Efficient photocatalytic degradation of acid fuchsin in aqueous solution using separate porous tetragonal-CuFe2O4 nanotubes

Journal of Hazardous Materials 284 (2015) 163–170

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Efficient photocatalytic degradation of acid fuchsin in aqueous solution using separate porous tetragonal-CuFe2 O4 nanotubes Panpan Jing, Jianan Li, Lining Pan, Jianbo Wang, Xiaojun Sun, Qingfang Liu ∗ Key Laboratory for Magnetism and Magnetic Materials of Ministry of Education, Lanzhou University, Lanzhou 730000, People’s Republic of China

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• The microstructure and component of the t-CuFe2 O4 are characterized in detail. • Porous t-CuFe2 O4 nanotubes perform a robust and durable photocatalytic activity. • Porous t-CuFe2 O4 nanotubes have significant component and structure stability. • An applied magnetic field can effectively recycle the used t-CuFe2 O4 nanotubes.

a r t i c l e

i n f o

Article history: Received 11 August 2014 Received in revised form 25 October 2014 Accepted 14 November 2014 Available online 15 November 2014 Keywords: Photocatalysis Porous nanotube Magnetic separation Tetragonal-CuFe2 O4

a b s t r a c t To develop a new promising magnetic photocatalyst, homogeneous tetragonal-CuFe2 O4 (t-CuFe2 O4 ) nanotubes were successfully synthesized via the electrospinning technique followed by heating treatment. The detailed investigation of chemical phase and microstructure reveals that the obtained samples are inversely spinel CuFe2 O4 nanotubes with an average diameter of about 272 ± 2 nm, which are assembled by numerous CuFe2 O4 single crystal nanoparticles with regular polyhedron structure and possess a very outstanding porous feature. Furthermore, element mapping, UV–vis adsorption spectrum, N2 adsorption–desorption isotherm, and magnetic hysteresis loop indicate that these t-CuFe2 O4 nanotubes have uniform component distribution, strong light response in the range of 200 nm–800 nm, considerable specific surface area of 12.8 m2 /g and porosity of 15.5 nm, and enough magnetization of about 18 emu/g. Therefore, the t-CuFe2 O4 nanotubes show an excellent catalytic activity and durability for the photodecomposition of acid fuchsin dye in aqueous solution under a simulated sunlight source. Furthermore, these CuFe2 O4 nanotubes could be acted as an eco-friendly and recyclable photocatalyst because they can be efficiently separated from the residual solution. Finally, a mechanism is presented for the significant photocatalytic performance of the porous CuFe2 O4 nanotubes. © 2014 Published by Elsevier B.V.

1. Introduction Million tons of synthetic dyestuffs and chemical pesticides, such as, acid fuchsin (AF, C20 H17 N3 Na2 O9 S3 ), rhodamine B (RhB, C28 H31 ClN2 O3 ), and monocrotophos are widely applied in industry

∗ Corresponding author. Tel.: +86 13321221629. E-mail address: [email protected] (Q. Liu). http://dx.doi.org/10.1016/j.jhazmat.2014.11.015 0304-3894/© 2014 Published by Elsevier B.V.

and agriculture each year [1,2]. Due to high toxicity and persistence, these harmful and external “essentials” usually cause a terrible pollution to the environment [3,4]. Some remedies including biodegradation [5], adsorption [6], and others have been implemented. Comparing with the strategies mentioned above, a more facile and eco-friendly photocatalytic degradation technique, which is based on “advanced oxidation process” (AOPs) [7], is proposed to manage environment pollution [8,9]. Since the pioneering photocatalytic experiment of water splitting using a

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TiO2 electrode was reported by Fujishima and Honda [10], a relatively large amount of research about photocatalysts has been conducted, which is chiefly focused on understanding the potential principles, enhancing their efficiency and achieving practical applications. For example, Ferguson studied the reaction kinetics of the photocatalyzed oxidation of As(III) on TiO2 in 2005 [11]. The highly efficient photocatalysis of novel CuS/ZnO heterostructures was reported by Lee and Yong [12]. It is common knowledge that the pure TiO2 (Eg = 3.0–3.2 eV [13], Eg is the band gap) and ZnO (Eg = 3.37 eV [12]) can only be excited by the UV-light, which impedes their availability. Accordingly, for the sufficient utilization of solar energy containing about 46% visible-light [14], other photocatalysts have been prepared, such as, Ag3 PO4 [15], Bi2 WO6 [16], and NiTiO3 [17]. These photocatalysts show favorable catalytic activity and applicability, however, their timely separation from purified aqueous solutions becomes a challenging problem. If the used catalysts could not be recovered completely, secondary pollution might be triggered by the residual photocatalyst powder [9]. What is more, recycling is also another important aspect for a practical photocatalyst as it can tremendously improve the cost-benefit [18]. Fortunately, some photocatalysts with good magnetism could overcome the recycling problem. Spinel ferrites (MFe2 O4 , M = Ni, Co, Cu, and Zn) with a closepacked structure of oxygen anions have been extensively studied due to their unique dielectric, optical, and magnetic properties [19–21]. As one of the typical magnetic semiconductor materials, copper ferrite (CuFe2 O4 ) shows potential applications in magnetic memory, high-frequency devices, sensors, drug delivery, and anode materials for lithium ion battery [22–24]. In general, CuFe2 O4 can either be described as a cubic Fd3m structure or a tetragonal I41 /amd structure depending upon the concentration of Cu2+ occupying the octahedral sites [25] (the tetragonal phase is more stable at room temperature and it can transform to cubic phase because of the Jahn–Teller effect [22]). In recent years, CuFe2 O4 nanomaterials also present a desirable catalytic performance in hydrogen (H2 ) generation and organic pollutants removal. For instance, Yang et al. fabricated the tetragonal CuFe2 O4 nanoparticles and evaluated their photocataytic activity through the H2 production under visible-light irradiation [26]. In 2013, Zhu et al. reported that the 4-chlorophenol (4-CP) was photocatalytically degraded using the pure CuFe2 O4 and Ag–CuFe2 O4 nanoparticles synthesized by the co-precipitation route [27]. Combining the advantages of superior light response and robust crystal structure, furthermore, researchers have always fabricated composites (or heterojunctions) of CuFe2 O4 and other materials, such as, CuFe2 O4 /MgFe2 O4 [28], CuFe2 O4 /CdS [29], CuFe2 O4 /SnO2 [30], and CuFe2 O4 /Bi4 Ti3 O12 [31]. It is very important to underline that most of explorations about photocatalysts are based on nanostructures because of their large specific surface ratio (S/M), special photoelectronic property [32,33]. Unfortunately, aggregation is a universal drawback for these granular nano-photocatalysts because of their highly active surfaces. One dimensional (1D) nanotubes with length of several micrometers, which have a high length-tube diameter ratio up to above 100 [34], work surprisingly well because they not only effectively avoid the aggregation phenomenon but also can acquire an outstanding rapid and long-distance electron-transport capability, an ion-exchangeable ability, and an enhanced light absorption [35]. Today, lots of CuFe2 O4 nanomaterials have been synthesized by different methods including hydrothermal method [36] and sol–gel combustion method [23,37] and so forth [38,39]. Nevertheless, there are hardly any results about photocatalytic activity of CuFe2 O4 nanotubes. In the present work, magnetic photocatalyst of tetragonalCuFe2 O4 (t-CuFe2 O4 ) nanotubes with outstanding polyporous structure were successfully synthesized via a standard singleneedle electrospinning technique followed by an accurate heating

treatment. Through a photodegradation experiment of AF dye in aqueous solution, it is clear that the prepared polyporous CuFe2 O4 nanotubes show a satisfying photocatalytic activity and endurance. More specifically, the used CuFe2 O4 powders can maintain a robust structure and component stability and be easily separated from the treated solution using a magnetism. 2. Experiment details 2.1. Preparation of the magnetic porous t-CuFe2 O4 nanotubes The magnetic polyporous t-CuFe2 O4 nanotubes were prepared using a single-needle electrospinning technique assisted by sol–gel method. All the reagents used in this work are chemically pure and without further purification. The precursor for electrospinning was synthesized by following two steps. Firstly, a transparent solution was obtained by dissolving about 0.242 g Cu(NO3 )2 ·3H2 O and about 0.808 g Fe(NO3 )3 ·9H2 O in a 10 mL binary co-solvent of N,N-dimethyl formamide (DMF) and ethanol at 1:1 under vigorous magnetic stirring for 2 h at room temperature. Secondly, about 0.764 g polyvinylpyrolidone (PVP, Mw = 1,300,000) was added into 8.786 g nitrate solution extracted from the above solution then after stirring for about 8 h, a clear nitrate/PVP sol was formed. In a typical electrospinning process, a certain amount of the nitrate/PVP sol was held in a glass syringe equipped with a flat-tipped stainless needle acting as a spinneret. The needle, whose inner diameter was about 0.4 mm, was connected to a +14 kV voltage supplied by a DC high-voltage source. Right below the tip of the needle about 14.5 cm, there was placed a steel frame used for collecting the asspun nanofibres. The spinning rate was 0.4 mL/h maintained by a micro-injection pump. When the electrospinning finished, a nonwoven nitrate/PVP fibrous membrane was transferred from the collector to a muffle and was annealed for 3 h at 650 ◦ C. The heating rate was 60 ◦ C h−1 . Finally, the ultimate CuFe2 O4 nanotubes were prepared. 2.2. Characterizations of the magnetic porous t-CuFe2 O4 nanotubes Field emission scanning electron microscopy (FESEM, Hitachi S-4800) and transmission electron microscopy (TEM, TecnaiTM G2 F30, FEI) equipped with a high-angle annular dark-field (HAADF) detector were applied to observe the morphology of the obtained CuFe2 O4 nanotubes. The crystal structure information was investigated by powder X-ray diffraction (XRD, D/max-2400,  = 1.5406 Å) patterns and high-resolution transmission electron microscopy (HRTEM) images. In order to determine the surface chemical states and the metal ions position and distribution, X-ray photoelectron spectroscopy (XPS, Kratos AXIS Ultra DLD, monochrome Al target, 600 W) and energy dispersive X-ray (EDX) mapping also were analyzed. Through measuring N2 adsorption–desorption isotherms, the specific surface area (SBET ) was calculated according to the multipoint Brunaure–Emmett–Teller (BET) model and the pore volume was calculated from the desorption branch based on the Barrett–Joyner–Halenda (BJH) equation. The magnetic parameters and UV–visible absorption spectrum at room temperature were obtained by a vibrating sample magnetometer (VSM, Lakeshore 7403) and a spectrophotometer (TU-1901), respectively. 2.3. Photocatalytic evaluation of the magnetic poroust-CuFe2 O4 nanotubes The photocatalytic activity of the as-prepared porous CuFe2 O4 nanotubes was assessed by degrading the AF dye in aqueous solution under simulated sunlight radiated by a halogen tungsten lamp (the power is 175 W and the wavelength of emitted light is mainly

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Fig. 1. Typical SEM images and diameter distribution histograms of samples: (a–c) as-spun nitrate/PVP precursor nanofibres; (d–f): as-annealed porous t-CuFe2 O4 nanotubes.

in the range of from 300 nm to 1000 nm). In a typical photocatalytic experiment, 30 mg CuFe2 O4 powders were dispersed in a 50 mL AF solution (15 mg/L) and stored in a glass beaker reactor. Before irradiation, the reactor was stirred in dark for 45 min to obtain a good adsorption–desorption balance between CuFe2 O4 nanotubes and dye molecules or other ions. At given intervals of 15 min during the photocatalytic reaction, ∼4 mL suspension was sampled and magnetically separated the used CuFe2 O4 powders to monitor the changes of dyes concentration by a TU-1901 spectrophotometer. To declare the endurance of polyporous CuFe2 O4 naotubes, the cycling test was also performed using a same butch photocatalyst. Furthermore, the self-degradation experiment of the AF without photocatalyst was also conducted. 3. Results and discussion Fig. 1(a) and (b) shows the typical SEM images of partial asspun Fe(NO3 )3 /Cu(NO3 )2 /PVP composite nanofibres with smooth surfaces. These nanofibres pile up together in a bottom–up order and develop a non-woven fabric. Their diameter distribution is presented in Fig. 1(c), and the average diameter is about 498 ± 2 nm. Our previous works have not only proved that CuFe2 O4 and CoFe2 O4 ferrites [40,41] could be well crystallized at temperatures of over 500 ◦ C but also some interesting nanostructures can be acquired by accurately controlling the annealing conditions in the post-treatment. As seen in Fig. 1(d) and (e), favorable porous nanotubes of several micrometers in length were

successfully prepared through sintering the as-spun nitrate/PVP precursor nanofibres at 650 ◦ C for 3 h with a heat rate of 1 ◦ C/min. Obviously, the outside diameters of these nanotubes are reduced to about 190–330 nm (Fig. 1(f)) and their average value is calculated to be about 272 ± 2 nm. The walls of these nanotubes are composed of nanoparticles along the axis direction of the nanotube. Impressive subsidence holes, which should be ascribed to the random decomposition of PVP and the crystallization of nanoparticles, are distributed on the walls and extend to the interior of the nanotubes. Such holes, actually, are the cracks among nanoparticles and they are also illustrated by the TEM images (Fig. 2(a) and (b)). More importantly, these pores are very favorable to increase the specific surface area of the sample and to the adsorption and permeability of the contaminants molecular and incident light [42]. The specific formation mechanism of the nanotubes prepared in this work has been clarified in the earlier study [43]. To reveal more information about components and microstructures of the synthesized nanotubes, XRD, and TEM characterizations were tested. It can be clearly observed that, as shown in Fig. 2(a), the porous products possess uniform morphology of hollow fibers, which is consistent with the SEM results. The average inner diameter and the wall thickness of these nanotubes are estimated to be about 165 ± 2 nm and 53 ± 1 nm, respectively. Fig. 2(b) displays one nanotube with an outside diameter of about 255 nm and a wall thickness of about 55 nm. The nanoparticles inside this nanotube are regular polygon structures, which can be observed in the magnified TEM image (Fig. 2(c)) of the selected area in Fig. 2(b).

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Fig. 2. (a) Low magnification TEM image, (b and c) high magnification TEM image (d) HRTEM image, (e) XRD pattern, (f) Cu 2p3/2 XPS spectrum and (g) Fe 2p3/2 XPS spectrum of the prepared porous CuFe2 O4 nanotubes.

If we assume that this nanotube is a single wall nanotube (i.e., these nanoparticles are non-overlapping along the vertical direction of the tube-wall), it can be deduced that the size of these nanoparticles equals to the wall thickness of about 55 nm. A HRTEM image (Fig. 2(d)) of the nanoparticle (shown in Fig. 2(c)) makes clear that it is a single crystal and the fringe spacing is about 2.42 Å which corresponds to the crystal spacing of the (2 0 2) plane of CuFe2 O4 . In the XRD pattern, all the diffraction peaks can be indexed well as the body-centered CuFe2 O4 (JCPDS Card No. 34-0425, space group I41 /amd) with inverse spinel structure [44]. These polycrystalline CuFe2 O4 porous nanotubes are constructed by single crystals of t-CuFe2 O4 . Formula for the tetragonal crystal system, the lattice parameters (c and a) are as follows: sin2  =

2 2 2 (h + k2 ) + 2 l2 2 4a 4c

(1)

where  and  are diffraction angle and wavelength of incidence Xray, respectively, h, k, and l are Miller indexes. Therefore, the unit cell parameters of the prepared CuFe2 O4 nanograins are calculated to be a = 5.831 Å and c = 8.60 Å, which basically is in agreement with the reported value (a = 5.844 Å, c = 8.630 Å). Moreover, the average crystallite size is estimated to be about 51 nm according to the Debye–Scherrer equation [45]. It is compatible with the result of TEM analysis. Apart from the good crystallinity observed in the XRD pattern, the prepared CuFe2 O4 nanotubes also display a high purity because of no significant peaks of other phases. X-ray photoelectron spectroscopy (XPS) was used to investigate the surface properties and the ions position in tetrahedral (A) and octahedral (B) sites of the prepared t-CuFe2 O4 . The highresolution spectra of Cu 2p3/2 peak and Fe 2p3/2 peak are displayed in Fig. 2(f) and (g), respectively. The Cu 2p3/2 spectra includes two

components at 932.2 eV and 933.7 eV for Cu+ ions on (A)-sites and Cu2+ ions on (B)-sites [46], respectively. Comparative analysis of the whole Cu 2p3/2 spectra shows that the Cu2+ (B) ions are in the majority. In the high binding energy side of the principal line, a satellite peak is also observed (Fig. 2(f)). Besides the satellite peak, in Fig. 2(g), the Fe 2p3/2 peak can also be divided into two major peaks at 710.1 eV and at 711.9 eV, corresponding to Fe3+ (A) ions and Fe3+ (B) ions, respectively [47]. And the number of Fe3+ ions on octahedral sites is basically equal to that on tetrahedral sites (n(Fe3+ (A))/n(Fe3+ (B)) = 0.45/0.46 ≈ 1). Consequently, the above results demonstrate that the porous t-CuFe2 O4 nanotubes with inverse spinel structure have been successfully synthesized. For an excellent 1D photocatalyst, elements distribution is also very important. EDX analysis of the as-prepared t-CuFe2 O4 nanotubes is shown in Fig. 3. In Fig. 3(a), a detail-rich HAADF-STEM image further proves the uniformly porous tubular morphology of the sample, which has been shown in Fig. 2(a). The EDX spectra (Fig. 3(f)) expound the three major elements (Cu, Fe, and O) in obtained products and the molar ratio of Cu/Fe/O is basically consistent with the stoichiometric ratio of CuFe2 O4 . The C and Ni peaks appearing in the EDX spectrum can be ascribed to the nickel–carbon grid used for EDX analysis. In order to determine the element distribution of the nanotubes, the EDX mapping is performed. Fig. 3(c–e) shows the EDX mapping of O, Fe, and Cu elements of the obtained nanotube in Fig. 3(b). It is clear that the required elements (O, Fe, and Cu) of CuFe2 O4 are evenly distributed in the nanotube, meaning that the t-CuFe2 O4 nanotubes possess uniform component which is good for the photocatalytic performance. Taking into account the nanostructure, good crystallinity, purity, and uniform component, it is expected that the porous tCuFe2 O4 nanotubes have an obvious efficiency in the photocatalytic

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Fig. 3. (a and b) Typical HAADF-STEM images of the prepared t-CuFe2 O4 nanotubes. (c–e) EDX mapping of O, Fe, and Cu elements of the selected nanotube shown in (b). (f) EDX spectrum of the sample shown in (a).

Fig. 4. (a) Light absorption spectra of AF solutions at different times by the porous t-CuFe2 O4 photocatalyst with 90 min. (b) Time dependent of relative concentration of residual AF solution under the light irradiation with or without photocatalyst. (c) Typical SEM image and (d) XRD pattern of the recycled t-CuFe2 O4 nanotubes; (e) cycling test of same batch t-CuFe2 O4 nanotubes for the photodecomposition of AF.

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degradation of organic contaminants. To evaluate the photocatalytic performance of the resultant t-CuFe2 O4 nanotubes, the catalytic decomposition process of the AF molecules in aqueous solution were conducted (the initial pH is ∼5.0 [48]) under simulated sunlight irradiation at given interval times. With increasing irradiation time, as displayed in Fig. 4(a), the absorption intensity of the strongest peak at  = 543 nm of AF gradually decreases and almost disappears after 90 min under the photocatalysis of CuFe2 O4 nanotubes. And there are no new peaks observed, which means that the AF molecules are mainly degraded into harmless H2 O, CO2 , and mineral acids (NO2 − , NO3 − , or SO4 2− ) [48]. To make a comparison, the self-degradation experiment of the AF without CuFe2 O4 nanotubes under the same light condition was also examined. When the pure AF solution (in the absence of CuFe2 O4 powders) was exposed to the light irradiation for the same time period, the decomposed concentration of AF (the red curve in Fig. 4(b)) in the solution was only about 15%. Consequently, the disintegration of the AF dye was attributed to the photolysis of CuFe2 O4 nanotubes. Fig. 4(b) shows the degradation curves of AF with (black) and without CuFe2 O4 photocatalysts (red), presented as the time-dependent normalized dye concentration (Ct /C0 , where Ct and C0 represent the final concentration at time t and the initial concentration of the AF solution, respectively). Evidently, this result suggests the high photocatalytic activity of the prepared porous t-CuFe2 O4 nanotubes in the degradation of AF. In addition, after the photocatalytic reaction finished, the residual CuFe2 O4 powders suspending in the solution should be recycled immediately. As seen in the inset of Fig. 5(c), they are completely separated from the liquid by a little magnet within about 1 min. After being washed with the deionized water and dried, these separated CuFe2 O4 powders were detected by the XRD and SEM again, and the result can be seen in Fig. 4(c) and (d). The

porous tubular nanostructures have been preserved largely intact except for the changes of length and no distinctive variations of crystal structure of the CuFe2 O4 nanotubes have occurred. In order to reveal the photocatalytic endurance of the used CuFe2 O4 nanotubes, cycling tests for the photodecomposition of AF were further performed, as shown in Fig. 4(e). It is distinct that although the catalysis rate of repeated cycles is slightly lower than that of the first cycles, the decomposition of AF is still remain approximately 80% over the reused CuFe2 O4 nanotubes in the fourth test. The slightly reduction of the catalytic efficiency of the CuFe2 O4 nanotubes is mainly ascribed to the little loss, which is, because the catalyst has a good dispersity in aqueous solution during every photocatalytic test. Therefore, our synthesized CuFe2 O4 nanotubes could be used as a durable and efficient photocatalyst because they possesses excellent features of stable structure and component. Porous t-CuFe2 O4 nanotubes perform outstanding photocatalytic activity and recovery, one point worth considering is that the photocatalysts should exhibit such properties as good light response and utilization, high specific surface area, remarkable magnetism, and amazing stability. As can be seen from Fig. 5(a), the UV–vis light adsorption spectrum indicates clearly that the t-CuFe2 O4 nanotubes have obvious response to both UV light ( < 400 nm) and visible-light (400 <  < 800 nm). From this spectrum, the Eg of the prepared t-CuFe2 O4 nanotubes is calculated to be about 1.53 eV, which is basically consistent with the earlier report [31], by using the Kebulka–Munk formula [49]: ahv = C(hv − Eg)

1/2

(2)

where a, h, , C, and Eg are respectively, optical absorption coefficient, Planck constant, incident photon frequency,

Fig. 5. (a) UV–vis light diffuse reflection spectrum, (b) nitrogen adsorption–desorption isotherm, (c) magnetic hysteresis loop and (d1–3) photocatalytic mechanism diagram of the porous t-CuFe2 O4 nanotubes. The insets in (c) are the photographs of the magnetic separation of t-CuFe2 O4 powders from the residual solution.

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a constant and direct energy band gap. In addition, the nitrogen adsorption–desorption isotherm of nanotubes is presented in Fig. 5(b). The Brunauer–Emmett–Teller (BET) surface area is measured to be about 12.8 m2 /g, which should be ascribed to its amazing porosity and hollow nanostructure. According to the pore size distribution curve (inset of Fig. 5(b)), the average size of the pores is about 15.5 nm. As we all know, the porous structure is beneficial to the absorption and diffusion of target molecules into the active sites of t-CuFe2 O4 nanoparticles, and thereby, promote the reaction between them. Moreover, the magnetic properties of prepared t-CuFe2 O4 nanotubes were studied by VSM in the field range from −12 to +12 kOe and the magnetic hysteresis loop is depicted in Fig. 5(c). The result shows that the porous samples exhibit a typical ferromagnetic behavior at room temperature, and the saturation magnetization (Ms), remanence (Mr), and coercivity (Hc) are about 18.0 emu/g, 11.4 emu/g and 1531 Oe, respectively. After the water purification, the used CuFe2 O4 powders could be completely separated from the residual solution by applying an external magnetic field within 1 min (insets in Fig. 5(c)). In order to understand well, the intrinsic principle of the AF dye in aqueous solution decomposed quickly by the photocatalysis of porous t-CuFe2 O4 nanotubes, an elaborate mechanism is illustrated by Fig. 5(d1–3). Firstly, in Fig. 5(d1), dyes molecules (AF), oxygen molecules (O2 ), and water molecules (H2 O) are absorbed on the surfaces of the nanotube photocatalyst (CuFe2 O4 ). Owing to the porous nanotube structures, these molecules can be fully absorbed not only by the outer surfaces but also by the internal surfaces of the CuFe2 O4 nanotubes. Secondly, AF molecules and CuFe2 O4 photocatalyst are irradiated by the incident lights. As shown in Fig. 5(d2), similarly, besides being directly captured by the CuFe2 O4 nanoparticles on the outer surface of the nanotubes, the incident lights can also be captured by the CuFe2 O4 nanoparticles on the inner surface by getting into the nanotubes through the pores. The multireflection effect in the interior of the nanotubes also promotes the capture of the incident lights to some extent. For the above two processes, the porous nanotube structures have made grate contributions to the absorption of the aforementioned molecules and the capture of the incident lights. In other words, they can offer much active sites for the photocatalytic reactions. Thirdly, AF molecules and CuFe2 O4 nanotubes are excited by the incident photons. On one hand, if the energy of some incident photons is capable for breaking the chemical bonds of the AF molecules, those excited AF molecules are self-disintegrated directly. On the other hand, as illustrated in Fig. 5(d3), when the ground state electrons of CuFe2 O4 absorb photons with a sufficient energy of h (namely, h = Eg ≈ 1.53 eV), these electrons (e− ) jump into the conduction band (CB) from the valence band (VB), meanwhile, the same number of holes (h+ ) in the VB are left. If these photogenerated carriers (e− and h+ ) become free, they shift instantaneously to the surface of the CuFe2 O4 nanoparticles and are captured by absorbed O2 , OH− . Of course, the absorbed AF molecules can also captured the carriers to ionize. After that, a lot of superoxide free radicals (O2 − ) and hydroxyl radicals (OH• ) are formed. Fourthly, these produced active radicals react with the ionized AF molecules to decompose them into the harmless H2 O, CO2 , and mineral acids (NO2 − , NO3 − , or SO4 2− ) [48,50,51]. During this step, partial strong oxidizing holes (h+ ) can also directly participate in the decomposition of the AF molecules. For the self-decomposition of AF molecules mentioned in the third step, the quantity is negligible according to the results of photocataltic experiment (Fig. 4(b)). Hence, the photocatalysis of the CuFe2 O4 nanotubes is the major reason for the decomposition of the AF molecules. In comparison to the photodecomposition of AF, however, the prepared t-CuFe2 O4 nanotubes could not accelerate the decomposition of RhB, which was also pretested in the present work (not shown here). For an availably catalytic decomposition process, the

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absorption of photocatalyst toward the containment molecules is one of the important factors. Researchers have pointed out that the CuFe2 O4 powder can effectively remove the organic dyes because it has a good adsorption capacity for the dye molecules under an acidic condition of pH < 5.5 [52]. In our experiment, the initial pH of the AF solution is within the above range but that of the RhB is not. Hence, the AF can be degraded by the photocatalysis of the photoacatalysis of CuFe2 O4 nanotubes. However, the pH affecting the adsorption capacity may be just one possible factor. Besides that, we also believe that once the CuFe2 O4 nanotubes are dipped in the AF solution, which offers an acid environment for catalytic reaction, the surface charge balance of catalyst grains will be broken and these surfaces will carry a positive charge, which electrostatically attracts the partial negatively charged electrons (e− ). It is good for the separation of the photogenerated carries (e− and h+ ) and enhances the photocatalytic activity of the t-CuFe2 O4 nanotubes. Based on the aforementioned analysis, the synergistic effect of the good light-response, the excellent porous nanotube structure, and the modest pH value of the dye solution contributes to the superior photocatalytic activity of the porous t-CuFe2 O4 nanotubes. Of course, there also exists possibility of some other circumstances contributing the photocatalytic efficiency of CuFe2 O4 . So far we have demonstrated excellent photocatalytic property of the prepared porous t-CuFe2 O4 nanotubes, however, some important factors influencing the enhancement of the photocatalytic activity, such as, annealing temperature, doping, recombination with other semiconductors, and etc., will be investigated in our next work. 4. Conclusion Magnetic photocatalyst of porous t-CuFe2 O4 nanotubes was successfully synthesized via a standard single-needle electrospinning process followed by an accurate heating treatment. According to the detailed characterization by different techniques, the prepared t-CuFe2 O4 nanotubes are assembled by numerous CuFe2 O4 single crystal nanoparticles with a regular polyhedron structure and an average size of about 51 nm along the long axis direction. It also indicates that these t-CuFe2 O4 nanotubes possess significant properties, such as, uniform component distribution, strong light response in the range of 200 nm–800 nm, considerable specific surface area of 12.8 m2 /g and porosity, and good magnetism of Ms = 18 emu/g and Hc = 1531 Oe. Due to the synergistic effect of the strong light-response, steady polyporous tubular structure, and uniform component, of the t-CuFe2 O4 nanotube photocatalyst shows a remarkable catalytic efficiency and endurance in the photodecomposition of the AF dye in aqueous solution. Moreover, the used t-CuFe2 O4 powders in the residual solution could be effectively recycled with the help of an external magnetic field, which could greatly promote their potential applications in eliminating organic pollutants and purifying waste water. Acknowledgements This work is supported by National Science Funds of China (NSFC) (51171075, 51371092) and the Fundamental Research Funds for the Central Universities (Lzujbky-2013-32). References [1] M. Ahmad, E. Ahmed, Z.L. Hong, W. Ahmed, A. Elhissi, N.R. Khalid, Photocatalytic, sonocatalytic and sonophotocatalytic degradation of rhodamine B using ZnO/CNTs composites photocatalysts, Ultrason. Sonochem. 21 (2014) 761–773. [2] F. Mazille, T. Schoettl, N. Klamerth, S. Malato, C. Pulgarin, Field solar degradation of pesticides and emerging water contaminants mediated by polymer films containing titanium and iron oxide with synergistic

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