key Laboratory of Low Dimensional Materials Physics and Preparation Technology,

School of Science, Tianjin University, Tianjin 300072, China Corresponding Author * [email protected] * [email protected]

KEYWORDS: photocatalytic, magnetoresistance, spin, magnetic field, photoelectrocatalysis

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ABSTRACT: The promotion of magnetic field on catalytic performance has attracted extensive attention for a long time and substantial improvements have been achieved in some catalysis fields. However, due to several orders of magnitude weaker of Zeeman energy, magnetic field seems impossible to alter the band structure and has negligible effect on semiconductor photocatalytic performance, which makes this task a great challenge. On the other hand, the spinrelated behavior usually plays an important role in determining catalytic performance. For example, in some molecular catalysis, such as photosystem II, ferromagnetic alignment of the active material results in spin oriented electrons, which are selected and accumulated at the interface, leading to great promotion of the oxygen evolution reaction activity. Here, we propose a magnetoresistance-related strategy to boost the carrier transfer efficiency and apply it in αFe2O3/reduced graphene oxide hybrid nanostructures (α-Fe2O3/rGO) to improve the photocatalytic performance under magnetic field. We show that both the degradation rate constant and photocurrent density of α-Fe2O3/rGO can be dramatically enhanced with the application of magnetic field, indicating the promotion of the photocatalytic performance.


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The capture of solar energy and its conversion to storable chemical fuel or elimination of organic pollutants by photocatalytic processes provides a promising route to overcome the current global energy crisis and environmental issues.1-3 Hence the promotion of photocatalytic performance has become a hot topic in this field. Based on the mechanism of semiconductor photocatalysis, it is pivotal to take full advantage of photogenerated charge carriers for enhancing the activity of photocatalysts and thus considerable efforts have been devoted to solving this problem.4 For instance, band gap engineering has been performed in many systems through ion implantation,5 metal loading,6 or other dopant-free methods7,8 to extend light absorption into the visible portion of the solar spectrum and produce many more photogenerated charge carriers to participate in the photocatalytic processes.9 Accelerating the transport and suppressing the recombination of charge carriers are other important approaches for this issue.4 It is reported that the state-of-the art semiconductor heterojunctions, for example Fe2O3/TiO2 composite photocatalysts,10 can obviously improve the separation of electrons and holes; while graphene, which acts as an excellent electron acceptor and transporter, can facilitate the charge migration owing to its high electron mobility.11 However, despite the huge success achieved during recent years, the light conversion efficiency is still lower than expected.12 Therefore, great efforts are needed to explore alternative strategies to further improve the utilization of solar energy and optimize the overall separation and transfer efficiency of electron-hole pairs. Recently, the external-fields-enhanced catalytic performance has received extensive attention. For example, non-redox, bond-forming process of Diels–Alder reaction can be accelerated by an oriented electric field,13 which offers an opportunity of precise manipulation of chemical reaction. Elastic strain can also tune the catalytic activity of platinum in a controlled and predictable way.14,15 In the field of photocatalysis, there are many reports about the external


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field enhancement, because it doesn’t change the geometries or components of the photocatalysts and needn’t complicated preparation processes. Up to now, the thermal,16 ultrasonic wave,17 microwave18 and electric fields19 have been successfully introduced into some photocatalytic systems, and consequently, the photocatalytic efficiency has been markedly improved. Externalmagnetic-field-tuned approach, as a kind of non-contact and environment-friendly methods, has been widely discussed and effectively contributed towards sustainable developments.20 In this sense, it is fascinating to utilize external magnetic fields in the photocatalysis process for performance improvement. Unfortunately, the Zeeman energy caused by the magnetic field is several orders of magnitude lower than the band gap of semiconductors and thus has slight influence on the photocatalytic performance.21,22 It seems that, in addition to the enlarged photocatalytic reaction areas caused by the magnetic Lorentz force,23 magnetic field seemingly can’t make further contribution to semiconductic photocatalytic performance improvement. Magnetoresistance (MR) is an essential spintronic phenomenon which manifests the change of resistance in a material under the magnetic field. It is usually defined as MR% = RH − R0⁄R0, where R(H) and R(0) are the resistance under a magnetic field H and zero field, respectively. In this sense, the realization of negative MR in a photocatalytic system means that, under a magnetic field, charge carriers can migrate to the surface reaction sites faster and more carriers can take part in surface reactions per unit time, which is helpful to boost the photocatalytic efficiency. In this work, with a proper design, the α-Fe2O3/reduced graphene oxide hybrid nanostructures (α-Fe2O3/rGO) are prepared. Our experimental results demonstrate that this magnetic system exhibits good photocatalytic performance. More importantly, thanks to the large negative MR effect in this system, more charge carriers can migrate from α-Fe2O3 to rGO per unit time through the interface between them under a magnetic field, which leads to a


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promotion of photocatalytic efficiency. This work not only provides a prototype for enhancing the photocatalytic performance but also introduces a universal method that is widely available in catalytic fields. RESULTS AND DISCUSSION Preparation, Structure arrangement and chemical compositions of α-Fe2O3/rGO. The nanostructure composed of α-Fe2O3 and rGO was prepared by a facile hydrothermal reaction as schematically illustrated in Figure S1. Figure 1a,b display the transmission electron microscopy (TEM) images of α-Fe2O3/rGO nanocomposites and α-Fe2O3 nanoparticles, respectively. As shown in Figure 1a, α-Fe2O3 nanoparticles with sizes of about 50-80 nm inlay and distribute homogeneously in the matrixes of corrugated rGO sheets. From the enlarged image shown in the inset of Figure 1a, a coated structure together with an interface between α-Fe2O3 nanoparticles and transparent rGO sheets are clearly observed, which is critical for the formation of the hybrid nanostructure. As presented in Figure 1b, the high-resolution TEM image of an α-Fe2O3 nanoparticle shows a single-crystalline structure with a lattice fringe spacing of 3.74 Å, which corresponds to the (012) plane of α-Fe2O3.24 The phase of α-Fe2O3/rGO is investigated by powder X-ray diffraction (XRD). From XRD patterns in Figure 2a, the iron oxide can be indexed as α-Fe2O3 (JCPDS: 33-0664). No shift of diffraction peak is observed between α-Fe2O3 and α-Fe2O3/rGO samples, indicating that the formation of nanocomposites with the present of rGO has a negligible effect on the crystal phase of α-Fe2O3. Raman spectrum of rGO is characterized by two main features: the G band arising from the first order scattering of E1g phonon of sp2 C atoms and the D band arising from a breathing mode


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of κ-point photons of A1g symmetry.25,26 Moreover, the increasing ratio of D band intensity to G band intensity (ID/IG) can be regarded as a characteristic of the reduction of GO to rGO.27 Figure 2b shows the Raman spectra of GO and α-Fe2O3/rGO in the range of 1000–3000 cm-1. It is obvious that GO displays two prominent Raman peaks at 1355 cm-1 and 1590 cm-1, respectively, while the value of ID/IG is about 1.1. After hydrothermal reduction, the Raman spectra of αFe2O3/rGO still displays two pronounced peaks at 1335 cm-1 and 1592 cm-1, manifesting the maintenance of the structure of graphene in the nanocomposite. However, the value of ID/IG in this case increases to 1.7, suggesting the elimination of the oxygen functional groups in the GO nanosheets and the reestablishment of the conjugated graphene network.28 The surface composition and chemical state of element in α-Fe2O3/rGO are further investigated by X-ray photoelectron spectroscopy (XPS) measurement. The wide scan XPS spectrum of α-Fe2O3/rGO is shown in Figure S3a. The photoelectron lines observed at binding energies of about 285, 530, and 711 eV, correspond to C 1s, O 1s, and Fe 2p, respectively. In the spectrum of Fe 2p (Figure 2c), the peaks of Fe 2p3/2 and Fe 2p1/2 locate at 710.5 and 724.0 eV while two satellite peaks are identified at 719.0 and 733.8 eV, indicating the formation of the αFe2O3 phase.29,30 In the α-Fe2O3/rGO nanocomposite, there may have three species of the C 1s spectra, which are assigned to C-C (284.5 eV), C-O (286.7 eV) and C=O (287.8 eV), respectively. In Figure 2d, the C 1s spectra shows mainly the non-oxygenated carbon and the peak for C=O is almost vanished, implying that GO is substantially reduced after the hydrothermal reaction. Magnetic and magnetotransport behavior of α-Fe2O3/rGO. Magnetic hysteresis loop (M-H) of α-Fe2O3/rGO is measured at room temperature (magnetic hysteresis loop of α-Fe2O3 and pure rGO are shown in Figure S8). As shown in Figure 3a, a typical ferromagnetic M-H


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curve with small coercivity is observed and saturated under a magnetic field of 6 kOe. It is well known that α-Fe2O3 nanoparticles can show ferromagnetic behavior due to the canted spin structure.31,32 Besides, there is an interface between α-Fe2O3 and rGO in our sample, which is likely to induce the magnetic moments in rGO due to the magnetic proximity effect.33,34 In order to verify this hypothesis, first principles calculation is performed in this α-Fe2O3/rGO nanostructure. As depicted in Figure S16, there are three different supercell models in terms of three different terminations of α-Fe2O3 (001) surface, for which model V3 with Fe-O3-Fe arrangement has the smallest distance between rGO and α-Fe2O3 surface (d value), indicating a strong interfacial hybridization between them (Figure 3b). The interaction mechanism of V3 system can be further manifested by an analysis of the density of state (DOS) as calculated in Figure 3c. For the interfacial Fe I↑ atom, it is observed that the partial DOS of C atom at the interface no longer remains symmetrical and passes through Fermi level mainly in the spin-down channel. The strong peak of polarization Fe I↑-d state get hybridized with C I-p orbital, which leads to the magnetic moment in rGO (see Supporting Information for details). During the photocatalytic process, the photogenerated carriers of α-Fe2O3 migrate to the rGO through the interface between them. Since both α-Fe2O3 and rGO possess magnetic moments, the transport property of carriers would be affected by the magnetic moments arrangement at interface. With the application of a magnetic field, the magnetic moments of both α-Fe2O3 and rGO become aligned in a parallel arrangement along the field direction and then increase the injection rate of polarized carriers transfer from α-Fe2O3 to rGO. As a result, many more carriers can transfer through the interface per unit time, leading to the increase of current density and decrease of resistivity. Figure 3d shows the magnetotransport behavior of αFe2O3/rGO at room temperature under a maximum magnetic field of 8 kOe. An obvious negative


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MR effect is observed in α-Fe2O3/rGO nanocomposites with a maximum value of about -9% at 8 kOe, which is consistent with the aforementioned analysis. For comparison, the magnetotransport behavior of α-Fe2O3 is also investigated and no obvious MR effect is observed. In Figure 3d, MR of α-Fe2O3/rGO shows a rapid decrease at low field, while in the high field larger than 6 kOe, it is almost saturated. This result is consistent with the magnetic behavior in M-H curve (Figure 3a), in which the magnetization is almost saturated under a magnetic field of 6 kOe. Here the saturation behavior of MR can be understood as following: the magnetic moments have almost become aligned at 6 kOe and the increase of magnetic field makes no further contribution to MR effect of α-Fe2O3/rGO nanocomposites. The negative MR effect of α-Fe2O3/rGO nanocomposites means that more photogenerated charge carriers of α-Fe2O3 can take part in the surface reactions per unit time. In order to verify the effect of magnetic field on the photocatalytic activity, we select the degradation of organic pollutants as an example to investigate photocatalytic process under controllable magnetic field (because this experimental apparatus can be conveniently placed in an electromagnet environment.). Figure 4a is a schematic image of the experimental apparatus, in which an electromagnet with magnetic field ranging from 0 to 10 kOe is supplied. Here quartz doublelayer beaker is chosen as the photocatalytic reactor and temperature-controlled water is circulated by a pump in order to keep the reaction temperature constant during the experiment process (20 °C). Photocatalytic properties of α-Fe2O3/rGO under magnetic field. The photocatalytic activities of α-Fe2O3 and α-Fe2O3/rGO are characterized by the removal of rhodamine B dye (RhB). Before irradiation, time-dependent absorbance changes at 554 nm for RhB of α-Fe2O3 and α-Fe2O3/rGO under dark condition have been investigated (Figure S9). In the case of α-


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Fe2O3 photocatalysts, only 6% of RhB is removed within 60 min without light irradiation. However, under the same condition, about 12% of RhB is absorbed by α-Fe2O3/rGO photocatalysts which is due to the fact that dye molecules can transfer from solution to the surface of photocatalysts and be absorbed via π−π conjugation between RhB and aromatic regions of rGO.35 Before light irradiation, 24 hours settling was carried out. After that, we compare the photocatalytic performance of α-Fe2O3 and α-Fe2O3/rGO without magnetic field as illustrated in Figure S10. It is observed that α-Fe2O3/rGO nanocomposites show significant progress in the photodegradation of RhB compared to both α-Fe2O3 and P25, a standard commercial photocatalyst, which is attributed to high electron mobility of rGO.37 According to the previous work of Okumura,36 long time settling can effectively suppress the impact of Lorentz force on photocatalysis. This is due to the fact that although external magnetic field can promote the adsorption performance owing to Lorentz force and enlarged reaction area, after long time settling, the formation of organic molecular and O2 complex is complete and key bonding of the molecule for decoloration is blocked by the existence of O2 molecules. As shown in Figure S11, without settling, the adsorption performances of both αFe2O3/rGO and rGO are evidently promoted by external magnetic fields and show no sign of saturation. However, after 24 h settling, the magnetic field has negligible effect on adsorption. In our work, before photocatalytic test, all the samples were settled 24 hours under corresponding magnetic field to exclude the impact of magnetic-field-induced Lorentz force on adsorption. Figure 4b shows the photocatalytic activities of α-Fe2O3/rGO nanocomposites under different magnetic fields. It is found that the degradation rates of α-Fe2O3/rGO are 59%, 74%, 83% and 84% for the magnetic field of 0 kOe, 3 kOe, 6 kOe and 8 kOe respectively, demonstrating a prominent enhancement of photocatalytic performance under the external magnetic field. The


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pseudo-first-order kinetics of the photocatalysts under various magnetic fields could be analyzed using the pseudo-first order model as follow:38 − lnC ⁄C = kt here k is the pseudo-first-order rate constant (min-1). As shown in Figure 4c, it is found that the photocatalytic degradation under various magnetic fields follows first-order reaction dynamics. The α-Fe2O3/rGO @ 6 kOe shows the rate constant k of 0.042 min-1, which is 1.75 times of α-Fe2O3/rGO NMF (0.024 min-1) and 1.2 times of α-Fe2O3/rGO @ 3 kOe (0.035 min-1). Figure 4d plots the comparison of photodegradation of some other organic pollutants between the zero magnetic field and 6 kOe, which further illustrates the magnetic-field-enhanced photocatalytic performance and verifies the availability in many other contaminants. Since the report of magnetic field effect in chemical kinetics by Steiner et al. in 1989,39 many efforts have been devoted to the study of magnetic field manipulation of chemical reactions.41,42 For example, according to the study by Kiwi,44 magnetic fields have a negative effect on H2 production of a typical TiO2-RuO2-Pt heterogeneous system. However, based on Zhang’s work,45 with a magnetic field of 0.5942 kOe, the photocatalytic conversion of benzene over Pt/TiO2 is increased by 2.5%. Wang also found that with magnetic field of 0.82 kOe, initial phenol concentration of 60 mg L-1, and TiO2 catalyst dosage of 1.0 g L-1, the highest degradation rate constant of 0.00672 min-1 was achieved.46 According to the experimental results mentioned above, many mechanisms attribute to the magnetic adsorption effect, e.g. the net Lorenz force on dissolved oxygen,36 and spin states of radical pairs.40 Among them, the model of short-rangeorder diffusion effect in the Helmholtz layer shows a time-related feature and more significant character at the early stage, which is also consistence with our experiment results. As shown in


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both Figure 4b and Figure S11, clearly change of first 10 min degradation is observed due to short-range-order diffusion, which manifests that magnetic adsorption plays a certain role in this magnetic-field-induced improvement. However, in our experiments, besides the long-time settling to diminish the impact of Lorentz under the field, more evidences have been provided to demonstrate that the effect of magnetic adsorption can not explain the whole magnetic field effect on photocatalytic process. In general, the higher the external magnetic field is, the larger the Lorentz force exerts on the objects. But in our case, there are no obvious difference between the effects of 6 kOe and 8 kOe on the photocatalytic performance of α-Fe2O3/rGO, which is shown in Figure S13a. Actually, this photocatalytic result is consistent well with the saturation behavior of MR shown in Figure 3b. On the other hand, Figure S12 displays the photocatalytic activities of pure rGO under 0 kOe, 6 kOe and 8 kOe and Figure S13b shows the photocatalytic activities of pure α-Fe2O3 under zero magnetic field and 6 kOe. It is observed that the photocatalytic performance is almost unaltered with the application of magnetic field, which can be attributed to the absence of MR effect of rGO and α-Fe2O3 (Figure 3d). The aforementioned results verify that apart from magnetic adsorption, negative MR plays an important role in improving the photocatalytic activity in αFe2O3/rGO nanocomposites. Photoelectrochemical performance of α-Fe2O3/rGO under magnetic field. The photoelectrochemical (PEC) results of α-Fe2O3/rGO film can give a substantial evidence for the magnetic field enhancement of its photocatalytic activity. The PEC investigation displays that photocurrent density of α-Fe2O3/rGO film is 6.2 µA cm-2 at 1.23 V under zero magnetic field. However, when a fixed NdFeB magnet with a magnetic field of 1 kOe is imposed to the photoelectrode, the photocurrent increases to 12.5 µA cm-2 (Figure 5a). The time dependence of


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open circuit potential decay (OCPD) is measured with the light illumination on and off. When the light is on, there is an obvious response with 0.16 V open circuit potential (OCP) under zero magnetic field. As a magnet is attached to α-Fe2O3/rGO film, an enhancement of 0.03 V OCP could be observed (Figure S15). It is evident that, under the magnetic field, our samples show a slower OCPD rate than that under zero field, suggesting a delaying recombination kinetic of electrons. The lifetime of photogenerated electrons (τ) could be calculated from OCPD (Figure 5b). It is observed that electrons lifetime of α-Fe2O3/rGO under magnetic fields is longer, which demonstrates the magnetic-field-enhanced photocatalytic efficiency as well. On the other hand, the MR of α-Fe2O3/rGO system won’t be affected, although nanoparticles are distributed randomly on FTO glass. This is due to the fact that, once the external magnetic field was imposed on the ferromagnetic nanocomposites, all spin moments become parallel to the field, as shown in magnetic hysteresis loop (in Figure 3a). Hence, both the spin moments of α-Fe2O3 and spin moments of rGO parallel to each other under the external magnetic field. As a result, many more polarized carriers can transfer from α-Fe2O3 to rGO through the interface, demonstrating a negative MR effect. The mechanism of magnetic field-enhanced photocatalytic performance of α-Fe2O3/rGO is sketched in Figure 6. In a typical semiconductor photocatalytic reaction, an electron is excited from the valence band (VB) of the semiconductor into its conduction band (CB) under the irradiation of sunlight, and then the photoexcited electrons and holes can be used to remove pollutants from the environment, which is shown in Figure 6a. By imposing an external field, the carriers can transfer from α-Fe2O3 to rGO through the interface between them more easily due to the negative MR effect in this nanostructure. As a result, more charge carriers can transfer to the active site to participate in the photocatalytic process per unit time, and consequently,


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promote the degradation efficiency of organic pollutants into nontoxic inorganic molecules (Figure 6b).

CONCLUSIONS In summary, nanocomposites of α-Fe2O3/rGO are prepared using a simple hydrothermal method, in which an interface between α-Fe2O3 and rGO is established. With the application of an external magnetic field, a negative MR effect is observed due to the parallel alignment of magnetic moments of both α-Fe2O3 and rGO. Taking advantage of this feature, the photocatalytic performance of α-Fe2O3/rGO is greatly improved, which is attributed to the increased injection rate of carrier from α-Fe2O3 to rGO under the magnetic field. The present results provide an insight into the magnetic field enhancement on the photocatalytic activity. Furthermore, with proper design, the external magnetic fields are promising to be widely used in catalytic fields for the purpose of increasing catalytic performance.

EXPERIMENTAL SECTION Material synthesis. The chemicals used in this work were of analytical reagent grade. Iron chloride hexahydrate (FeCl3·6H2O) and urea (CH4N2O) were purchased from Aladdin Chemistry Co., Ltd. (Shanghai, China). Solutions were freshly prepared with deionized water. Graphene oxide was prepared by using a modified Hummers method. The samples of α-Fe2O3/rGO were prepared by hydrothermal and reduction reactions. In a typical experiment, FeCl3·6H2O (0.81 g) and urea (1.08 g) were added to GO solution (2mg ml-1) under stirring. The weight ratio of GO to


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α-Fe2O3 was fixed at 1:10. After being sonicated for 30min, the above mixture was transferred into a Teflon-lined stainless steel autoclave (100 mL) and followed by hydrothermal treatment at 180 °C for 8 h. Then the samples were cooled to room temperature naturally. The resulting black products were centrifuged and washed with deionized water and absolute alcohol three times, respectively. The obtained products were dried in a vacuum oven at 60 °C for 4 h for the following characterization. For a comparison, pure α-Fe2O3 nanoparticles were prepared under the same condition except for the introduction of the aqueous dispersion of the GO and pure rGO were prepared under the same condition except for the introduction of the aqueous dispersion of the FeCl3·6H2O. Material characterization. XRD patterns of the powders were recorded using an X-ray diffraction spectrometer (Cu-Kα, TD−3500, Tongda). The transmission electron microscopic (TEM) images were acquired with a JEOL JEM 2100 microscope operating at 200 kV. X-ray photoelectron spectroscopy (XPS) was performed on a PHI5000 VersaProbe (ULVAC−PHI, Japan). Standard C 1s peak was used as a reference for correcting the shifts. Raman spectra were obtained by a Raman system (T64000, Jobin Yvon, USA) using a 514.5 nm laser as the light source. Tapping-mode AFM measurements were carried out under ambient conditions using a Dimension V (Veeco, USA). This thickness of the α-Fe2O3/rGO was estimated to be 3 nm (Figure S4). FTIR spectroscopy was conducted using a Nicolet NEXUS870 (USA) spectrometer. UV−Vis−NIR diffuse reflectance spectra (DRS) were obtained by using the UV−Vis−NIR spectrophotometer (TU−1900, Puxi) with an integrating sphere attachment and with BaSO4 as reflectance standard. The fluorescence spectra of the samples were taken by the photoluminescence (PL) spectrofluorometer (FLS920, Edinburgh, UK) with an excitation at 550 nm light. The magnetic properties of the samples were measured using a vibrating sample


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magnetometry (LakeShore 7407, Lake Shore, USA). The magnetoresistance properties of samples were measured by Keithley 2410 with the external magnetic field supplied by a selfdesigned electromagnet at room temperature. Before transport measurement, the samples were compressed to a pellet with a diameter of 1 cm. Photocatalytic degradation performance measurement. The photocatalytic activities of the as-obtained samples were evaluated in terms of the degradation of rhodamine B (RhB, 10 mg L-1). 30 mg of photocatalysts were added into a 200 mL double layer quartz photo-reactor containing 80% of RhB solution. Prior to photo-irradiation, the suspensions were magnetically stirred in the dark to attain the adsorption/desorption equilibrium between the dye and the surface of the catalyst under ambient conditions. A 300 W Xe lamp (with AM 1.5 air mass filter) was used as a simulated solar light source. Before irradiation, all samples were settled 24 h under the magnetic field. At the given time intervals, 4 ml of the samples were taken from the mixture and immediately centrifuged to remove the photocatalysts. The degradation rates were analyzed by recording variations in the absorption in UV−vis spectra of RhB using the ultraviolet visible spectrometer (TU−1900, Puxi). The percentage of degradation is defined as Ct/C0 (test concentration is denoted as Ct and initial concentration is C0). Each experiment was repeated three times to ensure accuracy. The degradation of RhB under various magnetic fields was also performed with a self-designed electromagnet circling around the photo-reactor under the same condition. Acid Orange 7 (20 mg L-1), Methyl Orange (10 mg L-1), Conge Red (50 mg L-1), Indigo Carmine (20 mg L-1), Patern Blue (10 mg L-1) and Phenol (5 mg L-1) were also used as pollutants to verify the effectiveness of this magnetic-field-enhanced effect. The phenol concentration was measured by 4-aminoantipyrene method. The dissolved oxygen concentration of deionized water was ~8.0 mg L-1 at 20 °C in ambient air atmosphere.


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Photoelectrochemical tests. Firstly, α-Fe2O3/rGO nanocomposites and terpineol (~98%, Aladdin) were mixed with the ratio of 200:1 w/v and stirred for several hours in order to achieve a uniform suspension. Then, the mixture was coated onto a fluorine-doped SnO2 (FTO) glass substrate (25 × 20 × 2 mm, sheet resistance 7 Ω sq-1) with doctor blade method to prepare the photoelectrodes under same condition. Finally, α-Fe2O3/rGO photoelectrodes were dried and transferred to a quartz tubular furnace to sinter in argon at 500 °C for 2 h. The photoelectrochemical (PEC) performance of the photoelectrodes was evaluated in a threeelectrode cell using an electrochemical analyzer (CHI-630D, Shanghai Chenhua) under AM 1.5G illumination (standard 100 mW cm-2) cast by an Oriel 92251A-1000 sunlight simulator calibrated by the standard reference of a Newport silicon solar cell. 1 M NaOH aqueous solution (pH 13.6) was used as electrolyte. α-Fe2O3/rGO nanocomposites was used as a working electrode. A Pt foil and a saturated Ag/AgCl electrode were used as a counter and a reference electrode. The RHE potential was calculated following the formula ERHE = EAg/AgCl + 0.059pH + E0Ag/AgCl where ERHE is the converted potential vs. RHE, EAg/AgCl is the applied potential against Ag/AgCl reference electrode, and E0Ag/AgCl = 0.1976 V (25 °C). Under PEC measurement, a fixed NdFeB magnet with magnetic field of 1 kOe was imposed to the photoelectrode (Figure S14). Calculation details. Our calculations are performed by Vienna ab initio simulation package code47 with the generalized gradient approximation (GGA) parameterized by Perdew-BurkeErnzerhof (PBE)48 together with the vdW-D2 correction.49 The Hubbard U term (U=5 eV and J=1 eV for Fe) is used to describe the on-site electron-electron Coulomb repulsion as suggested in the literature.50 In order to find the theoretical equilibrium static geometries, the plane-wave


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cutoff energy is set to 500 eV and the energy and force convergence criteria acting on each atom are less than 10-6 eV and 0.01 eV Å-1, respectively. For α-Fe2O3 bulk calculations, the hexagonal unit cell containing 30 atoms is used throughout our calculation progress. The Monkhorst-Pack k-point sampling method with a 5×5×2 grid is taken for the geometry optimization, while a 9×9×3 k-point mesh is adopted for the calculation of electronic properties. According to the opposite magnetic moments coupling on short distance pairs of Fe atoms and the equal magnetic moments placed on larger distance in rhombohedral primitive cell, the antiferromagnetic (AFM) ordering of α-Fe2O3 is arranged. Using the GGA+U method, corresponding lattice constants for the hexagonal unit cell are a=b=5.071 Å and c=13.882 Å, in good agreement with previous theoretical51 and experimental results.52,53 Moreover, the calculated band gap (2.1 eV) and the Fe magnetic moments (±4.152 µB) based on the optimized geometric structure are also consistent with previous results, which are applied to all the calculations in this work (see Supporting Information for details).

ASSOCIATED CONTENT Supporting Information. A listing of the contents of each file supplied as Supporting Information should be included. For instructions on what should be included in the Supporting Information as well as how to prepare this material for publications, refer to the journal’s Instructions for Authors. The following files are available free of charge. Calculation details. Necessary characterizations mentioned in the main text (PDF)


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AUTHOR INFORMATION Corresponding Author * [email protected] * [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Grant Nos. 51571108 and 51371095).

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FIGURES CAPTION

Figure 1. (a) The TEM image of α-Fe2O3/rGO hybrid nanocomposites. The inset shows the partly enlarged TEM image. (b) High-resolution TEM image of a single α-Fe2O3 nanoparticle.

Figure 2. (a) XRD patterns of α-Fe2O3/rGO nanocomposites. (b) Raman spectra of GO and α-Fe2O3/rGO nanocomposites. (c) High resolution Fe 2p and (d) High resolution C 1s XPS spectra of α-Fe2O3/rGO nanocomposites.

Figure 3. (a) The magnetic hysteresis loop for α-Fe2O3/rGO composites at room temperature. The inset shows the magnified views of the low-field region. (b) Schematic drawing of model V3 of α-Fe2O3 surface and the side view of charge difference density for graphene on the top of α-Fe2O3 surface. The isosurface value is 0.001 e Å-3. Yellow and blue regions represent net charge gain and loss, respectively. The dark brown, red and golden brown spheres represent C, Fe and O atoms, respectively. (c) Total DOS of V3 heterostructure and partial DOS of interfacial atoms. Fermi level is indicated by the vertical line and set to zero. (d) Magnetotransport properties of α-Fe2O3 and α-Fe2O3/rGO composites at room temperature.


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Figure 4. (a) Schematic diagram of the electromagnets-photocatalysis apparatus. (b) Photocatalytic degradation of RhB with various magnetic fields in the presence of αFe2O3/rGO hybrid nanostructures under Xe light irradiation. (c) Kinetic curves of the degradation of RhB by α-Fe2O3/rGO composites under different magnetic fields. (d) Photocatalytic degradation of several different pollutants between NMF and 6 kOe fields in the presence of α-Fe2O3/rGO hybrid nanostructures under Xe light irradiation of 40 minutes. The inner error bars correspond to the standard errors of the mean within each measurement.

Figure 5. (a) Profiles of time versus transient photocurrent density of α-Fe2O3/rGO under Xe light irradiation with or without magnetic field. (b) Electrons lifetime as a function of open circuit potential with or without magnetic field.

Figure 6. Schematic illustration of proposed photocatalytic mechanism in the α-Fe2O3/rGO composites without (a) and with (b) magnetic fields.


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Figure 2. 131x108mm (300 x 300 DPI)


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Figure 4. 124x97mm (300 x 300 DPI)


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