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Ag nanocomposites with enhanced photocatalytic activity under sunlight exposure
Author’s Accepted Manuscript Synthesis of Fe3O4 nanowire@CeO2/Ag nanocomposites with enhanced photocatalytic activity under sunlight exposure Jiali Chang, Qingliang Ma, Jianchao Ma, Hongzhu Ma www.elsevier.com/locate/ceri
PII: DOI: Reference:
S0272-8842(16)30510-7 http://dx.doi.org/10.1016/j.ceramint.2016.04.104 CERI12722
To appear in: Ceramics International Received date: 23 February 2016 Revised date: 19 April 2016 Accepted date: 19 April 2016 Cite this article as: Jiali Chang, Qingliang Ma, Jianchao Ma and Hongzhu Ma, Synthesis of Fe3O4 nanowire@CeO2/Ag nanocomposites with enhanced photocatalytic activity under sunlight exposure, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.04.104 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synthesis of Fe3O4 nanowire@CeO2/Ag nanocomposites with enhanced photocatalytic activity under sunlight exposure
Jiali Changa, Qingliang Mab, Jianchao Mac, Hongzhu Ma*a
a. School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi'an 710119, Shaanxi, P. R. China
b. Key Laboratory of Coal Science and Technology, Taiyuan University of Technology, Ministry of Education and
Shanxi Province, Taiyuan 030024, Shanxi, P. R.China
c. College of Mining Technology, Taiyuan University of Technology, Taiyuan 030024, Shanxi, P.R.China
Abstract Ternary magnetic Fe3O4 nanowire@CeO2/Ag nanocomposites have been firstly synthesized by means of hydrothermal and co–precipitation techniques, and their ability to adsorb, photocatalytic degradation organic
pollutants, methylene blue present in water, and separate, has been demonstrated. The results show that CeO2 and Ag nanoparticles are uniformly deposited on the surface of Fe3O4 nanowires. The photocatalytic experiments demonstrate that the Fe3O4@CeO2/Ag nanocomposites exhibit remarkably enhanced photocatalytic properties and stability compared to CeO2, CeO2/Ag, Fe3O4@CeO2, Fe3O4 under natural sunlight exposure. Moreover, excellent photocatalytic degradation efficiency for phenol and MO are also observed. The enhanced photocatalytic
performance may be attributed to the synergetic effect of Fe3O4 nanowire, CeO2 and Ag nanoparticles, which lead to the enhanced light harvesting, the promoted charge separation and enhanced adsorption capacity. In addition, the
Fe3O4@CeO2/Ag photocatalyst can be easily collected and separated by an external magnet. These results suggest that the nanocomposites could be exploited as potential candidates for solar photocatalysis.
*Corresponding author. Tel.: +86 29 81530726. Fax: +86 29 81530727. E–mail: [email protected].
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Keywords: Fe3O4 nanowire@CeO2/Ag; Photocatalysis; Sunlight; Surface plasmon resonance; Magnetic separation
1. Introduction Rapid industrialization and population growth have sparked a global crisis concerning energy shortage and environmental pollution. Semiconductor photocatalysis is a highly appealing process for water splitting for hydrogen generation and the degradation of organic pollutants [1, 2]. To date, many metal oxide semiconductors have been used in heterogeneous photocatalysis, including TiO2, ZnO, WO3, Cu2O, etc [3–6]. Among these materials, CeO2 has been paid more attention, due to the moderate redox potential of the Ce4+/Ce3+ couple and its resistance to photocorrosion [7]. Unfortunately pristine CeO2 can only be excited by ultraviolet light (UV) because of its wide band gap (about 3.2 eV), and limit its further application [8]. Thus it is expected to extend the light absorption of CeO2 to the visible light region, such as doping of metal elements, or coupling with other semiconductors [9, 10]. Noble metal nanoparticles, such as Ag, Au and Pt, have been extensively deposited on the surface of various metal oxides for improving the photocatalytic activity [11–13]. For example, Ag nanoparticles anchored at metal oxide semiconductors (TiO2, ZnO, and CeO2) show greatly enhanced photocatalytic activity than pure metal oxide under a simulated sunlight [14]. Novel Ag/CeO2 nanocubes and Ag nanowires@CeO2 core–shell structures are beneficial for enhancing photocatalytic activity owing to well–defined morphology with controllable optical properties from plasmonic effects
27
[15, 16]. The enhancements in photocatalytic ability are mainly attributed to the surface plasmon resonance (SPR) of noble metal nanostructures [17]. However, these photocatalysts are difficult to recycle in the industrial applications. In recent years, much effort has been focused on the incorporation of magnetic components into semiconductor oxide photocatalysts, which can be easily separated from the treated solution by an external magnetic field [18]. Iron oxides (γ–Fe2O3, or Fe3O4) have excellent conductivity, so it could act as an electron transfer channel and acceptor, which could suppress the photo–generated carrier recombination [19]. The Fe3O4/metal oxides consisted of one–dimensional (1D) nanostructures, such as Fe3O4/P(MAA–DVB)/TiO2nanochains [20], Fe3O4/C@ZnO microrods [21] and ZnO nanowire/Fe3O4@SiO2 [22], show excellent photocatalytic performances, owing to the fact that 1D nanostructures are benefit to the rapid transfer of free electron and the adsorption of organic molecules [23]. Therefore, it is expected that the photocatalytic performance can be further enhanced by incorporating both 1D magnetite nanostructures and noble metal nanoparticles into metal oxides to construct a ternary multifunctional heterostructures photocatalyst. However, there is few report on the synthesis of Ag, Fe3O4 co–doped CeO2 as a sunlight–driven photocatalyst. Here, a novel highly effective, recoverable Fe3O4 nanowire@CeO2/Ag heterojunction photocatalyst, with magnetic nanostructures, was successfully fabricated and synthesized by a combination of hydrothermal and co–precipitation techniques. The composition, morphology and microstructure of the
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as–synthesized samples were characterized and their photocatalytic performances were investigated on the degradation of methylene blue (MB) under natural sunlight exposure compared to pure CeO2, CeO2/Ag, Fe3O4@CeO2 and Fe3O4. For further establish the photocatalytic performance, the photocatalytic degradation of other substrates, such as phenols and methyl orange (MO) were also investigated. Moreover, the reusability of the nanocomposites was evaluated by several consecutive recycling tests, and a possible photocatalytic mechanism was proposed.
2. Experimental 2.1.Materials Cerium (III) nitrate hexahydrate (Ce(NO3)3·6H2O), ammonium ferrous sulfate ((NH4)2Fe(SO4)2·6H2O), silver nitrate (AgNO3), sodium citrate (Na3C6H5O7·2H2O), hexamethylenetetramine (C6H12N4), methylene blue (MB), phenol and methyl orange (MO) were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). All of these reagents were of analytical grade and used without further purification. Solutions were freshly prepared with deionized water, which was purged with high–purity N2 gas for 30 min to remove any dissolved oxygen before use. 2.2. Preparation of Fe3O4 nanowire@CeO2/Ag nanocomposites In a typical process, 20 mL of 1.02 g (NH4)2Fe(SO4)2·6H2O was put into a 100mL Teflon–lined stainless steel autoclave at room temperature. Then, 50 mL of 1.84 g Na3C6H5O7·2H2O was added dropwise with stirring vigorously for 30 min. The
29
autoclave was sealed and maintained at 473 K for 24 h, and then cooled to room temperature. A black solid product was obtained and collected by an external magnet, washed with deionized water and absolute ethanol, and finally dried at 333 K under vacuum. The preparation of the Fe3O4@CeO2/Ag nanocomposites was carried out by an ultrasonic–assisted in–situ precipitation method. The obtained 23 mg of Fe3O4 nanowires were dispersed in 20 mL deionized water. Meanwhile 0.43 g of Ce(NO3)3·6H2O and 0.06 g of AgNO3 were dissolved in 20 mL ethanol, assisted with an ultrasonic treatment process for 60 min, then followed by further stirring for 5 h in a water bath at 333 K. Finally, 0.07 g hexamethylenetetramine (HMT) dissolved in 2 mL deionized water was introduced and stirred for 4 h at 343 K. The final solid product was collected by a magnet, washed with deionized water and absolute ethanol for several times to remove any possible residual ion. The resulting precipitate was dried at 353 K under vacuum and finally calcined at 573 K for 2h in air. The preparation process of Fe3O4@CeO2/Ag is shown in Scheme 1. For comparison, pure CeO2was synthesized by adding HMT (0.07 g) into Ce(NO3)3 solution (0.43 g, 40 mL) under constant stirring at 343 K. CeO2/Ag and Fe3O4@CeO2 were obtained by a similar procedure in the absence of Fe3O4 or AgNO3, respectively. Scheme 1 2.3. Characterization The X–ray powder diffraction (XRD) pattern was obtained by using D/max–3c
30
diffractometer (Rigalcu Corporation, Japan) with Cu Kα radiation. Fourier transform infrared (FT–IR) spectra were taken on a Tensor 27 (Bruker, Germany) instrument in the range of 4000–400 cm-1. The microstructure of the obtained products was characterized by transmission electron microscopy (TEM) on a JEM–2100 (JEOL, Japan) at 300 kV and desktop scanning electron microscopy (SEM) using a Hitachi TM3030. The Brunauer–Emmett–Teller (BET) specific surface area was analyzed by a micrometerics ASAP 2020 apparatus (McKesson Corporation, America) by nitrogen adsorption at 77 K. UV–vis diffuse reflectance spectroscopy (DRS) was carried out using
a
TU–1950
spectrometer
(Purkinje
General
Instrument,
China).
Photoluminescence (PL) spectra were tested on a Hitachi F–7000 fluorescence spectrophotometer (Shimadzu, Japan). X–ray photoelectron spectroscopy (XPS) was performed using a PHI–5400 spectrometer (Shimadzu, Japan) equipped with a monochromatic Al Kα X–ray as the excitation source. The bulk chemical compositions were determined by X–ray fluorescence (XRF) spectrometer (Shimadzu, XRF–1800). The flat–band potentials of the samples were determined from the Mott–Schottky polts obtained by using a CHI electrochemical analyzer (CHI 660D, China). The Mott–Schottky analysis for samples was carried out by conducting a standard electrochemical measurement at 100 Hz in a 0.5 M Na2SO4 by scanning the potential from –0.6 V to 0 V (versus Ag/AgCl) at 50 mV/s. The flat band potential (Vfb) and carrier density (ND) of a semiconductor can be calculated using the Mott–Schottky equations, according to the literature [24].
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2.4. Photocatalysis experiments The photocatalytic behavior on the degradation of MB (10 mg L-1, 100 mL) with 50 mg of the photocatalyst under irradiation was explored. All the experiments were performed in the sunlight illumination of (500 ± 100) × 102 lx and fixed period of the daylight. The dimensions of the glass reactor were 10 cm (diameter) × 2.5 cm (height) and the evaluated effective surface area of the reactor was 90 cm2. Prior to irradiation, the aqueous suspension was mechanically stirred for 1 h in the dark to ensure the establishment of an adsorption–desorption equilibrium between photocatalyst and organic pollutants. Samples were withdrawn at regular intervals and the photocatalyst was separated immediately by a magnet for analysis. The photodegradation efficiency was monitored by measuring the absorbance of the solution samples at 665 nm (MB) with a UV–visible spectrophotometer (TU–1905, China). To further demonstrate the sunlight photocatalytic activity, phenol and MO were also chosen as the model pollutants. The mineralization of the substrates was estimated by COD removal measured by COD meter (5B–3(C), Lanzhou, China).
3.Results and discussion 3.1.Microstructure characterization For pure CeO2, all the diffraction peaks are readily indexed to a pure cubic phase (fluorite structure, JCPDS file no. 34–0394 space group Fm3m) of CeO2 (Fig. 1a–I) [25]. In the pattern of Fig. 1a–II, the extra diffraction peaks at 2θ values of 38.1°, 44.3°,
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64.4° and 77.4° can be indexed to the (111), (200), (220) and (311) faces of Ag nanoparticles, respectively, confirming the formation of Ag/CeO2 nanocomposites (JCPDS file no. 04–0783) [26]. The diffraction peaks located at 2θ value of 30.1°, 35.4°, 37.1°, 43.1°, 53.4°, 56.9° and 62.5° (Fig. 1a–III), ascribed to (220), (311), (222), (400), (422), (511) and (440) faces of the characteristic peaks of Fe3O4 (cubic inverse spinel structure, JCPDS file no. 19–0629) [27]. A novel weak diffraction peak at 2θ value of 35.4°, indexed to the (311) face of Fe3O4, was also observed in Fe3O4@CeO2 sample in Fig. 1a–IV, indicating that Fe3O4 is stable in the calcinations process with the depositing CeO2, which is consistent with the reference report [28]. Compared with Fig. 1a–II, Fe3O4@CeO2/Ag nanocomposites exhibited almost the same feature, except a minor peak at 35.4° of 2θ corresponding to Fe3O4 is observed (Fig. 1a–V). The diffraction profile of the sample containing the strongest (311) reflection peak of Fe3O4 was refined by Pearson–VII distribution (Fig. 1b). The diffraction peak of Fe3O4 is quite weak, comparing with that of CeO2. This is probably because that the quantity of Fe3O4 is too low and the coated of CeO2 and Ag nanoparticles on the surface of Fe3O4 nanowires, which further confirmed by the XRF elemental analysis, the mass content of Fe3O4 are about 9.14% and 7.24% for Fe3O4@CeO2 and Fe3O4@CeO2/Ag, respectively (Table S1). Fig.1 FTIR analysisis regarded as a useful tool in identifying the changes of M–O skeleton in metal oxides. Fig. 2 shows the FT–IR spectra of pure CeO2, CeO2/Ag,
33
Fe3O4@CeO2 and Fe3O4@CeO2/Ag. The absorption peaks at 495 cm-1, 541 cm-1 and 648 cm-1 are assigned to the stretching vibration of the Ce–O bond for pure CeO2 (Fig. 2a) [29]. The weak peaks at 1050 cm-1 and 1460 cm-1 can be attributed to residual organic moieties derived from HMT. Additionally, the characteristic peak at 1386 cm-1 is due to the typical bending vibration of residual nitrate ions [30]. The FT–IR spectra are almost the same after depositing Ag nanoparticles (Fig. 2b). As for Fe3O4@CeO2 (Fig. 2c), two new peaks appear at 588 cm-1 and 1120 cm-1, corresponding to the stretching vibration of Fe–O bond and residual sulfate ions, respectively [20, 31]. Obvious changes in the bands related to Fe3O4 and CeO2 are observed in the Fe3O4@CeO2/Ag (Fig. 2d): the Fe–O stretching vibration band at 588 cm-1 disappeared and the intensity of other characteristic peaks decreased dramatically, which might be caused by the relatively small amount of Fe3O4 and better dispersion of CeO2 and Ag nanoparticles in the nanocomposites. The above observations indicate that CeO2/Ag nanohybrids were successfully coated on the surface of Fe3O4 nanowires by the co–precipitation method. Fig.2 The morphology and microstructure of pure CeO2, CeO2/Ag, Fe3O4@CeO2, Fe3O4@CeO2/Ag and Fe3O4 are characterized by TEM and SEM (Fig. 3 and Fig. 4). The morphology of pure CeO2 shows spherical–shaped nanoparticles with uniform sizes (Fig. 3a), whereas CeO2 nanoparticles are gathered together when wrapping with Ag nanoparticles in Ag/CeO2 sample (Fig. 3b). The morphology of agglomerated particles
34
can be further confirmed by SEM images (Fig. 4b). Besides, the average size of CeO2/Ag nanospheres is about 200 nm in diameter and that of Ag is about 50 nm. Two distinct phases, CeO2 nanoparticles appearing black and the Fe3O4 nanowires with a lighter colour, were observed in Fe3O4@CeO2 sample (Fig. 3c and 4c). It is noticeable that CeO2 nanoparticles are uniformly coated on the surface of the Fe3O4 nanowires, and the diameter of the Fe3O4 nanowires and the size of CeO2 are about 150 and 50 nm, respectively. As shown in Fig. 3d, the as–prepared Fe3O4@CeO2/Ag nanocomposites have a relatively rough surface with average diameter of about 600 nm, and CeO2 and Ag nanoparticles are coated uniformly onto the surface of Fe3O4 nanowires with a mean thickness of 450 nm (Fig. 4d). From the results of XRD, FTIR, TEM and SEM, it can be concluded that Fe3O4 nanowire@CeO2/Ag heterojunction nanocomposites have been successfully prepared. This unique architecture plays a vital role in enhancing the adsorption of organic molecules and the separation of photo–induced charge carrier, and hence improving the photocatalytic efficiency. Fig.3 Fig.4 XPS was also employed to examine surface elements and their valence states (Fig. 5). From the survey spectra in Fig. 5a, the peaks of C, Ce, Ag, Fe and O elements are observed. The C elements are assigned to adventitious carbon contaminant. Therefore, it can be concluded that the Ce, Ag, Fe and O elements existed in the Fe3O4@CeO2/Ag nanocomposites. Regarding the typical signals of Ce3d spectra, the three main 3d5/2
35
peaks featured at around 880.1 eV, 884.6 eV and 895.6 eV correspond to the α1, α2, and α3 components, while the Ce3d3/2 located at 898.5 eV, 904.5 eV and 914.2 eV correspond to β1, β2 and β3, respectively (Fig. 5b). The signals α2 and β2, characteristics of Ce3+, are found in the Fe3O4@CeO2/Ag nanocomposites, while other peaks correspond to the Ce4+. The presence of Ce3+ can be contributed to the interaction between ceria and the surrounding sliver atoms that may have led to a transfer of lattice oxygen in the CeO2 lattice [32]. The regional XPS spectrum of Fe3O4@CeO2/Ag exhibit two peaks centered at 366.2 and 372.2 eV, assigned to Ag 3d5/2 and Ag 3d3/2, respectively, with obviously lower shift (368.2 eV and 374.2 eV for Ag0), indicating the interaction between CeO2 and Ag nanoparticles (Fig. 5c) [33]. In the spectrum of Fe2p (Fig. 5d), the Fe2p3/2 and Fe2p1/2 peaks are located at 710.4 and 725.1 eV, indicating the pure Fe3O4 phase in the Fe3O4@CeO2/Ag nanocomposites [34], further proving that Fe3O4 is stable in the calcination process. Fig. 5e presents the high–resolution XPS spectra of O1s, the peak located at around 529.1 eV is attributed to the lattice oxygen in CeO2, whereas the broad peak appeared around 531.3 eV is due to the chemisorbed oxygen [35]. XPS analysis further verifies the heterojunction formation of the novel ternary photocatalyst, and the synergistic effect among the three components result in the improvement of photodegradation. Fig. 5 The BET specific surface areas and porous structure of the as–prepared samples were investigated using nitrogen adsorption–desorption isotherms. Fig. 6 displays the
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isotherm curves and corresponding pore size distribution curves (inset of Fig. 6) of pure CeO2, CeO2/Ag, Fe3O4@CeO2 and Fe3O4@CeO2/Ag. The adsorption–desorption isotherms of all the aforementioned samples are type IV with an apparent H3 hysteresis loop at relatively higher P/P0, indicating the presence of mesopores and macropores structure [36]. The BET surface areas listed in Table 1 indicate that Fe3O4@CeO2 has larger surface areas than pure CeO2. Similarly, this is also suitable for Fe3O4@CeO2/Ag and CeO2@Ag, which may be due to the incorporation of the Fe3O4 nanowires. Compared to CeO2 and Fe3O4@CeO2, CeO2/Ag and Fe3O4@CeO2/Ag show a significant decrease in surface area, which may be caused by the introduction of Ag nanoparticles. This is consistent with the SEM images (Fig. 4b and 4d). The results show that the introduction of the Fe3O4 nanowires plays an important contributing role in achieving the higher BET surface area, which is beneficial for photocatalytic activity owing to the increased adsorption capacities and active sites to organic pollutants. Fig. 6 Table 1 3.2.Photocatalytic activity To investigate the synergistic effect of the ternary photocatalytic system, the photocatalytic activities of various photocatalysts were evaluated for the degradation of MB solution. The degradation efficiencies of MB with pure CeO2, CeO2/Ag, Fe3O4@CeO2, Fe3O4@CeO2/Ag and Fe3O4 photocatalysts, or without photocatalyst, under natural sunlight exposure are presented in Fig. 7a. The results show that the
37
degradation of MB without a photocatalyst is relatively slow, 0.55% and 14.23% of MB observed in the dark for 60 min and after 120 min exposure, respectively. Meanwhile, in the presence of photocatalysts, the MB concentration decreases steadily with increasing sunlight exposure time. It can be seen that 94.1% of MB photocatalytically degraded after 120 min exposure for the Fe3O4@CeO2/Ag nanocomposites. However, for the CeO2/Ag, Fe3O4@CeO2, pure CeO2 and Fe3O4 samples, only 81.4%, 67.1%, 64.6% and 17.6% MB are degraded, respectively. The kinetic curves in Fig. 7b show that the relationship between –ln(Ct/C0) (C0 and Ct are the initial concentration and the concentrations of MB after sunlight exposure time for t min, respectively) and the irradiation time is almost linear, suggesting that the photocatalytic reaction follows the pseudo–first–order
kinetics.
According to
the
Langmuir–Hinshelwood
model
(–(lnCt/C0)=kt) [37], the rate constant k is estimated to be about 0.0221 min-1 for Fe3O4@CeO2/Ag nanocomposites, higher than that of CeO2/Ag (0.0131 min-1), Fe3O4@CeO2 (0.0064 min-1), pure CeO2 (0.0065 min-1), Fe3O4 (0.0016 min-1), respectively.
The
results
demonstrate
that
the
photocatalytic
activity
of
Fe3O4@CeO2/Ag nanocomposites is much higher than those of the others. As it is well known that MB may absorb the natural sunlight, the sensitization possibility of samples should be considered. Therefore, the photocatalytic activity of the Fe3O4@CeO2/Ag nanocomposites was also evaluated by phenol (50 mg L-1) and MO (15 mg L-1) degradation, and the results are presented in Fig. 7c. As can be seen, no evident photodegradation of phenol and MO was observed in the absence of catalyst. In contrast,
38
the C/C0 for phenol and MO after 180 min in the presence of the Fe3O4@CeO2/Ag nanocomposites are up to 83.9% and 74.2%, respectively. It suggests that the Fe3O4 nanowire@CeO2/Ag nanocomposites possess an outstanding photocatalytic capacity for organic pollutants degradation. In addition, UV–Vis spectra show that the absorption peaks corresponding to the substrates (MB, phenol and MO) decrease rapidly as the exposure time increases, also indicating the photodegradation of the substrates (Fig. S1, ESI). The COD removal efficiencies of MB (120 min of reaction), phenol and MO (180 min of reaction) are 68.8%, 62.4% and 57.3%, respectively (Fig. 7d), further demonstrate
that
the
excellent
enhancement
of
photocatalytic
activity
for
Fe3O4@CeO2/Ag nanocomposites under sunlight exposure. Fig. 7 The stability and reusability of a photocatalyst are of great significance for its further practical application. The cycling stability of Fe3O4@CeO2/Ag nanocomposites as a magnetically recoverable photocatalyst was investigated. As shown in Fig. 8a, the adsorption capacity and photocatalytic activity are almost retainable after 5 cycles. The photocatalytic degradation efficiency for MB only decreases from 94.1% in the 1st cycle to 93.1% in the 5th cycle. And XRD patterns of the photocatalyst before and after the five recycles are essentially same, as shown in Fig. 8b. Moreover, the photocatalyst maintained good magnetic separation effect in the external magnet (Fig. 8b inset). These results indicate that Fe3O4@CeO2/Ag nanocomposites could be used as stable photocatalyst for decomposing organic pollutants exposed to sunlight.
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Fig. 8 The photocatalytic activity of the photocatalysts is closely related to their light absorption ability. Fig. 9 shows UV–vis DRS of pure CeO2, CeO2/Ag, Fe3O4@CeO2 and Fe3O4@CeO2/Ag nanocomposites. The absorption edge of CeO2 exists at around 400 nm, which is consistent with the reference reports [38]. In comparison with pure CeO2, there is a slight red shift in the absorption edge and an improved absorbance in the visible light region ranging from 450 to 650 nm for Fe3O4@CeO2, mainly resulting from the narrower band gap of Fe3O4. This phenomenon is analogous to the recent studies on Fe3O4/AgBr and Fe3O4/g–C3N4 [19, 39]. Furthermore, when Ag nanoparticles are selectively deposited on the surfaces, CeO2/Ag and Fe3O4@CeO2/Ag present more intense light absorption in the visible light region 450–800 nm, this may be attributed to the SPR effect of Ag nanoparticles [16]. The widen adsorption region of CeO2/Ag and Fe3O4@CeO2/Ag is beneficial for producing more photogenerated charge carriers. In addition, the direct band gap energy (Eg) of the nanoparticles can be calculated from the equation of (αhν)2=A (hν – Eg), where hν is the photon energy, α is the absorption coefficient, and A is a constant for the certain material [40]. The Eg values of pure CeO2, CeO2/Ag, Fe3O4@CeO2 and Fe3O4@CeO2/Ag are calculated to be 3.17, 2.35, 2.95 and 2.11 eV, respectively (Fig. 9b). Obviously, Fe3O4@CeO2/Ag exhibits a remarkable decreased Eg compared to CeO2/Ag, implying that the synergistic effect of Ag/CeO2 and Fe3O4 in the ternary nanocomposites, and further narrows the band gap of CeO2. Thus, the Fe3O4@CeO2/Ag nanocomposites have better light absorption ability.
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Fig. 9 PL technique is an effective way to study the efficiency of charge carrier trapping, immigration and transfer at the surface of photocatalysts. A lower PL intensity usually implies a lower electron–hole recombination rate and higher photocatalytic activity [41, 42]. The PL spectra of pure CeO2, CeO2/Ag, Fe3O4@CeO2 and Fe3O4@CeO2/Ag are presented in Fig. 10. It is clearly observed that pure CeO2 displays a strong PL intensity at around 451 nm, 468 nm and 483 nm. The mild variation may be attributed to synthetic route and calcination conditions [43]. After the Fe3O4 nanowires are introduced, the PL signal intensity shows an apparent decrease, indicating that the Fe3O4 nanowires improve the transfer of photogenerated electron–hole pairs. Moreover, owing to the addition of Ag nanoparticles, the PL intensity of the CeO2/Ag is distinctly lower than that of CeO2, which confirmed that the presence of Ag nanoparticles promotes photogenerated charge carriers. Moreover the PL intensity of ternary Fe3O4@CeO2/Ag nanocomposites is the lowest among all the samples, suggesting that the more photogenerated electrons are trapped by the cooperative effect of Ag nanoparticles and Fe3O4 nanowires and more effective charge separation are obtained. Fig. 10 Fig.11 shows the 1/C2 versus V plots for pure CeO2, CeO2/Ag, Fe3O4@CeO2, Fe3O4@CeO2/Ag and Fe3O4. The flat band potential shift from –0.453 V for pure CeO2 and –0.439 V for Fe3O4@CeO2 to –0.511 V for CeO2/Ag and –0.533 V for Fe3O4@CeO2/Ag nanocomposites, respectively (Table 2). The carrier density of pure
41
CeO2 (2.16×1018 cm-3) and Fe3O4@CeO2 (1.50×1018 cm-3) are lower than those of CeO2/Ag (6.31×1018 cm-3) and Fe3O4@CeO2/Ag nanocomposites (8.58×1018 cm-3), respectively. The results indicate that the addition of Ag nanoparticles lead to a more negative shift of Vfb and a higher carrier density. This is because the band bending at the semiconductor/electrolyte interface pushes electrons away from the interface while attracting holes, increasing the band bending while keeping the Fermi level equilibration [44]. The shift to a more negative flat–band potential indicates that Fe3O4@CeO2/Ag nanocomposites are better photocatalyst than the other samples [45]. However, the addition of Fe3O4 nanowire has little effect on the flat band potential. Fig. 11 3.3. Photocatalytic mechanism According to the above discussion, a possible mechanism for the Fe3O4@CeO2/Ag nanocomposites is proposed (Scheme 2). On one hand, the Fermi energy of Ag (4.26 eV) is lower than that of CeO2 (about 5.0 eV). This leads to the electrons transfer from the Fermi level of Ag to that of CeO2 until the energy thermodynamic equilibrium was established [46, 47]. The Ag nanoparticles can absorb resonance photons and generated hot electrons with energy 1.0–4.0 eV with respect to the metal Fermi level [16]. With natural sunlight exposure, the equilibrated Fermi level electrons are injected rapidly into the CeO2 conduction band via a SPR mechanism [37]. And DRS and Mott-Schottky analysis further proved that Ag/CeO2 heterojunction can effectively not only improve the light absorption ability but also drive the separation and transportation of the
42
electron–hole pairs. On the other hand, due to the large surface area and the conductivity of Fe3O4 nanowires [38], the interface between CeO2 and Fe3O4 effectively increases the electron–hole separation by transferring the photogenerated electrons from the conduction band of one CeO2 molecule to that of another CeO2 molecule through the Fe3O4 nanowires. Thus, electrons trapped in the Fe3O4 nanowires may have a long lifetime and can be transferred to the interface between the nanocomposites and solution. The photogenerated electrons react with O2 absorbed on the surface of Fe3O4@CeO2/Ag and produce the active species •O2-, which further react with H2O to form hydroxyl radicals (•OH), an extremely strong oxidant for the partial or complete mineralization of organic chemicals [14]. Therefore, Fe3O4@CeO2/Ag nanocomposites can be used as an effective sunlight active photocatalyst. Both Ag and CeO2 play important roles in improving the photocatalytic activity, and that the 1D structure of the Fe3O4 nanowires is also critical for the photocatalytic effect of the nanocomposites. Scheme 2
4. Conclusions In this study, we have successfully synthesized Fe3O4 nanowire@CeO2/Ag nanocomposites photocatalyst via simple hydrothermal and co–precipitation techniques. The photocatalytic activity of the Fe3O4@CeO2/Ag nanocomposites for MB is demonstrated to be superior to those of pure CeO2, CeO2/Ag, Fe3O4@CeO2, and Fe3O4. It also shows excellent photocatalytic activity and mineralizing ability in the phenol and MO degradation. The enhanced photocatalytic activity is closely associated with
43
electrons trapping of CeO2/Ag and electrons transferring of Fe3O4 nanowires, which leads to the extended light absorption range and higher separation efficiency of electron–hole pairs. In addition, the Fe3O4@CeO2/Ag nanocomposites show high stability, recoverable and reusability, which is very beneficial for practical applications. The present study provides a promising cost–effective photocatalytic material in environmental protection area.
Acknowledgements The authors are grateful to be supported by the Innovation Funds of Graduate Programs SNNU (2015CXS040), the Fundamental Research Funds for the Central Universities (GK201302013), the National Natural Science Foundation of China (No. 51204119), and the Natural Science Foundation of Shanxi (nos. 2014021015–4, 2015021107).
References [1] T. Hisatomi, J. Kubota, K. Domen, Recent advances in semiconductors for photocatalytic and
photoelectrochemical water splitting, Chem. Soc. Rev. 43 (2014) 7520–7535.
[2] M.N. Chong, B. Jin, C.W.K. Chow, C. Saint, Recent developments in photocatalytic water treatment technology:
A review, Water Res. 44 (2010) 2997–3027.
[3] H. Hou, M. Shang, L. Wang, W. Li, B. Tang, W. Yang, Efficient photocatalytic activities of TiO2 hollow fibers with mixed phases and mesoporous walls, Sci. Rep. 5 (2015) 15228.
44
[4] Y. Chen, C. Zhang, W. Huang, Y. Situ, H. Huang, Multimorphologies nano–ZnO preparing through a simple
solvothermal method for photocatalytic application, Mater. Lett. 141 (2015) 294–297.
[5] H. Chen, T. Tu, M. Wen, Q. Wu, Assembly synthesis of Cu 2O–on–Cu nanowires with visible–light–enhanced photocatalytic activity, Dalton Trans. 44 (2015) 15645–15652.
[6] Y.M. Hunge, M.A. Mahadik, S.S. Kumbhar, V.S. Mohite, K.Y. Rajpure, N.G. Deshpande, A.V. Moholkar, C.H.
Bhosale, Visible light catalysis of methyl orange using nanostructured WO3 thin films, Ceram. Int. 42 (2016) 789–798.
[7] E.M. Seftel, M.C. Puscasu, M. Mertens, P. Cool, G. Carja, Fabrication of CeO 2/LDHs self–assemblies with enhanced photocatalytic performance: A case study on ZnSn–LDH matrix, Appl. Catal. B: Environ. 164 (2015)
251–260.
[8] M.M. Khan, S.A. Ansari, D. Pradhan, D.H. Han, J. Lee, M.H. Cho, Defect–induced band gap narrowed CeO2 nanostructures for visible light activities, Ind. Eng. Chem. Res. 53 (2014) 9754–9763.
[9] G. Li, Z. Tang, Noble metal nanoparticle@metal oxide core/yolk–shell nanostructures as catalysts: recent progress
and perspective, Nanoscale. 6 (2014) 3995–4011.
[10] Z. Abbasi, M. Haghighi, E. Fatehifar, S. Saedy, Synthesis and physicochemical characterizations of nanostructured Pt/Al2O3–CeO2 catalysts for total oxidation of VOCs, J. Hazard. Mater. 186 (2011) 1445–1454. [11] D. Geetha, S. Kavitha, P.S. Ramesh, A novel bio–degradable polymer stabilized Ag/TiO2 nanocomposites and their catalytic activity on reduction of methylene blue under natural sun light, Ecotoxicol. Environ. Saf. 121 (2015)
126–134.
[12] M. Hoffmann, S. Kreft, G. Georgi, G. Fulda, M.M. Pohl, D. Seeburg, C. Berger Karin, E.V. Kondratenko, S.
Wohlrab, Improved catalytic methane combustion of Pd/CeO2 catalysts via porous glass integration, Appl. Catal. B:
45
Environ. 179 (2015) 313–320.
[13] D. Jiang, W. Wang, S. Sun, L. Zhang, Y. Zheng, Equilibrating the plasmonic and catalytic roles of metallic
nanostructures in photocatalytic oxidation over Au–modified CeO2, ACS Catal. 5 (2015) 613–621. [14] T. Liu, B. Li, Y. Hao, F. Han, L. Zhang, L. Hu, A general method to diverse silver/mesoporous–metal–oxide
nanocomposites with plasmon–enhanced photocatalytic activity, Appl. Catal. B: Environ. 165 (2015) 378–388.
[15] Q. Leng, D. Yang, Q. Yang, C. Hu, Y. Kang, M. Wang, M. Hashim, Building novel Ag/CeO 2 heterostructure for enhancing photocatalytic activity, Mater. Res. Bull. 65 (2015) 266–272.
[16] L. Wu, S. Fang, L. Ge, C. Han, P. Qiu, Y. Xin, Facile synthesis of Ag@CeO2 core–shell plasmonic photocatalysts with enhanced visible–light photocatalytic performance, J. Hazard. Mater. 300 (2015) 93–103.
[17] J. Zhan, H. Zhang, G. Zhu, Magnetic photocatalysts of cenospheres coated with Fe 3O4/TiO2 core/shell nanoparticles decorated with Ag nanopartilces, Ceram. Int. 40 (2014) 8547–8559.
[18] J. Rashid, M.A. Barakat, Y. Ruzmanova, A. Chianese, Fe3O4/SiO2/TiO2 nanoparticles for photocatalytic degradation of 2–chlorophenol in simulated wastewater, Environ. Sci. Pollut. Res. Int. 22 (2015) 3149–3157.
[19] Z. Zhu, Z. Lu, D. Wang, X. Tang, Y. Yan, W. Shi, Y. Wang, N. Gao, X. Yao, H. Dong, Construction of
high–dispersed Ag/Fe3O4/g–C3N4 photocatalyst by selective photo–deposition and improved photocatalytic activity, Appl. Catal. B: Environ. 182 (2016) 115–122.
[20] C. Li, J. Tan, X. Fan, B. Zhang, H. Zhang, Q. Zhang, Magnetically separable one dimensional
Fe3O4/P(MAA–DVB)/TiO2 nanochains: Preparation, characterization and photocatalytic activity, Ceram. Int. 41 (2015) 3860–3868.
[21] Y. Wang, J. Ning, E. Hu, C. Zheng, Y. Zhong, Y. Hu, Direct coating ZnO nanocrystals onto 1D Fe 3O4/C composite microrods as highly efficient and reusable photocatalysts for water treatment, J. Alloys Compd. 637 (2015)
46
301–307.
[22] X. Bian, K. Hong, X. Ge, R. Song, L. Liu, M. Xu, Functional hierarchical nanocomposites based on ZnO
nanowire and magnetic nanoparticle as highly active recyclable photocatalysts, J. Phys. Chem. C. 119 (2015)
1700–1705.
[23] J. Tian, Z. Zhao, A. Kumar, R.I. Boughton, H. Liu, Recent progress in design, synthesis, and applications of
one–dimensional TiO2 nanostructured surface heterostructures: a review, Chem. Soc. Rev. 43 (2014) 6920–6937. [24] K. Gelderman, L. Lee, S.W. Donne, Flat–band potential of a semiconductor: using the Mott–Schottky equation, J.
Chem. Edu, 84 (2007) 685–688.
[25] T.S. Sakthivel, D.L. Reid, U.M. Bhatta, G. Mobus, D.C. Sayle, S. Seal, Engineering of nanoscale defect patterns
in CeO2 nanorods via ex situ and in situ annealing, Nanoscale. 7 (2015) 5169–5177. [26] Z. Yang, K.T. Nguyen, H. Chen, H. Qian, L.P. Fernando, K.A. Christensen, J.N. Anker, Plasmonic silver
nanobelts via citrate reduction in the presence of HCl and their orientation–dependent scattering properties, J. Phys.
Chem. Lett. 2 (2011) 1742–1746.
[27] M. Abbas, B.P. Raoc, V. Reddya, C. G. Kima, Fe3O4/TiO2 core/shell nanocubes: Single–batch surfactantless synthesis, characterization and efficient catalysts for methylene blued egradation, Ceram. Int. 40 (2014)
11177–11186. [28] L. Zhao, N. Li, X. Li, W. Yan, Y. Chi, Q. Yuan, Y. Li, Magnetically separable Fe3O4@SiO2@TiO2–Ag microspheres with well–designed nanostructure and enhanced photocatalytic activity. J. Hazard. Mater, 262 (2013)
404–411.
[29] L. Wang, J. Ding, Y. Chai, Q. Liu, J. Ren, X. Liu, W.L. Dai, CeO2 nanorod/g–C3N4/N–rGO composite: enhanced visible–light–driven photocatalytic performance and the role of N–rGO as electronic transfer media, Dalton Trans. 44
47
(2015) 11223–11234.
[30] S. Narang, U.P. Singh, P. Venugopalan, Solvent–mediated supramolecular templated assembly of a metal
organophosphonate via a crystal–amorphous–crystal transformation, CrystEngComm, 18 (2016) 54–61.
[31] F. Chen, H. Ma, B. Wang, Simple and effective synthesis of methoxy–dimethylbenzene from electrochemical oxidation of p–xylene in methanol solvent catalyzed by SO42-/ZrO2–MxOy, J. Hazard. Mater. 162 (2009) 668–673. [32] N. Zhang, X. Fu, Y.J. Xu, A facile and green approach to synthesize Pt@CeO2 nanocomposite with tunable core–shell and yolk–shell structure and its application as a visible light photocatalyst, J. Mater. Chem. 21 (2011)
8152–8158. [33] D. Lin, H. Wu, R. Zhang, W. Pan, Enhanced photocatalysis of electrospun Ag−ZnO heterostructured nanofibers,
Chem. Mater. 21 (2009) 3479–3484. [34] J. Ma, S. Guo, X. Guo, H. Ge, A mild synthetic route to Fe3O4@TiO2–Au composites: preparation, characterization and photocatalytic activity, Appl. Surf. Sci. 353 (2015) 1117–1125.
[35] C. Lee, J.I. Park, Y.G. Shul, H. Einaga, Y. Teraoka, Ag supported on electrospun macro–structure CeO2 fibrous mats for diesel soot oxidation, Appl. Catal. B: Environ. 174–175 (2015) 185–192.
[36] X. Zhang, N. Zhang, Y.J. Xu, Z.R. Tang, One–dimensional CdS nanowires–CeO2 nanoparticles composites with boosted photocatalytic activity, New J. Chem. 39 (2015) 6756–6764.
[37] D. Channei, B. Inceesungvorn, N. Wetchakun, S. Ukritnukun, A. Nattestad, J. Chen, S. Phanichphant,
Photocatalytic degradation of methyl orange by CeO2 and Fe–doped CeO2 films under visible light irradiation, Sci. Rep. 4 (2014) 5757.
[38] M.M. Khan, S.A. Ansari, M.O. Ansari, B.K. Min, J. Lee, M.H. Cho, Biogenic fabrication of Au@CeO2 nanocomposite with enhanced visible light activity, J. Phys. Chem. C. 118 (2014) 9477–9484.
48
[39] Y. Cao, C. Li, J. Li, Q. Li, J. Yang, Magnetically separable Fe3O4/AgBr hybrid materials: highly efficient photocatalytic activity and good stability, Nanoscale Res. Lett. 10 (2015) 1–6.
[40] D.P.a.K.T. Leung, Controlled growth of two–dimensional and one–dimensional ZnO nanostructures on indium
tin oxide coated glass by direct electrodeposition, Langmuir, 24 (2008) 9707–9716.
[41] J. Su, Y. Zhang, S. Xu, S. Wang, H. Ding, S. Pan, G. Wang, G. Li, H. Zhao, Highly efficient and recyclable
triple–shelled Ag@Fe3O4@SiO2@TiO2 photocatalysts for degradation of organic pollutants and reduction of hexavalent chromium ions, Nanoscale, 6 (2014) 5181–5192. [42] W. Li, X. Liu, H. Li, Hydrothermal synthesis of graphene/Fe3+–doped TiO2nanowire composites with highly enhanced photocatalytic activity under visible light irradiation, J. Mater. Chem. A, 3 (2015) 15214–15224.
[43] M. Aslam, M.T. Qamar, M.T. Soomro, I.M.I. Ismail, N. Salah, T. Almeelbi, M.A. Gondal, A. Hameed, The effect
of sunlight induced surface defects on the photocatalytic activity of nanosized CeO2 for the degradation of phenol and its derivatives, Appl. Catal. B: Environ. 180 (2016) 391–402.
[44] Z. Zhang, J.T.Y. Jr, Band bending in semiconductors: chemical and physical consequences at surfaces and
interfaces, Chem. Rev, 112 (2012) 5520−5551.
[45] H. Cai, P. Liang, Z. Hu, L. Shi, X. Yang, J. Sun, N. Xu, J. Wu, Enhanced photoelectrochemical activity of
ZnO–Coated TiO2 nanotubes and its dependence on ZnO coating thickness, Nanoscale Res. Lett, 11(2016)1–11. [46] C. Zhang, A. Michaelides, D.A. King, S.J. Jenkins, Structure of gold atoms on stoichiometric and defective ceria
surfaces, J Chem Phys, 129 (2008) 100–103.
[47] L. Xu, F. Zhang, X. Song, Z. Yin, Y. Bu, Construction of reduced graphene oxide–supported Ag–Cu2O composites with hierarchical structures for enhanced photocatalytic activities and recyclability, J. Mater. Chem. A, 3
(2015) 5923–5933.
49
Table captions: Table 1 Porosity parameters of CeO2, CeO2/Ag, Fe3O4@CeO2 and Fe3O4@CeO2/Ag. Table 2 The semiconductor characteristic parameters of the samples.
50
Figure captions: Fig. 1. (a)XRD patterns of CeO2(I), CeO2/Ag(II), Fe3O4 (III), Fe3O4@CeO2 (IV) and Fe3O4@CeO2/Ag (V); (b) Profile refined by Pearson–VII distribution.
Fig. 2. FT–IR spectra of CeO2 (a), CeO2/Ag (b), Fe3O4@CeO2 (c) and Fe3O4@CeO2/Ag (d). Fig. 3. TEM images of CeO2 (a), CeO2/Ag (b), Fe3O4@CeO2 (c), Fe3O4@CeO2/Ag (d) and Fe3O4 (e). Fig. 4. SEM images of CeO2 (a), CeO2/Ag (b), Fe3O4@CeO2 (c), Fe3O4@CeO2/Ag (d) and Fe3O4 (e). Fig. 5. XPS patterns of the Fe3O4@CeO2/Ag nanocomposites: survey spectra (a), Ce 3d (b), Ag 3d (c), Fe 2p (d) and O 1s (e).
Fig. 6. N2 adsorption–desorption isotherm and corresponding pore–size distribution curves (inset) of the obtained pure CeO2, CeO2/Ag, Fe3O4@CeO2 and Fe3O4@CeO2/Ag. Fig. 7. (a) The photodegradation of MB with or without different catalysts: pure CeO2, CeO2/Ag, Fe3O4@CeO2, Fe3O4@CeO2/Ag and Fe3O4; (b) The kinetic curve of photocatalytic MB degradation using different catalysts; Photocatalytic degradation (c) and COD removal (d) of MB, phenol and MO solution in the presence or
absence of Fe3O4@CeO2/Ag. Fig. 8. (a) Recycles of Fe3O4@CeO2/Ag in the degradation of MB by Fe3O4@CeO2/Ag. (b) XRD patterns of Fe3O4@CeO2/Ag photocatalyst before (I) and after (II) the recycling experiment. Fig. 9. (a) UV–vis DRS of pure CeO2, CeO2/Ag, Fe3O4@CeO2 and Fe3O4@CeO2/Ag; (b) plots of (αhν)2 vs. photon energy (hν).
Fig. 10. Photoluminescence (PL) spectra of pure CeO2, CeO2/Ag, Fe3O4@CeO2 and Fe3O4@CeO2/Ag nanocomposites with an excitation wavelength of 325 nm.
51
Fig. 11. Mott–Schottky plots of pure CeO2, CeO2/Ag, Fe3O4@CeO2, Fe3O4@CeO2/Ag and Fe3O4.
Illustration captions Scheme 1. Schematic preparation of Fe3O4@CeO2/Ag. Scheme 2. Illustration of the mechanism for photogenerated charge carrier transfers in the Fe3O4@CeO2/Ag under natural sunlight exposure.
52
Table 1 Porosity parameters of CeO2, CeO2/Ag, Fe3O4@CeO2 and Fe3O4@CeO2/Ag
Samples
SBET (m2 g-1)
V (cm3 g-1)
Average pore size (nm)
CeO2
27.61
0.0894
24.10
CeO2/Ag
9.85
0.0304
37.77
Fe3O4@CeO2
36.20
0.0961
49.32
Fe3O4@CeO2/Ag
16.11
0.0402
36.69
53
Table 2 The semiconductor characteristic parameters of the samples.
Samples
Vbf (v)
ND × 1018 (cm-3)
CeO2
–0.453
2.16
CeO2/Ag
–0.511
6.31
Fe3O4@CeO2
–0.439
1.50
Fe3O4@CeO2/Ag
–0.533
8.58
Fe3O4
–0.411
0.45
54
Fig.1. (a)XRD patterns of CeO2(I), CeO2/Ag(II), Fe3O4 (III), Fe3O4@CeO2 (IV) and Fe3O4@CeO2/Ag (V); (b) Profile refined by Pearson-VII distribution.
55
Fig. 2. FT–IR spectra of CeO2 (a), CeO2/Ag (b), Fe3O4@CeO2 (c) and Fe3O4@CeO2/Ag (d).
56
Fig. 3. TEM images of CeO2 (a), CeO2/Ag (b), Fe3O4@CeO2 (c), Fe3O4@CeO2/Ag (d) and Fe3O4 (e).
57
Fig. 4. SEM images of CeO2 (a), CeO2/Ag (b), Fe3O4@CeO2 (c), Fe3O4@CeO2/Ag (d) and Fe3O4 (e).
58
Fig. 5. XPS patterns of the Fe3O4@CeO2/Ag nanocomposites: survey spectra (a), Ce 3d (b), Ag 3d (c), Fe 2p (d) and O 1s (e).
59
Fig. 6. N2 adsorption–desorption isotherm and corresponding pore–size distribution curves (inset) of the obtained pure CeO2, CeO2/Ag, Fe3O4@CeO2 and Fe3O4@CeO2/Ag.
60
Fig. 7. (a) The photodegradation of MB with or without different catalysts; (b) The kinetic curve of photocatalytic
MB degradation using different catalysts; Photocatalytic degradation (c) and COD removal (d) of MB, phenol and
MO solution in the presence or absence of Fe3O4@CeO2/Ag.
61
Fig. 8. (a) Recycles of Fe3O4@CeO2/Ag in the degradation of MB. (b) XRD patterns of Fe3O4@CeO2/Ag photocatalyst before (I) andafter (II) the recycling experiment.
62
Fig. 9. (a) UV–vis DRS of pure CeO2, CeO2/Ag, Fe3O4@CeO2 and Fe3O4@CeO2/Ag; (b) plots of (αhν)2 vs. photon energy (hν).
63
Fig. 10. PL spectra of pure CeO2, CeO2/Ag, Fe3O4@CeO2 and Fe3O4@CeO2/Ag nanocomposites with an excitation wavelength of 325 nm.
64
Fig. 11. Mott–Schottky plots of pure CeO2, CeO2/Ag, Fe3O4@CeO2, Fe3O4@CeO2/Ag and Fe3O4.
65
Scheme 1. Schematic preparation of Fe3O4@CeO2/Ag.
66
Scheme. 2. Illustration of the mechanism for photogenerated charge carrier transfers in the Fe3O4@CeO2/Ag under natural sunlight exposure.
67
PII: DOI: Reference:
S0272-8842(16)30510-7 http://dx.doi.org/10.1016/j.ceramint.2016.04.104 CERI12722
To appear in: Ceramics International Received date: 23 February 2016 Revised date: 19 April 2016 Accepted date: 19 April 2016 Cite this article as: Jiali Chang, Qingliang Ma, Jianchao Ma and Hongzhu Ma, Synthesis of Fe3O4 nanowire@CeO2/Ag nanocomposites with enhanced photocatalytic activity under sunlight exposure, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.04.104 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synthesis of Fe3O4 nanowire@CeO2/Ag nanocomposites with enhanced photocatalytic activity under sunlight exposure
Jiali Changa, Qingliang Mab, Jianchao Mac, Hongzhu Ma*a
a. School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi'an 710119, Shaanxi, P. R. China
b. Key Laboratory of Coal Science and Technology, Taiyuan University of Technology, Ministry of Education and
Shanxi Province, Taiyuan 030024, Shanxi, P. R.China
c. College of Mining Technology, Taiyuan University of Technology, Taiyuan 030024, Shanxi, P.R.China
Abstract Ternary magnetic Fe3O4 nanowire@CeO2/Ag nanocomposites have been firstly synthesized by means of hydrothermal and co–precipitation techniques, and their ability to adsorb, photocatalytic degradation organic
pollutants, methylene blue present in water, and separate, has been demonstrated. The results show that CeO2 and Ag nanoparticles are uniformly deposited on the surface of Fe3O4 nanowires. The photocatalytic experiments demonstrate that the Fe3O4@CeO2/Ag nanocomposites exhibit remarkably enhanced photocatalytic properties and stability compared to CeO2, CeO2/Ag, Fe3O4@CeO2, Fe3O4 under natural sunlight exposure. Moreover, excellent photocatalytic degradation efficiency for phenol and MO are also observed. The enhanced photocatalytic
performance may be attributed to the synergetic effect of Fe3O4 nanowire, CeO2 and Ag nanoparticles, which lead to the enhanced light harvesting, the promoted charge separation and enhanced adsorption capacity. In addition, the
Fe3O4@CeO2/Ag photocatalyst can be easily collected and separated by an external magnet. These results suggest that the nanocomposites could be exploited as potential candidates for solar photocatalysis.
*Corresponding author. Tel.: +86 29 81530726. Fax: +86 29 81530727. E–mail: [email protected].
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Keywords: Fe3O4 nanowire@CeO2/Ag; Photocatalysis; Sunlight; Surface plasmon resonance; Magnetic separation
1. Introduction Rapid industrialization and population growth have sparked a global crisis concerning energy shortage and environmental pollution. Semiconductor photocatalysis is a highly appealing process for water splitting for hydrogen generation and the degradation of organic pollutants [1, 2]. To date, many metal oxide semiconductors have been used in heterogeneous photocatalysis, including TiO2, ZnO, WO3, Cu2O, etc [3–6]. Among these materials, CeO2 has been paid more attention, due to the moderate redox potential of the Ce4+/Ce3+ couple and its resistance to photocorrosion [7]. Unfortunately pristine CeO2 can only be excited by ultraviolet light (UV) because of its wide band gap (about 3.2 eV), and limit its further application [8]. Thus it is expected to extend the light absorption of CeO2 to the visible light region, such as doping of metal elements, or coupling with other semiconductors [9, 10]. Noble metal nanoparticles, such as Ag, Au and Pt, have been extensively deposited on the surface of various metal oxides for improving the photocatalytic activity [11–13]. For example, Ag nanoparticles anchored at metal oxide semiconductors (TiO2, ZnO, and CeO2) show greatly enhanced photocatalytic activity than pure metal oxide under a simulated sunlight [14]. Novel Ag/CeO2 nanocubes and Ag nanowires@CeO2 core–shell structures are beneficial for enhancing photocatalytic activity owing to well–defined morphology with controllable optical properties from plasmonic effects
27
[15, 16]. The enhancements in photocatalytic ability are mainly attributed to the surface plasmon resonance (SPR) of noble metal nanostructures [17]. However, these photocatalysts are difficult to recycle in the industrial applications. In recent years, much effort has been focused on the incorporation of magnetic components into semiconductor oxide photocatalysts, which can be easily separated from the treated solution by an external magnetic field [18]. Iron oxides (γ–Fe2O3, or Fe3O4) have excellent conductivity, so it could act as an electron transfer channel and acceptor, which could suppress the photo–generated carrier recombination [19]. The Fe3O4/metal oxides consisted of one–dimensional (1D) nanostructures, such as Fe3O4/P(MAA–DVB)/TiO2nanochains [20], Fe3O4/C@ZnO microrods [21] and ZnO nanowire/Fe3O4@SiO2 [22], show excellent photocatalytic performances, owing to the fact that 1D nanostructures are benefit to the rapid transfer of free electron and the adsorption of organic molecules [23]. Therefore, it is expected that the photocatalytic performance can be further enhanced by incorporating both 1D magnetite nanostructures and noble metal nanoparticles into metal oxides to construct a ternary multifunctional heterostructures photocatalyst. However, there is few report on the synthesis of Ag, Fe3O4 co–doped CeO2 as a sunlight–driven photocatalyst. Here, a novel highly effective, recoverable Fe3O4 nanowire@CeO2/Ag heterojunction photocatalyst, with magnetic nanostructures, was successfully fabricated and synthesized by a combination of hydrothermal and co–precipitation techniques. The composition, morphology and microstructure of the
28
as–synthesized samples were characterized and their photocatalytic performances were investigated on the degradation of methylene blue (MB) under natural sunlight exposure compared to pure CeO2, CeO2/Ag, Fe3O4@CeO2 and Fe3O4. For further establish the photocatalytic performance, the photocatalytic degradation of other substrates, such as phenols and methyl orange (MO) were also investigated. Moreover, the reusability of the nanocomposites was evaluated by several consecutive recycling tests, and a possible photocatalytic mechanism was proposed.
2. Experimental 2.1.Materials Cerium (III) nitrate hexahydrate (Ce(NO3)3·6H2O), ammonium ferrous sulfate ((NH4)2Fe(SO4)2·6H2O), silver nitrate (AgNO3), sodium citrate (Na3C6H5O7·2H2O), hexamethylenetetramine (C6H12N4), methylene blue (MB), phenol and methyl orange (MO) were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). All of these reagents were of analytical grade and used without further purification. Solutions were freshly prepared with deionized water, which was purged with high–purity N2 gas for 30 min to remove any dissolved oxygen before use. 2.2. Preparation of Fe3O4 nanowire@CeO2/Ag nanocomposites In a typical process, 20 mL of 1.02 g (NH4)2Fe(SO4)2·6H2O was put into a 100mL Teflon–lined stainless steel autoclave at room temperature. Then, 50 mL of 1.84 g Na3C6H5O7·2H2O was added dropwise with stirring vigorously for 30 min. The
29
autoclave was sealed and maintained at 473 K for 24 h, and then cooled to room temperature. A black solid product was obtained and collected by an external magnet, washed with deionized water and absolute ethanol, and finally dried at 333 K under vacuum. The preparation of the Fe3O4@CeO2/Ag nanocomposites was carried out by an ultrasonic–assisted in–situ precipitation method. The obtained 23 mg of Fe3O4 nanowires were dispersed in 20 mL deionized water. Meanwhile 0.43 g of Ce(NO3)3·6H2O and 0.06 g of AgNO3 were dissolved in 20 mL ethanol, assisted with an ultrasonic treatment process for 60 min, then followed by further stirring for 5 h in a water bath at 333 K. Finally, 0.07 g hexamethylenetetramine (HMT) dissolved in 2 mL deionized water was introduced and stirred for 4 h at 343 K. The final solid product was collected by a magnet, washed with deionized water and absolute ethanol for several times to remove any possible residual ion. The resulting precipitate was dried at 353 K under vacuum and finally calcined at 573 K for 2h in air. The preparation process of Fe3O4@CeO2/Ag is shown in Scheme 1. For comparison, pure CeO2was synthesized by adding HMT (0.07 g) into Ce(NO3)3 solution (0.43 g, 40 mL) under constant stirring at 343 K. CeO2/Ag and Fe3O4@CeO2 were obtained by a similar procedure in the absence of Fe3O4 or AgNO3, respectively. Scheme 1 2.3. Characterization The X–ray powder diffraction (XRD) pattern was obtained by using D/max–3c
30
diffractometer (Rigalcu Corporation, Japan) with Cu Kα radiation. Fourier transform infrared (FT–IR) spectra were taken on a Tensor 27 (Bruker, Germany) instrument in the range of 4000–400 cm-1. The microstructure of the obtained products was characterized by transmission electron microscopy (TEM) on a JEM–2100 (JEOL, Japan) at 300 kV and desktop scanning electron microscopy (SEM) using a Hitachi TM3030. The Brunauer–Emmett–Teller (BET) specific surface area was analyzed by a micrometerics ASAP 2020 apparatus (McKesson Corporation, America) by nitrogen adsorption at 77 K. UV–vis diffuse reflectance spectroscopy (DRS) was carried out using
a
TU–1950
spectrometer
(Purkinje
General
Instrument,
China).
Photoluminescence (PL) spectra were tested on a Hitachi F–7000 fluorescence spectrophotometer (Shimadzu, Japan). X–ray photoelectron spectroscopy (XPS) was performed using a PHI–5400 spectrometer (Shimadzu, Japan) equipped with a monochromatic Al Kα X–ray as the excitation source. The bulk chemical compositions were determined by X–ray fluorescence (XRF) spectrometer (Shimadzu, XRF–1800). The flat–band potentials of the samples were determined from the Mott–Schottky polts obtained by using a CHI electrochemical analyzer (CHI 660D, China). The Mott–Schottky analysis for samples was carried out by conducting a standard electrochemical measurement at 100 Hz in a 0.5 M Na2SO4 by scanning the potential from –0.6 V to 0 V (versus Ag/AgCl) at 50 mV/s. The flat band potential (Vfb) and carrier density (ND) of a semiconductor can be calculated using the Mott–Schottky equations, according to the literature [24].
31
2.4. Photocatalysis experiments The photocatalytic behavior on the degradation of MB (10 mg L-1, 100 mL) with 50 mg of the photocatalyst under irradiation was explored. All the experiments were performed in the sunlight illumination of (500 ± 100) × 102 lx and fixed period of the daylight. The dimensions of the glass reactor were 10 cm (diameter) × 2.5 cm (height) and the evaluated effective surface area of the reactor was 90 cm2. Prior to irradiation, the aqueous suspension was mechanically stirred for 1 h in the dark to ensure the establishment of an adsorption–desorption equilibrium between photocatalyst and organic pollutants. Samples were withdrawn at regular intervals and the photocatalyst was separated immediately by a magnet for analysis. The photodegradation efficiency was monitored by measuring the absorbance of the solution samples at 665 nm (MB) with a UV–visible spectrophotometer (TU–1905, China). To further demonstrate the sunlight photocatalytic activity, phenol and MO were also chosen as the model pollutants. The mineralization of the substrates was estimated by COD removal measured by COD meter (5B–3(C), Lanzhou, China).
3.Results and discussion 3.1.Microstructure characterization For pure CeO2, all the diffraction peaks are readily indexed to a pure cubic phase (fluorite structure, JCPDS file no. 34–0394 space group Fm3m) of CeO2 (Fig. 1a–I) [25]. In the pattern of Fig. 1a–II, the extra diffraction peaks at 2θ values of 38.1°, 44.3°,
32
64.4° and 77.4° can be indexed to the (111), (200), (220) and (311) faces of Ag nanoparticles, respectively, confirming the formation of Ag/CeO2 nanocomposites (JCPDS file no. 04–0783) [26]. The diffraction peaks located at 2θ value of 30.1°, 35.4°, 37.1°, 43.1°, 53.4°, 56.9° and 62.5° (Fig. 1a–III), ascribed to (220), (311), (222), (400), (422), (511) and (440) faces of the characteristic peaks of Fe3O4 (cubic inverse spinel structure, JCPDS file no. 19–0629) [27]. A novel weak diffraction peak at 2θ value of 35.4°, indexed to the (311) face of Fe3O4, was also observed in Fe3O4@CeO2 sample in Fig. 1a–IV, indicating that Fe3O4 is stable in the calcinations process with the depositing CeO2, which is consistent with the reference report [28]. Compared with Fig. 1a–II, Fe3O4@CeO2/Ag nanocomposites exhibited almost the same feature, except a minor peak at 35.4° of 2θ corresponding to Fe3O4 is observed (Fig. 1a–V). The diffraction profile of the sample containing the strongest (311) reflection peak of Fe3O4 was refined by Pearson–VII distribution (Fig. 1b). The diffraction peak of Fe3O4 is quite weak, comparing with that of CeO2. This is probably because that the quantity of Fe3O4 is too low and the coated of CeO2 and Ag nanoparticles on the surface of Fe3O4 nanowires, which further confirmed by the XRF elemental analysis, the mass content of Fe3O4 are about 9.14% and 7.24% for Fe3O4@CeO2 and Fe3O4@CeO2/Ag, respectively (Table S1). Fig.1 FTIR analysisis regarded as a useful tool in identifying the changes of M–O skeleton in metal oxides. Fig. 2 shows the FT–IR spectra of pure CeO2, CeO2/Ag,
33
Fe3O4@CeO2 and Fe3O4@CeO2/Ag. The absorption peaks at 495 cm-1, 541 cm-1 and 648 cm-1 are assigned to the stretching vibration of the Ce–O bond for pure CeO2 (Fig. 2a) [29]. The weak peaks at 1050 cm-1 and 1460 cm-1 can be attributed to residual organic moieties derived from HMT. Additionally, the characteristic peak at 1386 cm-1 is due to the typical bending vibration of residual nitrate ions [30]. The FT–IR spectra are almost the same after depositing Ag nanoparticles (Fig. 2b). As for Fe3O4@CeO2 (Fig. 2c), two new peaks appear at 588 cm-1 and 1120 cm-1, corresponding to the stretching vibration of Fe–O bond and residual sulfate ions, respectively [20, 31]. Obvious changes in the bands related to Fe3O4 and CeO2 are observed in the Fe3O4@CeO2/Ag (Fig. 2d): the Fe–O stretching vibration band at 588 cm-1 disappeared and the intensity of other characteristic peaks decreased dramatically, which might be caused by the relatively small amount of Fe3O4 and better dispersion of CeO2 and Ag nanoparticles in the nanocomposites. The above observations indicate that CeO2/Ag nanohybrids were successfully coated on the surface of Fe3O4 nanowires by the co–precipitation method. Fig.2 The morphology and microstructure of pure CeO2, CeO2/Ag, Fe3O4@CeO2, Fe3O4@CeO2/Ag and Fe3O4 are characterized by TEM and SEM (Fig. 3 and Fig. 4). The morphology of pure CeO2 shows spherical–shaped nanoparticles with uniform sizes (Fig. 3a), whereas CeO2 nanoparticles are gathered together when wrapping with Ag nanoparticles in Ag/CeO2 sample (Fig. 3b). The morphology of agglomerated particles
34
can be further confirmed by SEM images (Fig. 4b). Besides, the average size of CeO2/Ag nanospheres is about 200 nm in diameter and that of Ag is about 50 nm. Two distinct phases, CeO2 nanoparticles appearing black and the Fe3O4 nanowires with a lighter colour, were observed in Fe3O4@CeO2 sample (Fig. 3c and 4c). It is noticeable that CeO2 nanoparticles are uniformly coated on the surface of the Fe3O4 nanowires, and the diameter of the Fe3O4 nanowires and the size of CeO2 are about 150 and 50 nm, respectively. As shown in Fig. 3d, the as–prepared Fe3O4@CeO2/Ag nanocomposites have a relatively rough surface with average diameter of about 600 nm, and CeO2 and Ag nanoparticles are coated uniformly onto the surface of Fe3O4 nanowires with a mean thickness of 450 nm (Fig. 4d). From the results of XRD, FTIR, TEM and SEM, it can be concluded that Fe3O4 nanowire@CeO2/Ag heterojunction nanocomposites have been successfully prepared. This unique architecture plays a vital role in enhancing the adsorption of organic molecules and the separation of photo–induced charge carrier, and hence improving the photocatalytic efficiency. Fig.3 Fig.4 XPS was also employed to examine surface elements and their valence states (Fig. 5). From the survey spectra in Fig. 5a, the peaks of C, Ce, Ag, Fe and O elements are observed. The C elements are assigned to adventitious carbon contaminant. Therefore, it can be concluded that the Ce, Ag, Fe and O elements existed in the Fe3O4@CeO2/Ag nanocomposites. Regarding the typical signals of Ce3d spectra, the three main 3d5/2
35
peaks featured at around 880.1 eV, 884.6 eV and 895.6 eV correspond to the α1, α2, and α3 components, while the Ce3d3/2 located at 898.5 eV, 904.5 eV and 914.2 eV correspond to β1, β2 and β3, respectively (Fig. 5b). The signals α2 and β2, characteristics of Ce3+, are found in the Fe3O4@CeO2/Ag nanocomposites, while other peaks correspond to the Ce4+. The presence of Ce3+ can be contributed to the interaction between ceria and the surrounding sliver atoms that may have led to a transfer of lattice oxygen in the CeO2 lattice [32]. The regional XPS spectrum of Fe3O4@CeO2/Ag exhibit two peaks centered at 366.2 and 372.2 eV, assigned to Ag 3d5/2 and Ag 3d3/2, respectively, with obviously lower shift (368.2 eV and 374.2 eV for Ag0), indicating the interaction between CeO2 and Ag nanoparticles (Fig. 5c) [33]. In the spectrum of Fe2p (Fig. 5d), the Fe2p3/2 and Fe2p1/2 peaks are located at 710.4 and 725.1 eV, indicating the pure Fe3O4 phase in the Fe3O4@CeO2/Ag nanocomposites [34], further proving that Fe3O4 is stable in the calcination process. Fig. 5e presents the high–resolution XPS spectra of O1s, the peak located at around 529.1 eV is attributed to the lattice oxygen in CeO2, whereas the broad peak appeared around 531.3 eV is due to the chemisorbed oxygen [35]. XPS analysis further verifies the heterojunction formation of the novel ternary photocatalyst, and the synergistic effect among the three components result in the improvement of photodegradation. Fig. 5 The BET specific surface areas and porous structure of the as–prepared samples were investigated using nitrogen adsorption–desorption isotherms. Fig. 6 displays the
36
isotherm curves and corresponding pore size distribution curves (inset of Fig. 6) of pure CeO2, CeO2/Ag, Fe3O4@CeO2 and Fe3O4@CeO2/Ag. The adsorption–desorption isotherms of all the aforementioned samples are type IV with an apparent H3 hysteresis loop at relatively higher P/P0, indicating the presence of mesopores and macropores structure [36]. The BET surface areas listed in Table 1 indicate that Fe3O4@CeO2 has larger surface areas than pure CeO2. Similarly, this is also suitable for Fe3O4@CeO2/Ag and CeO2@Ag, which may be due to the incorporation of the Fe3O4 nanowires. Compared to CeO2 and Fe3O4@CeO2, CeO2/Ag and Fe3O4@CeO2/Ag show a significant decrease in surface area, which may be caused by the introduction of Ag nanoparticles. This is consistent with the SEM images (Fig. 4b and 4d). The results show that the introduction of the Fe3O4 nanowires plays an important contributing role in achieving the higher BET surface area, which is beneficial for photocatalytic activity owing to the increased adsorption capacities and active sites to organic pollutants. Fig. 6 Table 1 3.2.Photocatalytic activity To investigate the synergistic effect of the ternary photocatalytic system, the photocatalytic activities of various photocatalysts were evaluated for the degradation of MB solution. The degradation efficiencies of MB with pure CeO2, CeO2/Ag, Fe3O4@CeO2, Fe3O4@CeO2/Ag and Fe3O4 photocatalysts, or without photocatalyst, under natural sunlight exposure are presented in Fig. 7a. The results show that the
37
degradation of MB without a photocatalyst is relatively slow, 0.55% and 14.23% of MB observed in the dark for 60 min and after 120 min exposure, respectively. Meanwhile, in the presence of photocatalysts, the MB concentration decreases steadily with increasing sunlight exposure time. It can be seen that 94.1% of MB photocatalytically degraded after 120 min exposure for the Fe3O4@CeO2/Ag nanocomposites. However, for the CeO2/Ag, Fe3O4@CeO2, pure CeO2 and Fe3O4 samples, only 81.4%, 67.1%, 64.6% and 17.6% MB are degraded, respectively. The kinetic curves in Fig. 7b show that the relationship between –ln(Ct/C0) (C0 and Ct are the initial concentration and the concentrations of MB after sunlight exposure time for t min, respectively) and the irradiation time is almost linear, suggesting that the photocatalytic reaction follows the pseudo–first–order
kinetics.
According to
the
Langmuir–Hinshelwood
model
(–(lnCt/C0)=kt) [37], the rate constant k is estimated to be about 0.0221 min-1 for Fe3O4@CeO2/Ag nanocomposites, higher than that of CeO2/Ag (0.0131 min-1), Fe3O4@CeO2 (0.0064 min-1), pure CeO2 (0.0065 min-1), Fe3O4 (0.0016 min-1), respectively.
The
results
demonstrate
that
the
photocatalytic
activity
of
Fe3O4@CeO2/Ag nanocomposites is much higher than those of the others. As it is well known that MB may absorb the natural sunlight, the sensitization possibility of samples should be considered. Therefore, the photocatalytic activity of the Fe3O4@CeO2/Ag nanocomposites was also evaluated by phenol (50 mg L-1) and MO (15 mg L-1) degradation, and the results are presented in Fig. 7c. As can be seen, no evident photodegradation of phenol and MO was observed in the absence of catalyst. In contrast,
38
the C/C0 for phenol and MO after 180 min in the presence of the Fe3O4@CeO2/Ag nanocomposites are up to 83.9% and 74.2%, respectively. It suggests that the Fe3O4 nanowire@CeO2/Ag nanocomposites possess an outstanding photocatalytic capacity for organic pollutants degradation. In addition, UV–Vis spectra show that the absorption peaks corresponding to the substrates (MB, phenol and MO) decrease rapidly as the exposure time increases, also indicating the photodegradation of the substrates (Fig. S1, ESI). The COD removal efficiencies of MB (120 min of reaction), phenol and MO (180 min of reaction) are 68.8%, 62.4% and 57.3%, respectively (Fig. 7d), further demonstrate
that
the
excellent
enhancement
of
photocatalytic
activity
for
Fe3O4@CeO2/Ag nanocomposites under sunlight exposure. Fig. 7 The stability and reusability of a photocatalyst are of great significance for its further practical application. The cycling stability of Fe3O4@CeO2/Ag nanocomposites as a magnetically recoverable photocatalyst was investigated. As shown in Fig. 8a, the adsorption capacity and photocatalytic activity are almost retainable after 5 cycles. The photocatalytic degradation efficiency for MB only decreases from 94.1% in the 1st cycle to 93.1% in the 5th cycle. And XRD patterns of the photocatalyst before and after the five recycles are essentially same, as shown in Fig. 8b. Moreover, the photocatalyst maintained good magnetic separation effect in the external magnet (Fig. 8b inset). These results indicate that Fe3O4@CeO2/Ag nanocomposites could be used as stable photocatalyst for decomposing organic pollutants exposed to sunlight.
39
Fig. 8 The photocatalytic activity of the photocatalysts is closely related to their light absorption ability. Fig. 9 shows UV–vis DRS of pure CeO2, CeO2/Ag, Fe3O4@CeO2 and Fe3O4@CeO2/Ag nanocomposites. The absorption edge of CeO2 exists at around 400 nm, which is consistent with the reference reports [38]. In comparison with pure CeO2, there is a slight red shift in the absorption edge and an improved absorbance in the visible light region ranging from 450 to 650 nm for Fe3O4@CeO2, mainly resulting from the narrower band gap of Fe3O4. This phenomenon is analogous to the recent studies on Fe3O4/AgBr and Fe3O4/g–C3N4 [19, 39]. Furthermore, when Ag nanoparticles are selectively deposited on the surfaces, CeO2/Ag and Fe3O4@CeO2/Ag present more intense light absorption in the visible light region 450–800 nm, this may be attributed to the SPR effect of Ag nanoparticles [16]. The widen adsorption region of CeO2/Ag and Fe3O4@CeO2/Ag is beneficial for producing more photogenerated charge carriers. In addition, the direct band gap energy (Eg) of the nanoparticles can be calculated from the equation of (αhν)2=A (hν – Eg), where hν is the photon energy, α is the absorption coefficient, and A is a constant for the certain material [40]. The Eg values of pure CeO2, CeO2/Ag, Fe3O4@CeO2 and Fe3O4@CeO2/Ag are calculated to be 3.17, 2.35, 2.95 and 2.11 eV, respectively (Fig. 9b). Obviously, Fe3O4@CeO2/Ag exhibits a remarkable decreased Eg compared to CeO2/Ag, implying that the synergistic effect of Ag/CeO2 and Fe3O4 in the ternary nanocomposites, and further narrows the band gap of CeO2. Thus, the Fe3O4@CeO2/Ag nanocomposites have better light absorption ability.
40
Fig. 9 PL technique is an effective way to study the efficiency of charge carrier trapping, immigration and transfer at the surface of photocatalysts. A lower PL intensity usually implies a lower electron–hole recombination rate and higher photocatalytic activity [41, 42]. The PL spectra of pure CeO2, CeO2/Ag, Fe3O4@CeO2 and Fe3O4@CeO2/Ag are presented in Fig. 10. It is clearly observed that pure CeO2 displays a strong PL intensity at around 451 nm, 468 nm and 483 nm. The mild variation may be attributed to synthetic route and calcination conditions [43]. After the Fe3O4 nanowires are introduced, the PL signal intensity shows an apparent decrease, indicating that the Fe3O4 nanowires improve the transfer of photogenerated electron–hole pairs. Moreover, owing to the addition of Ag nanoparticles, the PL intensity of the CeO2/Ag is distinctly lower than that of CeO2, which confirmed that the presence of Ag nanoparticles promotes photogenerated charge carriers. Moreover the PL intensity of ternary Fe3O4@CeO2/Ag nanocomposites is the lowest among all the samples, suggesting that the more photogenerated electrons are trapped by the cooperative effect of Ag nanoparticles and Fe3O4 nanowires and more effective charge separation are obtained. Fig. 10 Fig.11 shows the 1/C2 versus V plots for pure CeO2, CeO2/Ag, Fe3O4@CeO2, Fe3O4@CeO2/Ag and Fe3O4. The flat band potential shift from –0.453 V for pure CeO2 and –0.439 V for Fe3O4@CeO2 to –0.511 V for CeO2/Ag and –0.533 V for Fe3O4@CeO2/Ag nanocomposites, respectively (Table 2). The carrier density of pure
41
CeO2 (2.16×1018 cm-3) and Fe3O4@CeO2 (1.50×1018 cm-3) are lower than those of CeO2/Ag (6.31×1018 cm-3) and Fe3O4@CeO2/Ag nanocomposites (8.58×1018 cm-3), respectively. The results indicate that the addition of Ag nanoparticles lead to a more negative shift of Vfb and a higher carrier density. This is because the band bending at the semiconductor/electrolyte interface pushes electrons away from the interface while attracting holes, increasing the band bending while keeping the Fermi level equilibration [44]. The shift to a more negative flat–band potential indicates that Fe3O4@CeO2/Ag nanocomposites are better photocatalyst than the other samples [45]. However, the addition of Fe3O4 nanowire has little effect on the flat band potential. Fig. 11 3.3. Photocatalytic mechanism According to the above discussion, a possible mechanism for the Fe3O4@CeO2/Ag nanocomposites is proposed (Scheme 2). On one hand, the Fermi energy of Ag (4.26 eV) is lower than that of CeO2 (about 5.0 eV). This leads to the electrons transfer from the Fermi level of Ag to that of CeO2 until the energy thermodynamic equilibrium was established [46, 47]. The Ag nanoparticles can absorb resonance photons and generated hot electrons with energy 1.0–4.0 eV with respect to the metal Fermi level [16]. With natural sunlight exposure, the equilibrated Fermi level electrons are injected rapidly into the CeO2 conduction band via a SPR mechanism [37]. And DRS and Mott-Schottky analysis further proved that Ag/CeO2 heterojunction can effectively not only improve the light absorption ability but also drive the separation and transportation of the
42
electron–hole pairs. On the other hand, due to the large surface area and the conductivity of Fe3O4 nanowires [38], the interface between CeO2 and Fe3O4 effectively increases the electron–hole separation by transferring the photogenerated electrons from the conduction band of one CeO2 molecule to that of another CeO2 molecule through the Fe3O4 nanowires. Thus, electrons trapped in the Fe3O4 nanowires may have a long lifetime and can be transferred to the interface between the nanocomposites and solution. The photogenerated electrons react with O2 absorbed on the surface of Fe3O4@CeO2/Ag and produce the active species •O2-, which further react with H2O to form hydroxyl radicals (•OH), an extremely strong oxidant for the partial or complete mineralization of organic chemicals [14]. Therefore, Fe3O4@CeO2/Ag nanocomposites can be used as an effective sunlight active photocatalyst. Both Ag and CeO2 play important roles in improving the photocatalytic activity, and that the 1D structure of the Fe3O4 nanowires is also critical for the photocatalytic effect of the nanocomposites. Scheme 2
4. Conclusions In this study, we have successfully synthesized Fe3O4 nanowire@CeO2/Ag nanocomposites photocatalyst via simple hydrothermal and co–precipitation techniques. The photocatalytic activity of the Fe3O4@CeO2/Ag nanocomposites for MB is demonstrated to be superior to those of pure CeO2, CeO2/Ag, Fe3O4@CeO2, and Fe3O4. It also shows excellent photocatalytic activity and mineralizing ability in the phenol and MO degradation. The enhanced photocatalytic activity is closely associated with
43
electrons trapping of CeO2/Ag and electrons transferring of Fe3O4 nanowires, which leads to the extended light absorption range and higher separation efficiency of electron–hole pairs. In addition, the Fe3O4@CeO2/Ag nanocomposites show high stability, recoverable and reusability, which is very beneficial for practical applications. The present study provides a promising cost–effective photocatalytic material in environmental protection area.
Acknowledgements The authors are grateful to be supported by the Innovation Funds of Graduate Programs SNNU (2015CXS040), the Fundamental Research Funds for the Central Universities (GK201302013), the National Natural Science Foundation of China (No. 51204119), and the Natural Science Foundation of Shanxi (nos. 2014021015–4, 2015021107).
References [1] T. Hisatomi, J. Kubota, K. Domen, Recent advances in semiconductors for photocatalytic and
photoelectrochemical water splitting, Chem. Soc. Rev. 43 (2014) 7520–7535.
[2] M.N. Chong, B. Jin, C.W.K. Chow, C. Saint, Recent developments in photocatalytic water treatment technology:
A review, Water Res. 44 (2010) 2997–3027.
[3] H. Hou, M. Shang, L. Wang, W. Li, B. Tang, W. Yang, Efficient photocatalytic activities of TiO2 hollow fibers with mixed phases and mesoporous walls, Sci. Rep. 5 (2015) 15228.
44
[4] Y. Chen, C. Zhang, W. Huang, Y. Situ, H. Huang, Multimorphologies nano–ZnO preparing through a simple
solvothermal method for photocatalytic application, Mater. Lett. 141 (2015) 294–297.
[5] H. Chen, T. Tu, M. Wen, Q. Wu, Assembly synthesis of Cu 2O–on–Cu nanowires with visible–light–enhanced photocatalytic activity, Dalton Trans. 44 (2015) 15645–15652.
[6] Y.M. Hunge, M.A. Mahadik, S.S. Kumbhar, V.S. Mohite, K.Y. Rajpure, N.G. Deshpande, A.V. Moholkar, C.H.
Bhosale, Visible light catalysis of methyl orange using nanostructured WO3 thin films, Ceram. Int. 42 (2016) 789–798.
[7] E.M. Seftel, M.C. Puscasu, M. Mertens, P. Cool, G. Carja, Fabrication of CeO 2/LDHs self–assemblies with enhanced photocatalytic performance: A case study on ZnSn–LDH matrix, Appl. Catal. B: Environ. 164 (2015)
251–260.
[8] M.M. Khan, S.A. Ansari, D. Pradhan, D.H. Han, J. Lee, M.H. Cho, Defect–induced band gap narrowed CeO2 nanostructures for visible light activities, Ind. Eng. Chem. Res. 53 (2014) 9754–9763.
[9] G. Li, Z. Tang, Noble metal nanoparticle@metal oxide core/yolk–shell nanostructures as catalysts: recent progress
and perspective, Nanoscale. 6 (2014) 3995–4011.
[10] Z. Abbasi, M. Haghighi, E. Fatehifar, S. Saedy, Synthesis and physicochemical characterizations of nanostructured Pt/Al2O3–CeO2 catalysts for total oxidation of VOCs, J. Hazard. Mater. 186 (2011) 1445–1454. [11] D. Geetha, S. Kavitha, P.S. Ramesh, A novel bio–degradable polymer stabilized Ag/TiO2 nanocomposites and their catalytic activity on reduction of methylene blue under natural sun light, Ecotoxicol. Environ. Saf. 121 (2015)
126–134.
[12] M. Hoffmann, S. Kreft, G. Georgi, G. Fulda, M.M. Pohl, D. Seeburg, C. Berger Karin, E.V. Kondratenko, S.
Wohlrab, Improved catalytic methane combustion of Pd/CeO2 catalysts via porous glass integration, Appl. Catal. B:
45
Environ. 179 (2015) 313–320.
[13] D. Jiang, W. Wang, S. Sun, L. Zhang, Y. Zheng, Equilibrating the plasmonic and catalytic roles of metallic
nanostructures in photocatalytic oxidation over Au–modified CeO2, ACS Catal. 5 (2015) 613–621. [14] T. Liu, B. Li, Y. Hao, F. Han, L. Zhang, L. Hu, A general method to diverse silver/mesoporous–metal–oxide
nanocomposites with plasmon–enhanced photocatalytic activity, Appl. Catal. B: Environ. 165 (2015) 378–388.
[15] Q. Leng, D. Yang, Q. Yang, C. Hu, Y. Kang, M. Wang, M. Hashim, Building novel Ag/CeO 2 heterostructure for enhancing photocatalytic activity, Mater. Res. Bull. 65 (2015) 266–272.
[16] L. Wu, S. Fang, L. Ge, C. Han, P. Qiu, Y. Xin, Facile synthesis of Ag@CeO2 core–shell plasmonic photocatalysts with enhanced visible–light photocatalytic performance, J. Hazard. Mater. 300 (2015) 93–103.
[17] J. Zhan, H. Zhang, G. Zhu, Magnetic photocatalysts of cenospheres coated with Fe 3O4/TiO2 core/shell nanoparticles decorated with Ag nanopartilces, Ceram. Int. 40 (2014) 8547–8559.
[18] J. Rashid, M.A. Barakat, Y. Ruzmanova, A. Chianese, Fe3O4/SiO2/TiO2 nanoparticles for photocatalytic degradation of 2–chlorophenol in simulated wastewater, Environ. Sci. Pollut. Res. Int. 22 (2015) 3149–3157.
[19] Z. Zhu, Z. Lu, D. Wang, X. Tang, Y. Yan, W. Shi, Y. Wang, N. Gao, X. Yao, H. Dong, Construction of
high–dispersed Ag/Fe3O4/g–C3N4 photocatalyst by selective photo–deposition and improved photocatalytic activity, Appl. Catal. B: Environ. 182 (2016) 115–122.
[20] C. Li, J. Tan, X. Fan, B. Zhang, H. Zhang, Q. Zhang, Magnetically separable one dimensional
Fe3O4/P(MAA–DVB)/TiO2 nanochains: Preparation, characterization and photocatalytic activity, Ceram. Int. 41 (2015) 3860–3868.
[21] Y. Wang, J. Ning, E. Hu, C. Zheng, Y. Zhong, Y. Hu, Direct coating ZnO nanocrystals onto 1D Fe 3O4/C composite microrods as highly efficient and reusable photocatalysts for water treatment, J. Alloys Compd. 637 (2015)
46
301–307.
[22] X. Bian, K. Hong, X. Ge, R. Song, L. Liu, M. Xu, Functional hierarchical nanocomposites based on ZnO
nanowire and magnetic nanoparticle as highly active recyclable photocatalysts, J. Phys. Chem. C. 119 (2015)
1700–1705.
[23] J. Tian, Z. Zhao, A. Kumar, R.I. Boughton, H. Liu, Recent progress in design, synthesis, and applications of
one–dimensional TiO2 nanostructured surface heterostructures: a review, Chem. Soc. Rev. 43 (2014) 6920–6937. [24] K. Gelderman, L. Lee, S.W. Donne, Flat–band potential of a semiconductor: using the Mott–Schottky equation, J.
Chem. Edu, 84 (2007) 685–688.
[25] T.S. Sakthivel, D.L. Reid, U.M. Bhatta, G. Mobus, D.C. Sayle, S. Seal, Engineering of nanoscale defect patterns
in CeO2 nanorods via ex situ and in situ annealing, Nanoscale. 7 (2015) 5169–5177. [26] Z. Yang, K.T. Nguyen, H. Chen, H. Qian, L.P. Fernando, K.A. Christensen, J.N. Anker, Plasmonic silver
nanobelts via citrate reduction in the presence of HCl and their orientation–dependent scattering properties, J. Phys.
Chem. Lett. 2 (2011) 1742–1746.
[27] M. Abbas, B.P. Raoc, V. Reddya, C. G. Kima, Fe3O4/TiO2 core/shell nanocubes: Single–batch surfactantless synthesis, characterization and efficient catalysts for methylene blued egradation, Ceram. Int. 40 (2014)
11177–11186. [28] L. Zhao, N. Li, X. Li, W. Yan, Y. Chi, Q. Yuan, Y. Li, Magnetically separable Fe3O4@SiO2@TiO2–Ag microspheres with well–designed nanostructure and enhanced photocatalytic activity. J. Hazard. Mater, 262 (2013)
404–411.
[29] L. Wang, J. Ding, Y. Chai, Q. Liu, J. Ren, X. Liu, W.L. Dai, CeO2 nanorod/g–C3N4/N–rGO composite: enhanced visible–light–driven photocatalytic performance and the role of N–rGO as electronic transfer media, Dalton Trans. 44
47
(2015) 11223–11234.
[30] S. Narang, U.P. Singh, P. Venugopalan, Solvent–mediated supramolecular templated assembly of a metal
organophosphonate via a crystal–amorphous–crystal transformation, CrystEngComm, 18 (2016) 54–61.
[31] F. Chen, H. Ma, B. Wang, Simple and effective synthesis of methoxy–dimethylbenzene from electrochemical oxidation of p–xylene in methanol solvent catalyzed by SO42-/ZrO2–MxOy, J. Hazard. Mater. 162 (2009) 668–673. [32] N. Zhang, X. Fu, Y.J. Xu, A facile and green approach to synthesize Pt@CeO2 nanocomposite with tunable core–shell and yolk–shell structure and its application as a visible light photocatalyst, J. Mater. Chem. 21 (2011)
8152–8158. [33] D. Lin, H. Wu, R. Zhang, W. Pan, Enhanced photocatalysis of electrospun Ag−ZnO heterostructured nanofibers,
Chem. Mater. 21 (2009) 3479–3484. [34] J. Ma, S. Guo, X. Guo, H. Ge, A mild synthetic route to Fe3O4@TiO2–Au composites: preparation, characterization and photocatalytic activity, Appl. Surf. Sci. 353 (2015) 1117–1125.
[35] C. Lee, J.I. Park, Y.G. Shul, H. Einaga, Y. Teraoka, Ag supported on electrospun macro–structure CeO2 fibrous mats for diesel soot oxidation, Appl. Catal. B: Environ. 174–175 (2015) 185–192.
[36] X. Zhang, N. Zhang, Y.J. Xu, Z.R. Tang, One–dimensional CdS nanowires–CeO2 nanoparticles composites with boosted photocatalytic activity, New J. Chem. 39 (2015) 6756–6764.
[37] D. Channei, B. Inceesungvorn, N. Wetchakun, S. Ukritnukun, A. Nattestad, J. Chen, S. Phanichphant,
Photocatalytic degradation of methyl orange by CeO2 and Fe–doped CeO2 films under visible light irradiation, Sci. Rep. 4 (2014) 5757.
[38] M.M. Khan, S.A. Ansari, M.O. Ansari, B.K. Min, J. Lee, M.H. Cho, Biogenic fabrication of Au@CeO2 nanocomposite with enhanced visible light activity, J. Phys. Chem. C. 118 (2014) 9477–9484.
48
[39] Y. Cao, C. Li, J. Li, Q. Li, J. Yang, Magnetically separable Fe3O4/AgBr hybrid materials: highly efficient photocatalytic activity and good stability, Nanoscale Res. Lett. 10 (2015) 1–6.
[40] D.P.a.K.T. Leung, Controlled growth of two–dimensional and one–dimensional ZnO nanostructures on indium
tin oxide coated glass by direct electrodeposition, Langmuir, 24 (2008) 9707–9716.
[41] J. Su, Y. Zhang, S. Xu, S. Wang, H. Ding, S. Pan, G. Wang, G. Li, H. Zhao, Highly efficient and recyclable
triple–shelled Ag@Fe3O4@SiO2@TiO2 photocatalysts for degradation of organic pollutants and reduction of hexavalent chromium ions, Nanoscale, 6 (2014) 5181–5192. [42] W. Li, X. Liu, H. Li, Hydrothermal synthesis of graphene/Fe3+–doped TiO2nanowire composites with highly enhanced photocatalytic activity under visible light irradiation, J. Mater. Chem. A, 3 (2015) 15214–15224.
[43] M. Aslam, M.T. Qamar, M.T. Soomro, I.M.I. Ismail, N. Salah, T. Almeelbi, M.A. Gondal, A. Hameed, The effect
of sunlight induced surface defects on the photocatalytic activity of nanosized CeO2 for the degradation of phenol and its derivatives, Appl. Catal. B: Environ. 180 (2016) 391–402.
[44] Z. Zhang, J.T.Y. Jr, Band bending in semiconductors: chemical and physical consequences at surfaces and
interfaces, Chem. Rev, 112 (2012) 5520−5551.
[45] H. Cai, P. Liang, Z. Hu, L. Shi, X. Yang, J. Sun, N. Xu, J. Wu, Enhanced photoelectrochemical activity of
ZnO–Coated TiO2 nanotubes and its dependence on ZnO coating thickness, Nanoscale Res. Lett, 11(2016)1–11. [46] C. Zhang, A. Michaelides, D.A. King, S.J. Jenkins, Structure of gold atoms on stoichiometric and defective ceria
surfaces, J Chem Phys, 129 (2008) 100–103.
[47] L. Xu, F. Zhang, X. Song, Z. Yin, Y. Bu, Construction of reduced graphene oxide–supported Ag–Cu2O composites with hierarchical structures for enhanced photocatalytic activities and recyclability, J. Mater. Chem. A, 3
(2015) 5923–5933.
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Table captions: Table 1 Porosity parameters of CeO2, CeO2/Ag, Fe3O4@CeO2 and Fe3O4@CeO2/Ag. Table 2 The semiconductor characteristic parameters of the samples.
50
Figure captions: Fig. 1. (a)XRD patterns of CeO2(I), CeO2/Ag(II), Fe3O4 (III), Fe3O4@CeO2 (IV) and Fe3O4@CeO2/Ag (V); (b) Profile refined by Pearson–VII distribution.
Fig. 2. FT–IR spectra of CeO2 (a), CeO2/Ag (b), Fe3O4@CeO2 (c) and Fe3O4@CeO2/Ag (d). Fig. 3. TEM images of CeO2 (a), CeO2/Ag (b), Fe3O4@CeO2 (c), Fe3O4@CeO2/Ag (d) and Fe3O4 (e). Fig. 4. SEM images of CeO2 (a), CeO2/Ag (b), Fe3O4@CeO2 (c), Fe3O4@CeO2/Ag (d) and Fe3O4 (e). Fig. 5. XPS patterns of the Fe3O4@CeO2/Ag nanocomposites: survey spectra (a), Ce 3d (b), Ag 3d (c), Fe 2p (d) and O 1s (e).
Fig. 6. N2 adsorption–desorption isotherm and corresponding pore–size distribution curves (inset) of the obtained pure CeO2, CeO2/Ag, Fe3O4@CeO2 and Fe3O4@CeO2/Ag. Fig. 7. (a) The photodegradation of MB with or without different catalysts: pure CeO2, CeO2/Ag, Fe3O4@CeO2, Fe3O4@CeO2/Ag and Fe3O4; (b) The kinetic curve of photocatalytic MB degradation using different catalysts; Photocatalytic degradation (c) and COD removal (d) of MB, phenol and MO solution in the presence or
absence of Fe3O4@CeO2/Ag. Fig. 8. (a) Recycles of Fe3O4@CeO2/Ag in the degradation of MB by Fe3O4@CeO2/Ag. (b) XRD patterns of Fe3O4@CeO2/Ag photocatalyst before (I) and after (II) the recycling experiment. Fig. 9. (a) UV–vis DRS of pure CeO2, CeO2/Ag, Fe3O4@CeO2 and Fe3O4@CeO2/Ag; (b) plots of (αhν)2 vs. photon energy (hν).
Fig. 10. Photoluminescence (PL) spectra of pure CeO2, CeO2/Ag, Fe3O4@CeO2 and Fe3O4@CeO2/Ag nanocomposites with an excitation wavelength of 325 nm.
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Fig. 11. Mott–Schottky plots of pure CeO2, CeO2/Ag, Fe3O4@CeO2, Fe3O4@CeO2/Ag and Fe3O4.
Illustration captions Scheme 1. Schematic preparation of Fe3O4@CeO2/Ag. Scheme 2. Illustration of the mechanism for photogenerated charge carrier transfers in the Fe3O4@CeO2/Ag under natural sunlight exposure.
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Table 1 Porosity parameters of CeO2, CeO2/Ag, Fe3O4@CeO2 and Fe3O4@CeO2/Ag
Samples
SBET (m2 g-1)
V (cm3 g-1)
Average pore size (nm)
CeO2
27.61
0.0894
24.10
CeO2/Ag
9.85
0.0304
37.77
Fe3O4@CeO2
36.20
0.0961
49.32
Fe3O4@CeO2/Ag
16.11
0.0402
36.69
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Table 2 The semiconductor characteristic parameters of the samples.
Samples
Vbf (v)
ND × 1018 (cm-3)
CeO2
–0.453
2.16
CeO2/Ag
–0.511
6.31
Fe3O4@CeO2
–0.439
1.50
Fe3O4@CeO2/Ag
–0.533
8.58
Fe3O4
–0.411
0.45
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Fig.1. (a)XRD patterns of CeO2(I), CeO2/Ag(II), Fe3O4 (III), Fe3O4@CeO2 (IV) and Fe3O4@CeO2/Ag (V); (b) Profile refined by Pearson-VII distribution.
55
Fig. 2. FT–IR spectra of CeO2 (a), CeO2/Ag (b), Fe3O4@CeO2 (c) and Fe3O4@CeO2/Ag (d).
56
Fig. 3. TEM images of CeO2 (a), CeO2/Ag (b), Fe3O4@CeO2 (c), Fe3O4@CeO2/Ag (d) and Fe3O4 (e).
57
Fig. 4. SEM images of CeO2 (a), CeO2/Ag (b), Fe3O4@CeO2 (c), Fe3O4@CeO2/Ag (d) and Fe3O4 (e).
58
Fig. 5. XPS patterns of the Fe3O4@CeO2/Ag nanocomposites: survey spectra (a), Ce 3d (b), Ag 3d (c), Fe 2p (d) and O 1s (e).
59
Fig. 6. N2 adsorption–desorption isotherm and corresponding pore–size distribution curves (inset) of the obtained pure CeO2, CeO2/Ag, Fe3O4@CeO2 and Fe3O4@CeO2/Ag.
60
Fig. 7. (a) The photodegradation of MB with or without different catalysts; (b) The kinetic curve of photocatalytic
MB degradation using different catalysts; Photocatalytic degradation (c) and COD removal (d) of MB, phenol and
MO solution in the presence or absence of Fe3O4@CeO2/Ag.
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Fig. 8. (a) Recycles of Fe3O4@CeO2/Ag in the degradation of MB. (b) XRD patterns of Fe3O4@CeO2/Ag photocatalyst before (I) andafter (II) the recycling experiment.
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Fig. 9. (a) UV–vis DRS of pure CeO2, CeO2/Ag, Fe3O4@CeO2 and Fe3O4@CeO2/Ag; (b) plots of (αhν)2 vs. photon energy (hν).
63
Fig. 10. PL spectra of pure CeO2, CeO2/Ag, Fe3O4@CeO2 and Fe3O4@CeO2/Ag nanocomposites with an excitation wavelength of 325 nm.
64
Fig. 11. Mott–Schottky plots of pure CeO2, CeO2/Ag, Fe3O4@CeO2, Fe3O4@CeO2/Ag and Fe3O4.
65
Scheme 1. Schematic preparation of Fe3O4@CeO2/Ag.
66
Scheme. 2. Illustration of the mechanism for photogenerated charge carrier transfers in the Fe3O4@CeO2/Ag under natural sunlight exposure.
67