GO nanocomposites for visible light-driven degradation of ciprofloxacin

Journal Pre-proofs One-step Synthesis of TiO2/γ-Fe2O3/GO nanocomposites for visible light-driven Degradation of Ciprofloxacin Feifei Wang, Xiaolin Yu, Maofa Ge, Sujun Wu PII: DOI: Reference:

S1385-8947(19)32794-9 https://doi.org/10.1016/j.cej.2019.123381 CEJ 123381

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

16 July 2019 6 October 2019 3 November 2019

Please cite this article as: F. Wang, X. Yu, M. Ge, S. Wu, One-step Synthesis of TiO2/γ-Fe2O3/GO nanocomposites for visible light-driven Degradation of Ciprofloxacin, Chemical Engineering Journal (2019), doi: https://doi.org/ 10.1016/j.cej.2019.123381

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One-step Synthesis of TiO2/γ-Fe2O3/GO nanocomposites for visible light-driven Degradation of Ciprofloxacin Feifei Wanga,b,c, Xiaolin Yu b*, Maofa Ge b, Sujun Wua,c a School b State

of Materials Science and Engineering, Beihang University, Beijing 100191, P. R. China

Key Laboratory for Structural Chemistry of Unstable and Stable Species, Beijing National Laboratory for

Molecular Sciences, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China c

Intl. Research Centre for Advanced Structural and Bio-Materials, Beihang University, Beijing 100191, P. R.

China

Abstract: A series of TiO2 and γ-Fe2O3 co-doped graphene oxide (GO) nanosheets (xTiO2/γ-Fe2O3/GO) were prepared by a facile self-assembly method. The characterization results show that the xTiO2/γ-Fe2O3/GO have the advantages of narrow band gap energies (< 2.47eV), and high structure stability. The incorporation of TiO2 into γ-Fe2O3/GO composite dramatically enhances the catalytic activity, and the 0.03TiO2/γ-Fe2O3/GO composite exhibits the highest catalytic efficiency (99%) for ciprofloxacin (CIP) degradation (10mg/L, 50 mL) within 140 min under visible light, which is ~1.5 times higher than that of γ-Fe2O3/GO composite without TiO2. Furthermore, the photo-Fenton like mechanism was proposed by quenching experiment and electrochemistry method. The intermediates of the CIP degradation were analyzed by the assistance of LC-MS technology, which provides insight into the pathway of CIP degradation. Keywords: Photo-Fenton like process, TiO2/γ-Fe2O3/GO, Visible light, Ciprofloxacin 1. Introduction Nowadays, antibiotics are widely used in human and animal remedy to treat bacterial infections. Ciprofloxacin (CIP) is a second-generation fluoroquinolone antibiotics, which processes a carboxylic acid group in the quinolone moiety (pKa1=6.1) and an amine group in the piperazine moiety (pKa2=8.8) [1]. CIP exists as

Corresponding

author. tel: +86 1082316326; fax: +86 1082317108.

E-mail: [email protected] , [email protected]

different species: anionic, cationic, or zwitterionic form at different pH [1]. The interaction of C6 flurine and C7 piperazine exhibits enhanced Gram-negative and Gram-positive antibacterial activity, which has been extensively used in the treatment of human and livestock infectious diseases [2]. However, the CIP usually cannot be completely absorbed or metabolized in human and animal body [3]. Most of CIP are excreted in their pharmacologically active forms into the environment via urine and faeces. In addition, wastewater from hospitals and drug manufacture also contains a large amount of CIP. More seriously, the excessive use and relatively high chemical stability result in a high residual concentration (40 ng/L) of CIP in groundwater samples [4]. Those groundwater containing CIP are harmful to human health because CIP directly affects bacterial resistance. Therefore, it is necessary to explore a promising method for CIP removal in wastewater treatment. Advanced oxidation processes (AOPs) and photo-catalysis as well as their combination have been used for degradation of CIP and other antibiotics. One of the most widely applied and efficient AOPs is Fenton-like process, which can generate free hydroxyl radicals (•OH) as primary active species to oxidize most pollutants [5,6]. Moreover, this process can be operated at different pH and the catalyst can be recycled, which are very important for practical wastewater treatment [7]. However, the drawbacks of high transportation and storage costs of concentrated H2O2, high energy inputs and secondary metal ions pollution limit the application of the AOPs technology [8]. Therefore, the introduction of visible light into AOPs is highly desired for their high efficiency, economy and environmental-friendliness. Magnehematite (γ-Fe2O3) has demonstrated great potentials in photo-Fenton like process, owing to its low cost, excellent magnetic properties, great biocompatibility, and suitable band gap energies (2.0-2.3 eV) to absorb a large fraction of the solar spectrum [9,10]. The developed γ-Fe2O3 heterogeneous catalyst can be used as iron source for Fenton catalyst, which greatly widens the using range of the pH and reduces the substantial iron sludge. However, the γ-Fe2O3 is still not used widely in photo-catalysis due to its low carrier mobility, poor electron-hole lifetime (~10 ps) and short diffusion distance (2-4 nm) [11]. In this regard, constructing heterojunction

between γ-Fe2O3 and other semiconductors is an effective strategy for promoting charge carriers migration [10]. Among several of semiconductors, titanium dioxide (TiO2) is considered as an economical photo-catalyst because of its high photo-catalytic activity in the ultraviolet (UV) region (<5% of the solar energy), chemical stability and slow recombination rate of photo-generated charge carriers [12-14]. It has been shown that coupling TiO2 with Fe2O3 not only improve the light absorption of TiO2 towards visible light, but also can utilize the magnetic separation of the γ-Fe2O3 [12]. Li et al. confirmed that the Fe-incorporation induced the red-shift of the absorption edge of TiO2 microsphere into the visible light range [15]. Muñoz et al. found that the Fe2O3/TiO2 composite exhibits higher Fenton activity than commercial P25 catalyst, which is attributed to the broader absorbance spectrum and generation of larger amounts of HOx• of the Fe2O3/TiO2 materials [16]. Xing et al. prepared an interphase boundaries of Fe2TiO2 between TiO2 phase and Fe2O3 phase, which could improve the migration of photo-generated electrons [17]. However, there still remained some problems in practice, such as long reaction time, fast recombination of photo-generated electron and hole. Graphene oxide (GO) is a promising electron mediator to capture or shuttle the photo-generated electrons due to oxygen functional groups, large surface area and high electron mobility. Hybridizing γ-Fe2O3 with GO can enhance the charge separation efficiency due to its high electron mobility [18]. As previously reported, the GO support is a key factor for sufficient utilization of electron conductivity [19]. Jo et al. provided that the adding GO into TiO2/α-Fe2O3 could improve the degradation rate of methylene blue. However, this ternary nanocomposite suffers from the drawback of time-consuming, complicated process and high calcination temperature for the removal of surfactant [20,21]. Herein, we developed a facile self-assembly method for preparing the ternary nanocomposites

of

TiO2/γ-Fe2O3/GO.

The

catalytic

properties

of

the

TiO2/γ-Fe2O3/GO with different TiO2 content were explored. The results showed that the introduction of TiO2 into γ-Fe2O3/GO composite could promote the catalytic performance in CIP degradation. In this regard, the role of the TiO2 in photo-Fenton process was explored and the related CIP decomposition mechanism was investigated.

2. Experimental 2.1 Materials Graphite powder (99.998 wt.%, 200mesh) was purchased from Alfa Aesar Ltd. (Shanghai, China). TiO2 (P25, Acros) was purchased from Acros (Beijing, China). Potassium permanganate (KMnO4, 99.5 wt.%), hydrogen peroxide (H2O2, 30 wt.%), isopropyl alcohol ((CH3)2CHOH, 99.9 wt.%), ammonium oxalate((NH4)2C2O4, 98 wt.%) and silver nitrate (AgNO3) were all obtained from Beijing Chemical Works (Beijing, China). The ciprofloxacin (CIP) with a molecular formula C17H18FN3O3 and γ-Fe2O3 NPs were purchased from Aladdin (Shanghai, China). Pure iron plates (99.9 wt.%) were purchased from Anyang Iron & Steel Group Co Ltd. All chemicals were of analytical grade and used without further purification. 2.2. Fabrication of the TiO2/γ-Fe2O3/GO composite Graphene oxide (GO) was prepared from graphite powder using a modified Hummers method [22]. The TiO2/γ-Fe2O3/GO composite was obtained by self-assembly method [18]. First, a series of TiO2/GO with TiO2 concentration of 0.005 M, 0.02 M, 0.03 M and 0.05 M (labeled 0.005, 0.02, 0.03 and 0.05TiO2/GO, respectively) were prepared by mixing these TiO2 suspensions with 1mg/ml GO suspensions. After that, the even and stable suspensions were formed by ultrasonic for 30 min. Second, iron plates substrates were mechanically polished by a series of silicon carbide papers followed by polishing with alumina compounds. Then, the plates were ultrasonically cleaned in alcohol and deionized water in sequence for 15 min. Third, the iron plates were immersed in the above TiO2/GO suspensions at 40 ºC where the TiO2/γ-Fe2O3/GO films were gradually formed on the surface of the iron. After 4 hr, the iron plates together with the TiO2/γ-Fe2O3/GO films were taken out of the suspensions and then oven dried at 60 ºC for a further 2 hr. Finally, the TiO2/γ-Fe2O3/GO films could be readily peeled from the iron substrate and then ground into powder for catalytic evaluation. The obtained samples were denoted as xTiO2/γ-Fe2O3/GO, in which the x represents the concentration of TiO2 in suspensions.

2.3. Characterization xTiO2/γ-Fe2O3/GO were characterized by x-ray diffraction (XRD, PANalytical, Empyrean) using Cu Kα radiation (λ = 1.54184 Å) at room temperature. Surface morphology and elemental composition were observed by transmission electron microscopy (TEM, Jeol, JSM 2100F) coupled with electron dispersive spectroscopy (EDS). An investigation of the surface components of the catalyst was carried out on x-ray photoelectron spectroscope (XPS, Thermo Scientific, Escalab 250Xi) using Al Kα monochromatized radiation. Photoluminescence (PL) spectra were obtained on an Edinburgh FLS980 fluorescence spectrometer. Iron ions were tested by an inductively coupled plasma mass (Thermo iCAP RQ, Thermo Fisher). Electrochemical impedance spectroscopy (EIS) was employed to analyze the electron transfer ability using a CHI660D electrochemical workstation (CH instruments, Shanghai, China), with the measurements conducted in a three-electrode system. A cleaned ITO glass after loading the catalyst powders was used as the working electrode, while the Pt electrode, calomel electrode and 1 M Na2SO4 were employed as the counter electrode, reference electrode and electrolyte, respectively. The impedance spectra of the catalysts were recorded over the frequency range from 100 kHz to 0.01Hz. 2.4. Photo-Fenton like catalytic evaluation The catalytic activities of the samples were evaluated by catalytic degradation in CIP aqueous solution. A 300 W xenon lamp with a 420 nm cutoff filter was located 15 cm above the center of the solution as the visible light irradiation source. 20 mg of catalysts was added to the 50 ml CIP solution (10 mg/L, pH=6.60). Prior to irradiation, the catalytic systems without H2O2 were stirred in the dark for 30 min to establish adsorption-desorption equilibrium. The reaction was initiated by adding 100 μL H2O2 and the xenon lamp was turned on at the same time. The CIP concentration was analyzed by using a high performance liquid chromatography (HPLC, Shimadzu, LC20AD) with an Shimadzu INERTSUSTAIN C18 column (4.6 × 250 mm, 5 μm) and a UV detector at 274 nm. The mobile phase was a mixture of 4 % acetic/methanol (V/V, 60:40) with a flow rate of 0.5 ml/min. The intermediates of CIP degradation were analyzed by liquid chromatography-mass spectromrtry (LC-MS, Thermo

Scientific, Orbitrap Fusion) with a electrospray ionization in positive ionization mode, and the mobile phase was a mixture of 1 ‰ acetic/methanol (V/V, 50:50) with a flow rate of 0.2 ml/min. 3. Results and Discussion 3.1 Materials characterization The crystalline structure of the TiO2/γ-Fe2O3/GO composite with different TiO2 content was investigated by XRD as shown in Fig. 1a. It can be seen that there are two phases of TiO2 and γ-Fe2O3 in the XRD spectrum. The peaks at 2θ=30.5°, 35.8°, 43.5°, 53.8°, 57.4° and 63.0° are indexed to the (220), (311), (400), (422), (511) and (440) planes of γ-Fe2O3, respectively. The diffraction peaks of TiO2 at 25.2° and 48.0° are attributed to the planes of anatase structure, while the peak at 27.4° can be indexed to the (110) planes of rutile structure. The results indicate that the TiO2/γ-Fe2O3/GO composite was fabricated successfully. In addition, it is noted that with increasing x, the peak intensities of TiO2 increased, while the intensities of γ-Fe2O3 peaks decreased, indicating the high crystallinity or high levels of TiO2 were incorporated into the TiO2/γ-Fe2O3/GO composite and the decrease in the crystallinity of γ-Fe2O3 during self-assembly process. Fig. 1b shows the Raman spectra of GO and xTiO2/γ-Fe2O3/GO. The Raman peak of TiO2 at 154 cm-1(Eg) was observed in 0.02, 0.03, 0.05 TiO2/γ-Fe2O3/GO, while it was not found in 0.005 TiO2/γ-Fe2O3/GO owing to the low amount of TiO2. The GO curve shows two typical peaks at 1340 cm-1 and 1596 cm-1, which are attributed to the D band and G band, respectively. The intensity ratio of the two bands (ID/IG) was calculated to explore the crystal structure, disorder and defects of carbon [23]. Compared with GO (ID/IG), the ID/IG values of xTiO2/γ-Fe2O3/GO composites increased, which suggests that GO was partially reduced by iron and the reduction of GO could increase the electron density and extend the lifetime of electrons [20].

Fig. 1 XRD patterns and Raman spectra of xTiO2/γ-Fe2O3/GO composite

Fig. 2 shows that the morphology and microstructure of 0.03TiO2/γ-Fe2O3/GO composite by TEM. As can be seen from Fig. 2a, GO presents wrinkled and folded 2D nanosheets morphology, the naonoparticles (NPs) size of γ-Fe2O3 is about 10 nm and the TiO2 NPs is estimated in a range of 20-50 nm. TEM image further demonstrates that the γ-Fe2O3 and TiO2 are anchored on the GO nanosheets. In addition, the high-resolution transmission electron microscopy (HRTEM) image in Fig. 2b shows that the lattice fringes are ascribed to the (101), (112) facet of TiO2, and (311) facet of γ-Fe2O3. The energy dispersive X-ray spectroscopy (EDS) revealed the existence of O, Ti and Fe elements in the composites (Fig. 2c), which confirmed the TiO2 and γ-Fe2O3 NPs are tightly wrapped by GO during the self-assembly process.

Fig. 2 TEM image (a), high-resolution transmission electron microscopy (HRTEM) image (b) and

(c) corresponding element mapping image of 0.03TiO2/γ-Fe2O3/GO composite. Table 1 Sample compositions and the bonding information detected by XPS. Sample

Ti content (at%)

Fe content (at%)

Ti/Fe atomic ratio

0.005TiO2/γ-Fe2O3/GO

0.55

12.55

0.04

0.02TiO2/γ-Fe2O3/GO

2.05

16.15

0.13

0.03TiO2/γ-Fe2O3/GO

3.14

14.56

0.22

0.05TiO2/γ-Fe2O3/GO

4.41

15.35

0.28

XPS is used to further confirm the chemical compositions and chemical state of the resulting composites. Fig. 3a shows the Ti 2p high resolution spectra of the TiO2/γ-Fe2O3/GO. The two peaks at 464.6 and 458.8 eV are corresponding to Ti 2p1/2 and Ti 2p3/2, respectively [24]. The intensities of the two Ti 2p peaks increased with the increase of added TiO2 suspension, suggesting a higher incorporated Ti element into the composites. The characteristic peaks of Fe at 724.6 and 711.0 eV in Fig. 3b are ascribed to the Fe 2p1/2 and Fe 2p3/2, respectively [25]. The satellite peak of Fe 2p3/2 is located at about 719.2 eV, proving the formation of γ-Fe2O3 rather than Fe3O4 during self-assembly process, which is consistent with the XRD result. Table 1 shows the changes of the Ti/Fe atomic ratios of the various TiO2/γ-Fe2O3/GO composites. It can be seen that the atomic percentage of Ti was almost consistent with the nominal amount, while the atomic percentage of Fe had no conspicuous changes, indicating that the formation of γ-Fe2O3 in the composites cannot be affected by the concentrations of TiO2 suspension. In fact, the formation of γ-Fe2O3 in TiO2/γ-Fe2O3/GO

nanocomposites

was

dependent

on

the

GO,

and

the

oxygen-containing functional groups in GO can oxide iron species into γ-Fe2O3 [26]. Therefore, the Ti/Fe atomic ratios had an increase tendency. The XPS O1s could be fitted to three peaks at 529.9, 531.2 and 532.7 eV (Fig. 3c). The peak located at 529.9 eV can be assigned to the Ti-O bonds in TiO2 and Fe-O bonds in γ-Fe2O3 [17]. The peak at 531.2 eV and 532.7 eV are deduced to be hydroxyl with Fe or Ti and oxygen containing groups bonded with C atoms in GO, respectively. Fig. 3d shows the C 1s spectra that can be deconvoluted into three peaks at about 284.4, 285.4 and 288.6 eV,

corresponding to C-C/C=C, C-OH and O-C=O, respectively [27]. It was noted that the peak intensity of O-C=O in the TiO2/γ-Fe2O3/GO composites was very weak, indicating that the GO was partially reduced into graphene by iron plates during self-assembly process. This is in accordance with the aforementioned Raman results.

Fig. 3 XPS spectra of (a) Ti 2p, (b) Fe 2p, (c) O 1s and (d) C 1s for xTiO2/γ-Fe2O3/GO composite

The absorption edge and Tauc plots of the xTiO2/γ-Fe2O3/GO composites were analyzed to evaluate the band-gap energy. As can be seen from Fig. 4a, it is notable that the xTiO2/γ-Fe2O3/GO composites exhibit the stronger absorption in comparison with pure TiO2 (Fig. S1a) in the visible light region (400-760 nm), although slightly decreased absorption was observed in comparison to γ-Fe2O3/GO composites. This is due to the coupling of narrow band gap semiconductors of γ-Fe2O3 to enhance the visible light absorption [11]. On the other hand, the pure TiO2 have almost no visible light absorption, thus the incorporation of TiO2 weakens the visible light absorption [28]. The band gap energies of the xTiO2/γ-Fe2O3/GO composites were calculated by the reported equation: αhν = A (hν-Eg)n/2. The values of n for pure TiO2 and xTiO2/γ-Fe2O3/GO are 4 and 1, respectively. Fig. 4b shows the Tauc plots of the xTiO2/γ-Fe2O3/GO composites. The band gap energies of 0.005, 0.02, 0.03 and 0.05 TiO2/γ-Fe2O3/GO are about 2.29, 2.37, 2.43 and 2.47 eV, respectively. Those values

are smaller than 3.41 eV of the pure TiO2 (Fig. S1b) and larger than 2.24 eV of the γ-Fe2O3/GO composites, which proves that all the xTiO2/γ-Fe2O3/GO composites could be activated by visible light, although the introduction of TiO2 broadens the band gaps.

Fig. 4 (a)UV-vis-diffuse reflectance spectra of the samples and (b) Plots of (hv)2 vs. photon energy (hv) for the band gap energy.

3.2 Photo-Fenton process activity The photo-Fenton like performance of the prepared xTiO2/γ-Fe2O3/GO composites was investigated. As shown in Fig. 5a, the system containing only H2O2 has no catalytic activity for CIP degradation, which demonstrates that the H2O2 hardly generated •OH under visible light irradiation. The introduction of γ-Fe2O3/GO composite into the above system efficiently promotes the CIP degradation to 3.7 mg/L at 140 min. It is worth noting that the concentration of the CIP can be further reduced to a lower level (1.7 mg/L) when a small amount of TiO2 was incorporated into γ-Fe2O3/GO composite (x=0.005). Further increasing the amount of TiO2 to 0.02 and 0.03 resulted in the decrease of CIP concentrations to 1.1 and 0.1 mg/L, respectively. However, 0.05TiO2/γ-Fe2O3/GO composite exhibited the increased CIP concentration to 0.7 mg/L. This results from the “shading effect” of the higher loading TiO2, which can lead to less absorption of visible light [29,30]. It is notable that the 0.03TiO2/γ-Fe2O3/GO composite manifested the optimal reactivity and 99% CIP degradation was achieved at 140 min under visible light. However, the TiO2/GO composite in the absence of γ-Fe2O3 exhibited about 38% CIP degradation, which could be attributed to the weak absorption in visible light region and the low-efficiency Fenton-like process. Likewise, the CIP degradation rate of

TiO2/γ-Fe2O3 is lower than that of 0.03TiO2/γ-Fe2O3/GO, indicating the importance of GO as an electron acceptor and transporter [24]. Moreover, to ensure the major role of the heterogeneous catalytic reaction, the homogeneous catalytic reaction of CIP degradation rate was studied. The iron ions leached from 0.03TiO2/γ-Fe2O3/GO are 93.78 μg/L and Fig. S2 shows that the leaking iron ions show very low activity compared with γ-Fe2O3/GO+H2O2+visible system. Obviously, the removal of the CIP by xTiO2/γ-Fe2O3/GO catalysts follows a heterogeneous catalytic reaction. To exclude the effect of light or Fenton-like process, we performed the degradation experiment in the dark or without H2O2 by 0.03TiO2/γ-Fe2O3/GO composite. It is apparent that the CIP concentration at 140 min can be decreased to 4.9 mg/L for Fenton-like process and 5.5 mg/L for photo-catalysis, which are higher than that in photo-Fenton like reaction. Thus, it is inferred that synergetic effect between photo-catalysis and Fenton-like reaction should play a vital role in the CIP degradation, improving the catalytic

activity

of

TiO2/γ-Fe2O3/GO

composite.

The

corresponding

pseudo-first-order rate constant for degradation of CIP by 0.005, 0.02, 0.03 and 0.05TiO2/γ-Fe2O3/GO composites are 0.010, 0.014, 0.019 and 0.015 min-1, respectively, higher than that of γ-Fe2O3/GO (0.0048 min-1) composite (as can be seen from Fig. S3), demonstrating the introduction of certain TiO2 could result in an enhancement in CIP degradation. The rate constants of TiO2/γ-Fe2O3 and TiO2/GO are 0.013 and 0.0018 min-1, respectively, which are both lower than that of 0.03TiO2/γ-Fe2O3/GO. Moreover, the pH of the solution before and after reaction is measured. It can be seen from Fig. 5b that the pH increased from 6.60 to 8.58 after adsorption, which is attributed to the hydrophobic interactions, π bonding and weak van der Waals forces [31]. However, the solution pH decreased with the increase of catalytic reaction time. This may result from the generation of H+ and SO42- during the photo-Fenton catalytic process [32]. TOC removal efficiencies were used to determine the mineralization degree of antibiotics in the solution. Fig. S4 shows that the TOC removal efficiency of CIP by 0.005, 0.02, 0.03 and 0.05 TiO2/γ-Fe2O3/GO are 45%, 49%, 49% and 30% after 160 min, respectively. The stability of 0.03TiO2/γ-Fe2O3/GO was further evaluated through recycling the catalyst toward

photo-Fenton like degradation of CIP. As described in Fig. 5c, low concentration of CIP (0.5 mg/L) still can be achieved over four successive runs, suggesting its high stability. The slight decrease in catalytic activity might be attributed to the passivation of catalyst caused by residual contaminations, or the loss of catalyst during the recycling process [33]. The XRD pattern of used catalyst in Fig. S5 confirmed no obvious change in the crystalline phase, indicating the high structural stability.

Fig. 5 (a) Comparison of the Photo-Fenton like catalytic efficiencies of different systems, (b) the pH of the solution before and after reaction of the 0.03TiO2/γ-Fe2O3/GO composite (c) Cycle runs of photo-Fenton degradation of CIP with 0.03TiO2/γ-Fe2O3/GO composite.

3.3 Mechanism for CIP degradation The main active species formed in the photo-Fenton like process were explored for investigating the mechanism of CIP degradation. The scavengers of isopropyl alcohol (IPA, •OH scavenger), AgNO3 (electron scavenger), (NH4)2C2O4 (holes scavenger) are used to trap the major active species involved in degradation of CIP under photo-Fenton like process [34-36]. As shown in Fig. 6a, the reaction rate of the photo-Fenton like process is obviously inhibited after addition of IPA and (NH4)2C2O4, indicating that •OH and h+ as the relevant oxidative species play an important role in the degradation reaction. Also, the introduction of AgNO3 significantly suppressed the CIP degradation, suggesting that electrons play a dominant role in the photo-Fenton like process. The trapping experiment indicated that • OH and h+ should participate in the degradation process, and the electron dominated the degradation efficiencies. In addition, electrochemical impedance spectroscopy (EIS) Nyquist plots are tested to estimate the photo-generate charge separation and transfer properties of the composites. The arc radius on Nyquist plots represents the charge transfer resistance at the interface between electrode and solution. As shown in Fig. 6b, the arc radius of 0.03TiO2/γ-Fe2O3/GO composite is the

smallest among the xTiO2/γ-Fe2O3/GO composites, implying the low interfacial charge transfer resistance, high charge separation efficiency and slow charge recombination [27,37]. Besides, the PL intensity of the 0.03TiO2/γ-Fe2O3/GO composite in Fig. 6S is weaker than that of γ-Fe2O3/GO. The results indicated that the addition of TiO2 into the γ-Fe2O3/GO composite could effectively promote the electron transfer from γ-Fe2O3 to TiO2 via GO nanosheets, therefore inhibiting the recombination charge carriers [24].

Fig. 6 CIP degradation in the presence of radical scavengers (a) and EIS Nyquist plots of xTiO2/γ-Fe2O3/GO

Based on the above results of various characterization analysis and photo-Fenton like degradation experiments, a possible synergistic mechanism of enhanced catalytic performance over TiO2/γ-Fe2O3/GO composite was proposed in Fig. 7. Under the visible light irradiation, the γ-Fe2O3 can be easily excited to generate the electrons-hole pairs, while TiO2 cannot absorb the visible light due to the band gap of 3.41 eV (Fig. 7a). In this case, GO can serve as the electron reservoir to rapidly capture or shuttle the photo-generated electrons from conduction band (CB) of γ-Fe2O3. Subsequently, electrons are migrated to the CB of TiO2 via the conductive network of GO due to the coupling interfacial contact between GO and TiO2, while the photo-generated holes are accumulated in valence band (VB) of γ-Fe2O3, finally restraining the recombination of photo-generated electron-hole pairs. The electrons in CB of TiO2 can further react with H2O2 to produce •OH. Therefore, the incorporation of TiO2 into γ-Fe2O3/GO system could enhance the degradation efficiency. However, the nanocomposites with excessive or without TiO2 in the photo-Fenton system cannot degrade CIP effectively, which is due to the decreased absorption in visible

light, the broadened band gap of the TiO2/γ-Fe2O3/GO system and fast charge recombination of the γ-Fe2O3/GO system (Fig. 5a). Meanwhile, Fenton-like process is further facilitated on the surface of the catalyst because of the acceleration of the Fe(III)/Fe(II) cycle efficiency, which is confirmed by the degradation reaction in the dark (Fig. 5a). Fe(II) can be first formed by the reduction of Fe(III) in γ-Fe2O3 with H2O2 or with the electrons captured by GO, and partial generated Fe(II) further react with H2O2 to form •OH via the Fenton process (Fig. 7b). Thus, the introduction of the moderate amount of TiO2 into the γ-Fe2O3/GO composite can promote photo-Fenton like process for CIP degradation, which is benefited from the high separation efficiency, low charge recombination, long lifetime of the photo-generated electron-hole pairs.

Fig. 7 photo-Fenton catalytic mechanism of xTiO2/γ-Fe2O3/GO for CIP degradation under visible light irradiation

For better insight into the CIP degradation pathway of the 0.03TiO2/γ-Fe2O3/GO composite, the intermediates generated during the catalytic reaction were identified by LC-MS. Based on the comparative analysis of original, reaction 120 min and reaction 160 min of the samples, some intermediates in the catalytic process were thereby identified as: P1 (m/z=308.14), P2 (m/z=280.14), P3 (m/z=291.07), and P4 (m/z=263.08). It could be clearly found that CIP was effectively degraded in catalytic process because more intermediates with small molecular mass were produced after 160 min reaction (Fig. S7). Furthermore, the molecular structure of the four primary intermediates was analyzed by combining with above-obtained results and previous study. The degradation of CIP was usually occurred through hydroxylation, defluorination, carboxyl group substitution, and cleavage of cyclopropyl and

piperazine ring [38,39]. In present study, two main pathways for CIP degradation were proposed and displayed in Fig. 8. In pathway I, the piperazine ring and cyclopropyl were broken, where the formed P1 (m/z=308.14) was the dominant intermediates of CIP. This similar pathway was also observed in Martins’ study [3]. Then the P2 (m/z=280.14) was formed through substitution reaction of -COOH with •OH [40]. In pathway II, the CIP degradation was only initiated by the cleavage of the piperazine ring. The loss of -C2H2- at the piperazine ring promoted the oxidation of the terminal substituent of amino groups, then P3 (m/z=291.07) was formed [41]. Afterwards, the P3 transformed into P4 (m/z=263.08) through further oxidation of amino groups, leading to the total destruction of the piperazine ring [42,43].

Fig. 8 Proposed degradation pathways for CIP degradation in the 0.03TiO2/γ-Fe2O3 + H2O2 +vis system.

4. Conclusions In this work, a novel xTiO2/γ-Fe2O3/GO composite has been successfully prepared by a facile self-assembly method and the influence of TiO2 amount on catalysis of the xTiO2/γ-Fe2O3/GO

composites

was

explored.

It

was

found

that

the

0.03TiO2/γ-Fe2O3/GO composite exhibits optimal catalytic efficiency for CIP degradation, since combination of charge carriers could be highly suppressed at the interfaces of the γ-Fe2O3 and TiO2 NPs as well as the synergistic effect occurred. The repeat cycles experiment and XRD patterns of the composite before and after four cycles proved that the 0.03TiO2/γ-Fe2O3/GO composite owns a high structure stability.

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Highlights 1. Novel xTiO2/γ-Fe2O3/GO composites were prepared by a facile self-assembly method. 2. The addition of TiO2 in the composites enhanced the catalytic activity. 3. The possible synergistic mechanism and degradation pathway of CIP were discussed.