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ZnO-GO nanocomposites with improved photocatalytic degradation methyl orange under visible light irradiation
Journal of Alloys and Compounds 737 (2018) 197e206
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Synthesis and characterization of Fe3O4/ZnO-GO nanocomposites with improved photocatalytic degradation methyl orange under visible light irradiation Qi Feng a, b, Shaoyuan Li a, b, *, Wenhui Ma a, b, Hua-Jun Fan c, Xiaohan Wan b, Yun Lei a, b, Zhengjie Chen b, Jia Yang b, Bo Qin b a
State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming University of Science and Technology, Kunming 650093, China Institute of New Energy/Silicon Metallurgy and Silicon Material Engineering Research Center of Universities in Yunnan Province, Kunming University of Science and Technology, Kunming 650093, China c Department of Chemistry, Prairie View A&M University, Prairie View, TX 77446, United States b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 August 2017 Received in revised form 7 November 2017 Accepted 7 December 2017 Available online 9 December 2017
To improve the photocatalytic property and recovery rate of existing ZnO materials, a magnetically separable heterostructure photocatalytic composed of GO sheets and nanohybrid Fe3O4/ZnO material was synthesized by a simple low-temperature chemical synthesis process. This synthesis enables simultaneous decoration of Fe3O4/ZnO spheres on both sides of the GO sheets. As a result, the Tauc's plot revealed the band gap of Fe3O4/ZnO-GO sample decreases to 2.07 eV from 3.38 eV of ZnO. This smaller energy band gap and slower recombination rate of electron-hole pairs greatly improve the photocatalytic activity of the Fe3O4/ZnO-GO complex through the wider visible light absorption range. The photocatalytic reaction rate of the pure ZnO photocatalyst is 4.84 103 min1, for Fe3O4/ZnO composite is 1.564 102 min1 and for Fe3O4/ZnO-GO composite is 5.558 102 min1. The stability and recovery rate and photocatalytic property retention also greatly improved from ZnO-only photocatalyst. The photocatalytic reactivity of Fe3O4/ZnO-GO nanocomposite structure reaches 92.8% efficiency on the first run and was at 75% efficiency after the four cycling. © 2017 Elsevier B.V. All rights reserved.
Keywords: Ternary nanohybrid compounds Magnetical photocatalyst Graphene oxide sheets Methyl orange degradation
1. Introduction In recent decades, environmental pollution and its related effects have posed significant problems that increasingly confront human societies [1]. In addition to the drinkable water shortage and drought in the various region of the global, such water contamination threats human race survival [2,3]. To find an efficient and economically viable solution to eliminating the water contaminants becomes a pressing issue for every nation. More and more research turn to the solar energy, the radiant light, and heat from the sun, which is the most abundant clean energy source on the Earth. Extensive research studies focus on the development of new materials that can efficiently harvest solar irradiation and use it for green environmental pollution management. Namely, the
* Corresponding author. 253 Xuefu Road, Kunming 650093, Yunnan Province, China. E-mail addresses: [email protected] (S. Li), [email protected] (W. Ma). https://doi.org/10.1016/j.jallcom.2017.12.070 0925-8388/© 2017 Elsevier B.V. All rights reserved.
photocatalysis process that utilizes such renewable solar energy to activate the chemical reactions by oxidation and reduction process becomes a sustainable and attractive technology to provide a feasible solution for environmental problems [4]. Several types of promising photocatalysts, such as TiO2, ZnO, ZrO2, vanadium oxide and WO3 have been studied for their application in the environmental science [5]. Among these semiconductors, ZnO becomes one of the excellent and promising candidates as a green photocatalytic environmental reactor [6,7] because of its easy synthesis and environmentally friendly. In addition, ZnO possesses many other properties such as direct and wide band gap in the near-UV spectral region, strong oxidation ability, good photocatalytic property, and a large free-exciton binding energy [8], which makes its excitonic emission processes continuum at or even above room temperature. However, two major drawbacks hinder its wider application as a photoelectrode: one is the low photocatalytic efficiency, and the other is difficult to separate and recovery after the photocatalysis [9]. Such low recovery and reusability of ZnO particles are critical to their further
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applications for economic reasons. Centrifugation and filtration separation methods not only are impractical in the field but also still lead to catalyst loss and added energy consumption. Fixation the photocatalysts on thin films showed that the catalytic activity was considerably reduced because the effective surface area has been significantly decreased upon fixation [10]. For the sake of practicality, immobilizing ZnO catalysts onto the surface of magnetic nano- or micro particles becomes a trend because now the catalyst can be easily separated and recovered with an external magnet and the loss of catalysts can be minimized [11e14]. Several materials with different elemental compositions have been investigated as the magnetic core, such as Fe3O4, gFe2O3, NiFe2O4 and Co3O4. Among these materials, Fe3O4 has drawn great attention due to its remarkable magnetic properties, low toxicity, and biocompatibility [15e22]. Studies found that, after introducing Fe3O4 as the core of Fe3O4/ZnO hybrid nanostructures, photodissolution may occur in the hybrid nanoparticles, which would decrease the photocatalytic efficiency of ZnO materials [23]. To remedy this issue, we decided to introduce graphene oxide (GO) to be the third component of our carrier platform for Fe3O4/ZnO nanocomposite because GO possesses not only a unique electronic property but also is environmentally friendly. Graphene oxide (GO) is one of the most promising precursors of graphene and has attracted tremendous attention for its unique physicochemical properties [24e28]. Graphene, a novel twodimensional carbon material, has been recognized as an ideal candidate for a variety of application owing to its high surface area, striking electronic, mechanical and thermal properties [29], while the surface of GO are bedecked with diverse hydrophilic oxygencontaining functional groups [30,31]. Inorganic materials modified with partially GO have been reported to show enhanced photocatalytic properties [32]. In recent years, nanocomposites based on graphene have been widely investigated and their photocatalytic properties have been demonstrated [33,34], including TiO2/graphene [35,36], BiVO4/graphene [37,38], ZnO/Graphene [39,40] and like materials. In this study, non-toxic Fe3O4/ZnO-GO composite material was synthesized via a mild hydrothermal process, and the photocatalytic property was monitored through the degradation of methyl orange. Various characterization tools such as X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM) energy dispersive X-ray analysis (EDS), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) spectra, and UVevisible diffuse reflectance spectra was employed in this study. This work investigates (1) the structure and composition of the Fe3O4/ZnO-GO nanocomposite materials; (2) the photocatalytic activities of Fe3O4/ ZnO-GO composite materials; and (3) the recovery rate and photocatalytic reusability of Fe3O4/ZnO-GO material.
50 C. Afterward, the mixed solution was transferred into a Teflonlined autoclave reactor and heated to 200 C for 3 h. The produced Fe3O4 particles were collected in a magnetic field, washed with DI water until neutral and dried to a constant weight at 60 C under vacuum. To prepare the Fe3O4/ZnO nanocomposites, 1 g of aforementioned Fe3O4 solid was mixed with 20 mL DI water and 30 mL ethanol by ultrasonication until forming a uniform solution. Then, a mixture of 10 mmol Zn(Ac)2$2H2O with Tris (2-Hydroxyethyl) amine were dropwise added into the Fe3O4-DI-Ethanol mixture under stirring. This solution was transferred into a Teflon-lined autoclave reactor at 90 C for 1 h. The Fe3O4/ZnO nanocomposites were then filtered, washed with DI and ethanol and finally dried in an oven at 60 C. 2.3. Sensitization of Fe3O4/ZnO with GO nanosheets The GO was synthesized from natural flake graphite following a modified Hummer method [41]. Coating of the GO sheets with Fe3O4/ZnO was carried out by one-step chemical bath deposition method. Specifically, 1 mg/mL of GO dispersion was created through sonication. After stirring for 30 min at room temperature, 80 mg Fe3O4/ZnO was dispersed into this GO solution and stirred for additional 3 h at 100 C. The final product Fe3O4/ZnO-GO was centrifuged and washed several times with DI water, followed by applying an external magnet for isolation and drying at 60 C. 2.4. Characterization Phase information and the crystalline quality of the prepared nanostructures were investigated using X-ray diffraction (XRD; BrukerD4) with monochromatized Cu Ka radiation at the wavelength l ¼ 1.5406 Å. The surface morphology and structure were studied using a scanning electron microscope (SEM, FEI Quanta 200), transmission electron microscopy and energy dispersive Xray analysis (TEM/EDS, Tecnai G2 TF30 S-Twin). The specific surface area of the catalysts was measured from N2 adsorption at 77 K using a Micrometric ASAP 2020 system. Magnetization measurements were obtained at room temperature using a vibrating sample magnetometer (VSM, Lakeshore 7307) under the magnetic field range of 8.0e8.0 kOe. Raman spectroscopy of the composite was collected on a Dilor Labram-1B microspectrometer with 632.8 nm laser excitation. The core electron configuration of samples was determined by X-ray photoelectron spectroscopy (XPS, PHI-5500). The photoluminescence (PL) spectra of the sample were recorded at room temperature using a fluorescence spectrophotometer (Dong Woo Optron, South Korea) with an attached He-Cd laser at an excitation wavelength of 325 nm. The UVevisible diffuse reflectance spectra of samples, recorded in the range of 200e800 nm, were obtained using an ultravioletevisible (UVeVis, TU-1901) spectrophotometer.
2. Experimental procedure 2.5. Photocatalytic degradation of methyl orange 2.1. Preparation of ZnO A mixture of 10 mmol Zn(Ac)2$2H2O with Tris (2-Hydroxyethyl) amine was mixed thoroughly under stirring before the mixture was transferred into a Teflon-lined reactor and heated at 90 C for 1 h. The ZnO was then filtered, washed with DI and ethanol, and finally dried in an oven at 60 C until it is ready to be used. 2.2. Preparation of Fe3O4/ZnO As illustrated in Scheme 1, 2.7 g FeCl3$6H2O was dissolved in 100 mL of ethylene glycol before the mixture of 7.2 g NaAC and 2 g polyethyleneglycol was added under a nitrogen atmosphere at
The photocatalytic activity of Fe3O4/ZnO-GO nanocomposite material was evaluated by the degradation of Methyl Orange (1.0 105 M, 100 mL) under a 300 W Xe lamp with 420 nm light filter (custom-made) and monitored via UVeVis spectrophotometer. Particularly, 20 mg Fe3O4/ZnO-GO nanocomposite material was dispersed in the Methyl Orange dye solution with stirring in the dark for 1 h to allow absorption-desorption equilibrium before light irradiation. Then the dye solution containing the dispersed catalyst was illuminated by the custom-made Xe lamp while being continuously stirred. After given time intervals (30 min), 3 mL solution was withdrawn, centrifuged, and analyzed by a UVeVis spectrophotometer. The photocatalytic kinetic rate was calculated
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Scheme 1. Illustration for the preparation of Fe3O4/ZnO-GO nanocomposites.
based on the Langmuir-Hinshelwood kinetic model using the empirical Equation:
Ct ¼ C0 ekt Through this first order kinetic model, we can correlate UVeVis spectra with the change in concentration of the Methyl Orange dye before (C0) and after light irradiation (Ct), as well as derive the energy band gap. As a comparison, the photocatalytic activity of ZnO and Fe3O4/ZnO composite material were evaluated under the same condition. 3. Results and discussion 3.1. Morphology and surface area analysis Due to the abundance of oxygen-containing functional groups (hydroxyl, epoxide and carboxylic groups) and vast p-orbitals pdelocalization of carbon atoms on the basal planes of GO nanosheets, the GO sheets have high electron density on its surface and prone to local polarization. This unique electronic property enables GO sheets easily attract Fe3O4/ZnO nanoparticles on its surface. This can be identified and characterized through the morphology characterization tools such as scanning electron microscopy (SEM) as shown in Fig. 1(a). This SEM image of GO sheet itself demonstrates relatively flat layer structure and contains a unique fold
structure. For comparison, Fig. 1(b) shows the Raman spectra of between graphite (blue) and GO (red). Fig. 1(b) showed that graphite has a unique sharp and strong absorption peak (G peak) at 1576 cm1 and a relatively strong peak (2D band or G band) 2711 cm1, indicating a neat structure of sp2 graphite. These bands correspond to the stretching of the C]C bond, which is typical of all graphitic materials [42]. However, the Raman spectra of GO is quite different from that of graphite. For example, not only did 2D band intensity of GO diminish dramatically and the G peak (1578 cm1) broadened, but also there is a drastic increase the intensity of absorption peak (D band) at 1345 cm1. This difference is because when the graphite was oxidized, the sp2 hybrid carbon atoms in graphite were transformed into the sp3 hybrid structure, thus destroys the C]C double bond in the graphite layer. Raman spectroscopy is sensitive enough to characterize such distortion and shows such as D band [43]. In addition, the ratio of the G band with the D band also corresponds to the sp2/sp3 carbon atom ratio [44]. The transmission electron microscopy (TEM) images of prepared Fe3O4 and ZnO are shown in Fig. 2(a) and (b), respectively. We can find that the Fe3O4 particles have regular and loose spherical structure, and the sizes range from 160 to 220 nm. The ZnO particles show an irregular geometry and the size distribution has a main concentration of 20e50 nm, which is significantly smaller than that of Fe3O4. As a comparison, the SEM and TEM images of the Fe3O4/ZnO composite nanoparticles are shown in Fig. 2(c) and (d),
Fig. 1. SEM image (a) and Raman spectra (b) of GO.
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Fig. 2. TEM image of Fe3O4 (a) and ZnO (b), SEM image (c) and TEM image (d) of Fe3O4/ZnO, SEM image (e) of Fe3O4/ZnO-GO and the corresponding EDS image (f).
respectively. Compared to the Fe3O4 images in Fig. 2(a), the Fe3O4/ ZnO composite images in Fig. 2(c) and (d) exhibited small ZnO crystals on top of spherical Fe3O4. After the ZnO modification, structure of Fe3O4 becomes irregular, and the sizes of Fe3O4/ZnO have slightly increased according to Fig. 2(d). These images confirm the distribution and attachment of ZnO particles onto the Fe3O4 surface. To illustrate the distribution of Fe3O4/ZnO composite nanoparticle onto the GO sheets, the SEM image is illustrated in Fig. 2(e). The elemental analysis of the Fe3O4/ZnO-GO composites is confirmed using spot energy dispersive X-ray spectroscopy (EDS) in Fig. 2(f). The EDS spectra show Fe, Zn and O peaks, which was consistent with Graphene, ZnO and Fe3O4 composition of the Fe3O4/ZnO-GO nanostructure. After the hybridization process, the specific surface area of Fe3O4/ZnO-GO was investigated through N2 adsorptionedesorption isotherm (Fig. 3), which displays a typical type IV isotherm according to IUPAC, indicating the presence of mesopores. The BET surface area is 96.8 m2/g, and the pore sizes of the Fe3O4/ ZnO-GO are mainly focus on 2.7 nm, which might be attributed to the loose Fe3O4 particles or interspaces between the layers of GO sheets [45]. The relatively large surface area should be better for mass diffusion and transmission, which improves the
Fig. 3. N2 adsorptionedesorption isotherms of Fe3O4/ZnO-GO with the inset containing pore size distribution curves for Fe3O4/ZnO-GO.
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photocatalytic property of Fe3O4/ZnO-GO.
covered nature, the peak intensity of Fe3O4 nanocomposite materials is relatively lower than those of ZnO [49].
3.2. XRD
3.3. XPS characterization
The X-ray diffraction (XRD) is employed to study the crystalline phase and structure features. Fig. 4 shows the XRD of (a) GO powders, (b) ZnO, (c) Fe3O4, (d) Fe3O4/ZnO, and (e) and Fe3O4/ZnOGO. Through the improved Hummers synthesis procedure, the XRD pattern of the GO in Fig. 4(a) displays a clean diffraction peak at 2q angle of 11.2 . This corresponds to a d-spacing of 0.83 nm in the lamellar structure of the GO. The XRD spectra of ZnO are shown in Fig. 4(b), which shows eight diffraction peaks of (100), (002), (101), (102), (110), (103), (112) and (203). These peaks correspond to the wurtzite phase of ZnO(JCPDS 36-1451). The XRD spectra of Fe3O4 in Fig. 4(c) shows (220), (311), (400), (422), (511) and (440), and (533) peaks that can be attributed to magnetite Fe3O4(JDPDS 65-7) and previous research results [46]. The XRD spectra of Fe3O4/ZnO and Fe3O4/ ZnO-GO are shown in Fig. 4(d) and (e), respectively. These two spectra 4(e) and 4(d) are strikingly alike, both figures demonstrate two sets of diffraction peaks, one set for hexagonal wurtzite ZnO and the other set for magnetite Fe3O4with no obvious graphene peaks observed. We believe a few factors contribute to this phenomenon: 1) the sensitization of graphene did not affect the orientation and structure of the core-shell Fe3O4/ZnO; 2) the GO content/weight is relative low in the overall Fe3O4/ZnO-GO composite material, and 3) XRD peaks of GO are much lower in intensities compared with that for the Fe3O4/ZnO peak. Similar observations have been reported in the literature [47,48]. Furthermore, the major XRD diffraction peaks of Fe3O4/ZnO samples (Fig. 4(d)) resemble more of ZnO sample (Fig. 4(b)), which is consistent with SEM and TEM observation of ZnO-covered Fe3O4 nanoparticles. The noise-to-signal ratio is relatively low with ZnO sample and broadening in the 70-degree region. Because of ZnO-
The XPS spectrum is a great tool for identifying the materials composition and monitoring the surface chemistry evolution via core electron detection. Fig. 5(a) is the overall XPS spectrum of the nanocomposite material Fe3O4/ZnO-GO, which shows the peaks corresponding to Zn, Fe, O, and C. The close-up of Zn 2p orbital region (1015e1050 eV) is observed in Fig. 5(b): one at a binding energy of 1021.9 eV corresponds to Zn 2p3/2, and the other at 1045.1 eV corresponds to Zn 2p1/2. This is consistent with the zinc oxide formation [50]. The close-up of the three-peak structure of the Fe 2p core region (704e736 eV) was shown in Fig. 5(c). Two dominant peaks located at 710.6 and 724 eV are in good accordance with Fe 2p3/2 and Fe 2p1/2 spin orbit peaks accompanied by their satellite peaks between 718.2 and 732.4 eV, respectively. The results are consistent with the standard Fe3O4 XPS spectrum [51]. The close-up XPS spectra of C region (280e292 eV) in Fig. 5(d) illustrates a strong peak centered at 284.7 eV, which corresponded to the C]C group. Additional C ls peaks at 286.5eV and 288.7 eV correspond to C-O and C]O [52,53]. This close-upXPS region for oxygen (525e535 eV) demonstrates two O 1s peaks at 529.0eV, 530.8 eV as shown in Fig. 5(e). This morphology, XRD, and XPS spectrum characterizations successfully confirm the formation of the Fe3O4/ZnO-GO composite. To investigate the photocatalytic property, the degradation of methyl orange (MO) dye with ZnO, Fe3O4/ZnO, and Fe3O4/ZnO-GO were performed through monitoring UVeVis absorption spectra.
Fig. 4. XRD patterns of GO (a), ZnO (b), Fe3O4 (c), Fe3O4/ZnO (d), Fe3O4/ZnO-GO (e).
3.4. Band gap and UVevis absorption To determine the band gap of these nanocomposite materials, the well-established Tauc's plot is used by utilizing the UVevis absorption spectra [54]. Here we employed the UVevis spectra of methyl orange (MO) dye to monitor the photocatalytic activity and its effectiveness of various photocatalysts such as ZnO, Fe3O4/ZnO, and Fe3O4/ZnO-GO composite materials. The UVevis absorption spectra of MO dye with the corresponding photocatalysts ZnO, Fe3O4/ZnO or Fe3O4/ZnO-GO were recorded after 30 min reaction time and shown in Fig. 6(a). One can see that UVeVis spectrum of ZnO had strong optical absorption only in the 200e400 nm regions, which suggests the pure ZnO effectively response to UV light. On the contrary, UV spectra of both Fe3O4/ZnO catalyst (red) and Fe3O4/ ZnO-GO (black) had a strong absorption around 500 nm, which suggests both composite materials have broader absorption in visible light region. This increased and enhanced absorption in the visible region can be attributed to the multilayer core-shell structure of these nanocomposite materials. For example, the lattice parameters of ZnO are ɑ, b ¼ 3.2498 Å, c ¼ 5.2066 Å and the lattice parameters of Fe3O4 are ɑ, b, c ¼ 8.384 Å [55]. These crystal structure differences cause the stress at the interface where two materials are to form a multilayer structure, namely the tensile strain. This stress-strain effect eventually leads to a red-shift of the band gap of the composite complex, which explains the broader absorption to visible light of Fe3O4/ZnO and Fe3O4/ZnO-GO [56]. Compared to the UV spectrum of Fe3O4/ZnO (red), one can see that the further enhancement and broader absorption of the UV spectrum of Fe3O4/ZnO-GO nanocomposite. This feature suggests that the Fe3O4/ZnO-GO photocatalyst would be the most effective photocatalyst among three in degradation of MO dye. To quantitatively derive the band gap of these three photo 2 catalysts, the formula Ahv ¼ hv Eg [57] is employed, where A K
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Fig. 5. XPS spectrum of Fe3O4/ZnO-GO (a), high-resolution spectrum of the Zn2p region (b), high-resolution spectrum of the Fe2p region (c), high-resolution spectrum of the C 1s region (d), and high-resolution spectrum of the O 1s region (e).
represents the absorption, K is the absorption constant, and Eg corresponds to the band gap energy. By plotting the (Ahn/K) 2 vs. the photon energy (hn) as shown in Fig. 6(b),(c) and (d), one could calculate the band gap of ZnO, Fe3O4/ZnO, and Fe3O4/ZnO-GO, respectively, by extrapolating the slope of the linear region to the xaxis intersection. The results show that the band gap of ZnO, Fe3O4/ ZnO and Fe3O4/ZnO-GO is 3.38 eV, 2.88 eV, and 2.07 eV, respectively. The continuous red-shift of these band gaps from pure ZnO to Fe3O4/ZnO and to Fe3O4/ZnO-GO is consistent with the strong response to visible light greater than 400 nm, as evidenced in the UVevis spectra.
3.5. Photoluminescence The photoluminescence (PL) spectrum can be used to characterize the transfer behavior of the photon-generated electronsholes pair, which reflects the separation and recombination of photon-generated charge carriers. The fluorescence quenching spectra of ZnO and Fe3O4/ZnO were shown in Fig. 7. By creating the multilayer structure of Fe3O4/ZnO nanocomposite, its spectra demonstrated drastic quenching effect where the peaks at lower energy region (525 nm, 550 nm, 575 nm and 600 nm) show a much lower intensity, while the high-energy region (around 400 nm)
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Fig. 6. UVeVis absorption spectra of ZnO,Fe3O4/ZnO and Fe3O4/ZnO-GO nanocomposites (a), Tauc's plots for ZnO (b), Fe3O4/ZnO (c) and Fe3O4/ZnO-GO (d).
thus directly weakening the photoluminescence intensity of ZnO. This is consistent with the photocatalytic improvement observed in the next section.
3.6. Photocatalytic performance
Fig. 7. PL spectra of ZnO, Fe3O4/ZnO, and Fe3O4/ZnO-GO.
shows little impact. However, once this multiplayer structure combined with GO surface, a significant fluorescence quenching can be observed on the spectrum for Fe3O4/ZnO-GO. This quenching feature suggests that the combination of ZnO, Fe3O4 and GO can reduce the recombination probability of photogenerated electronhole pairs [58]. By prolonging the lifetime of electron-hole pairs, electrons cannot return from the excited state to the ground state,
The photocatalytic effectiveness of various photocatalysts was measured through the photodegradation performance of methyl orange (MO) under visible illumination. The photodegradation performance was monitored through the UV spectra as described in experiment section 2.5. For comparison, a blank MO solution without photocatalyst was evaluated under the exact experimental conditions and procedure, and its UV spectrum was shown in Fig. 8(a). One can see that the spectrum showed little change before and after the illumination which indicates the light itself has no effect on the degradation of the dye. However, once the pure ZnO samples were introduced to the MO solution, the spectrum started to show the degradation of MO solution upon illumination. The most effective degradation of the MO molecules was demonstrated when Fe3O4/ZnO and Fe3O4/ZnO-GO composites were introduced. The most degradation occurred in the first 30-min of the introduction of composite materials. Compared to the ZnO material, these two composite structures double its catalytic performance and platoon after 120 min. The pseudo-first-order reaction can be best demonstrated through the kinetics model by plotting ln (C0/C) vs. time as shown in Fig. 8(b). Through this model, one can calculate the reaction rate constant k for the pure ZnO photocatalyst is 4.84 103 min1, for Fe3O4/ZnO composite is 1.564 102 min1and for Fe3O4/ZnO-GO
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Fig. 8. Photodegradation of MO with no catalyst, ZnO, Fe3O4/ZnO and Fe3O4/ZnO-GO nanocomposites under visible-light irradiation (a), and the corresponding degradation rate constants (b).
composite is 5.558 102 min1. One can see that the rate constant for graphene composite Fe3O4/ZnO-GO was 3.6 times larger than that for the Fe3O4/ZnO and 11.5 times larger than that of ZnO sample. Because the photodegradation performance mainly depends on photoelectron separation before recombination and rapid transport, the improved catalytic performance found in Fe3O4/ZnOGO composite most likely is due to their effective separation and transport character such as photoluminescence quenching effect, increased optical absorbance, and increased surface area and conductivity upon binding with graphene oxide sheet. 3.7. Magnetic properties and reusability of Fe3O4/ZnO-GO To investigate the separation of the as-prepared Fe3O4/ZnO/GO nanocomposites, the magnetic hysteresis loop of the Fe3O4/ZnO/GO nanocomposites is shown in Fig. 9(a), measured at 300 K under an applied magnetic field sweeping from 8.0 to 8.0 kOe. The existence of hysteresis loop in the M-H curve shows the ferromagnetic behavior of the Fe3O4@ZnO/GO nanocomposites. The saturation magnetization (Ms), coercivity (Hc), and remnant magnetization (Mr) of the sample are 60.5 emu/g, 456 Oe, and 18.3 emu/g, respectively. The relatively low saturation magnetization is most likely attributed to the existence of nonmagnetic ZnO and GO. However, compared with previous research results [59], it is still large enough for effectively magnetic separation. To study the reusability of the Fe3O4/ZnO/GO photocatalyst, cycling photocatalytic experiments were carried out over four cycles under visible light, applying a magnet to remove the photocatalysts from solution after each cycle. The reusability, stability, and retention of Fe3O4/ZnO-GO samples were evaluated through the UV spectra of methyl orange under illumination with the recycled photocatalyst as shown in Fig. 9(b) and (c). To be consistent, the comparison experiments were all run for 120 min. Although some loss of photocatalytic activity was observed in the fourth run (green), the Fe3O4/ZnO-GO composite maintained a relatively high photocatalytic activity throughout the recycling process. For example, the degradation efficiency was 92.8% for the first-turn test and 76.5% after fourth-turn. The decrease of the photodegradation of MO during the recycling reactions could be attributed to the loss of some of the catalysts (1) through withdrawn for testing, (2) transfer from one run to another and (3) consumption during the photocatalytic processes. Considering these possible loss venues, the reusability, stability, and retention of
Fe3O4/ZnO-GO sample are excellent. One can attribute this improved reusability and retention of Fe3O4/ZnO-GO structure to the magnetic component Fe3O4 [60]. 3.8. The proposed mechanism Based on the above observation and experimental results, an electron transport mechanism for the photodegradation of MO over Fe3O4/ZnO-GO composites is proposed. The ZnO species with wide band gap of ~3.38 eV, is difficult to reach excited states under visible light irradiation. In this work, the Fe3O4/ZnO hybrid materials has a band gap of ~2.88 eV, which can be partially excited by visible light with the energy of 1.6e3.2 eV. However, It is found that the addition of GO can sufficiently narrow the band gap of ZnO and Fe3O4/ZnO to the visible light region of ~2.07 eV. Thus, in this case, GO in the Fe3O4/ZnO-GO nanocomposites acts as photosensitizer, like the reduced graphene oxide (GR) in ZnS-GR nanocomposites in literature [61]. Moreover, the photo-generated electrons form the Fe3O4/ZnO semiconductor can be separated by GO, which thus further contributes to the semiconductor photoactivity enhancement. The GO with large surface area further enhances such reaction with the enhanced absorption of MO. With the effective separation of electrons and holes by Fe3O4/ZnO-GO composites, coupled with the superior conductivities and absorption abilities of GO sheet, these properties concertedly contribute to the light absorbance enhancement and the improved photocatalytic performance of Fe3O4/ZnO-GO composites. 4. Conclusions Magnetically separable Fe3O4/ZnO-GO photocatalyst was successfully fabricated through a low-temperature hydrothermal method. The prepared Fe3O4/ZnO-GO catalyst displayed significantly enhanced photocatalytic activity toward MO degradation under visible light irradiation compared to that of ZnO and Fe3O4/ ZnO catalysts. The Tauc's plot revealed the band gap of Fe3O4/ZnOGO sample decreases to 2.07 eV from 3.38 eV of ZnO. The introduction of GO and Fe3O4 broadened the visible light response range of the composite catalyst. The quenching of the visible emission peak in the room-temperature PL spectra suggests effective separation of photogenerated electron-hole pairs. The measured photocatalytic reaction rate of the pure ZnO photocatalyst is 4.84 103 min1, for Fe3O4/ZnO composite is 1.564 102 min1
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Fig. 9. Magnetic hysteresis loop of the as-prepared Fe3O4/ZnO-GO nanocomposites (a) and cyclic photocatalysis of the Fe3O4/ZnO-GO nanocomposite (b) and (c).
and for Fe3O4/ZnO-GO composite is 5.558 102 min1. Meanwhile, the composite can be easily separated by applying an external magnetic field due to the magnetism of the Fe3O4. The photocatalytic reactivity of Fe3O4/ZnO-GO nanocomposite structure reaches 92.8% efficiency on the first run and was at 75% efficiency after the fourth recovery. Thus, this enhanced photoelectron chemical performance could make this Fe3O4/ZnO-GO nanocomposite material a promising candidate for the removal of organic materials from wastewater. Acknowledgements Financial support of this work from the NSFC (51504117, 61764009, 51762043), Yunnan Applied Basic Research Project (Y0120150138), the Program for Innovative Research Team in University of Ministry of Education of China (No. IRT_17R48) and H.J. Fan would acknowledge the partial financial support from the U.S. Department of Energy, National Nuclear Security Administration grant (DE-NA 0001861 & DE-NA 0002630) References
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Synthesis and characterization of Fe3O4/ZnO-GO nanocomposites with improved photocatalytic degradation methyl orange under visible light irradiation Qi Feng a, b, Shaoyuan Li a, b, *, Wenhui Ma a, b, Hua-Jun Fan c, Xiaohan Wan b, Yun Lei a, b, Zhengjie Chen b, Jia Yang b, Bo Qin b a
State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming University of Science and Technology, Kunming 650093, China Institute of New Energy/Silicon Metallurgy and Silicon Material Engineering Research Center of Universities in Yunnan Province, Kunming University of Science and Technology, Kunming 650093, China c Department of Chemistry, Prairie View A&M University, Prairie View, TX 77446, United States b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 August 2017 Received in revised form 7 November 2017 Accepted 7 December 2017 Available online 9 December 2017
To improve the photocatalytic property and recovery rate of existing ZnO materials, a magnetically separable heterostructure photocatalytic composed of GO sheets and nanohybrid Fe3O4/ZnO material was synthesized by a simple low-temperature chemical synthesis process. This synthesis enables simultaneous decoration of Fe3O4/ZnO spheres on both sides of the GO sheets. As a result, the Tauc's plot revealed the band gap of Fe3O4/ZnO-GO sample decreases to 2.07 eV from 3.38 eV of ZnO. This smaller energy band gap and slower recombination rate of electron-hole pairs greatly improve the photocatalytic activity of the Fe3O4/ZnO-GO complex through the wider visible light absorption range. The photocatalytic reaction rate of the pure ZnO photocatalyst is 4.84 103 min1, for Fe3O4/ZnO composite is 1.564 102 min1 and for Fe3O4/ZnO-GO composite is 5.558 102 min1. The stability and recovery rate and photocatalytic property retention also greatly improved from ZnO-only photocatalyst. The photocatalytic reactivity of Fe3O4/ZnO-GO nanocomposite structure reaches 92.8% efficiency on the first run and was at 75% efficiency after the four cycling. © 2017 Elsevier B.V. All rights reserved.
Keywords: Ternary nanohybrid compounds Magnetical photocatalyst Graphene oxide sheets Methyl orange degradation
1. Introduction In recent decades, environmental pollution and its related effects have posed significant problems that increasingly confront human societies [1]. In addition to the drinkable water shortage and drought in the various region of the global, such water contamination threats human race survival [2,3]. To find an efficient and economically viable solution to eliminating the water contaminants becomes a pressing issue for every nation. More and more research turn to the solar energy, the radiant light, and heat from the sun, which is the most abundant clean energy source on the Earth. Extensive research studies focus on the development of new materials that can efficiently harvest solar irradiation and use it for green environmental pollution management. Namely, the
* Corresponding author. 253 Xuefu Road, Kunming 650093, Yunnan Province, China. E-mail addresses: [email protected] (S. Li), [email protected] (W. Ma). https://doi.org/10.1016/j.jallcom.2017.12.070 0925-8388/© 2017 Elsevier B.V. All rights reserved.
photocatalysis process that utilizes such renewable solar energy to activate the chemical reactions by oxidation and reduction process becomes a sustainable and attractive technology to provide a feasible solution for environmental problems [4]. Several types of promising photocatalysts, such as TiO2, ZnO, ZrO2, vanadium oxide and WO3 have been studied for their application in the environmental science [5]. Among these semiconductors, ZnO becomes one of the excellent and promising candidates as a green photocatalytic environmental reactor [6,7] because of its easy synthesis and environmentally friendly. In addition, ZnO possesses many other properties such as direct and wide band gap in the near-UV spectral region, strong oxidation ability, good photocatalytic property, and a large free-exciton binding energy [8], which makes its excitonic emission processes continuum at or even above room temperature. However, two major drawbacks hinder its wider application as a photoelectrode: one is the low photocatalytic efficiency, and the other is difficult to separate and recovery after the photocatalysis [9]. Such low recovery and reusability of ZnO particles are critical to their further
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applications for economic reasons. Centrifugation and filtration separation methods not only are impractical in the field but also still lead to catalyst loss and added energy consumption. Fixation the photocatalysts on thin films showed that the catalytic activity was considerably reduced because the effective surface area has been significantly decreased upon fixation [10]. For the sake of practicality, immobilizing ZnO catalysts onto the surface of magnetic nano- or micro particles becomes a trend because now the catalyst can be easily separated and recovered with an external magnet and the loss of catalysts can be minimized [11e14]. Several materials with different elemental compositions have been investigated as the magnetic core, such as Fe3O4, gFe2O3, NiFe2O4 and Co3O4. Among these materials, Fe3O4 has drawn great attention due to its remarkable magnetic properties, low toxicity, and biocompatibility [15e22]. Studies found that, after introducing Fe3O4 as the core of Fe3O4/ZnO hybrid nanostructures, photodissolution may occur in the hybrid nanoparticles, which would decrease the photocatalytic efficiency of ZnO materials [23]. To remedy this issue, we decided to introduce graphene oxide (GO) to be the third component of our carrier platform for Fe3O4/ZnO nanocomposite because GO possesses not only a unique electronic property but also is environmentally friendly. Graphene oxide (GO) is one of the most promising precursors of graphene and has attracted tremendous attention for its unique physicochemical properties [24e28]. Graphene, a novel twodimensional carbon material, has been recognized as an ideal candidate for a variety of application owing to its high surface area, striking electronic, mechanical and thermal properties [29], while the surface of GO are bedecked with diverse hydrophilic oxygencontaining functional groups [30,31]. Inorganic materials modified with partially GO have been reported to show enhanced photocatalytic properties [32]. In recent years, nanocomposites based on graphene have been widely investigated and their photocatalytic properties have been demonstrated [33,34], including TiO2/graphene [35,36], BiVO4/graphene [37,38], ZnO/Graphene [39,40] and like materials. In this study, non-toxic Fe3O4/ZnO-GO composite material was synthesized via a mild hydrothermal process, and the photocatalytic property was monitored through the degradation of methyl orange. Various characterization tools such as X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM) energy dispersive X-ray analysis (EDS), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) spectra, and UVevisible diffuse reflectance spectra was employed in this study. This work investigates (1) the structure and composition of the Fe3O4/ZnO-GO nanocomposite materials; (2) the photocatalytic activities of Fe3O4/ ZnO-GO composite materials; and (3) the recovery rate and photocatalytic reusability of Fe3O4/ZnO-GO material.
50 C. Afterward, the mixed solution was transferred into a Teflonlined autoclave reactor and heated to 200 C for 3 h. The produced Fe3O4 particles were collected in a magnetic field, washed with DI water until neutral and dried to a constant weight at 60 C under vacuum. To prepare the Fe3O4/ZnO nanocomposites, 1 g of aforementioned Fe3O4 solid was mixed with 20 mL DI water and 30 mL ethanol by ultrasonication until forming a uniform solution. Then, a mixture of 10 mmol Zn(Ac)2$2H2O with Tris (2-Hydroxyethyl) amine were dropwise added into the Fe3O4-DI-Ethanol mixture under stirring. This solution was transferred into a Teflon-lined autoclave reactor at 90 C for 1 h. The Fe3O4/ZnO nanocomposites were then filtered, washed with DI and ethanol and finally dried in an oven at 60 C. 2.3. Sensitization of Fe3O4/ZnO with GO nanosheets The GO was synthesized from natural flake graphite following a modified Hummer method [41]. Coating of the GO sheets with Fe3O4/ZnO was carried out by one-step chemical bath deposition method. Specifically, 1 mg/mL of GO dispersion was created through sonication. After stirring for 30 min at room temperature, 80 mg Fe3O4/ZnO was dispersed into this GO solution and stirred for additional 3 h at 100 C. The final product Fe3O4/ZnO-GO was centrifuged and washed several times with DI water, followed by applying an external magnet for isolation and drying at 60 C. 2.4. Characterization Phase information and the crystalline quality of the prepared nanostructures were investigated using X-ray diffraction (XRD; BrukerD4) with monochromatized Cu Ka radiation at the wavelength l ¼ 1.5406 Å. The surface morphology and structure were studied using a scanning electron microscope (SEM, FEI Quanta 200), transmission electron microscopy and energy dispersive Xray analysis (TEM/EDS, Tecnai G2 TF30 S-Twin). The specific surface area of the catalysts was measured from N2 adsorption at 77 K using a Micrometric ASAP 2020 system. Magnetization measurements were obtained at room temperature using a vibrating sample magnetometer (VSM, Lakeshore 7307) under the magnetic field range of 8.0e8.0 kOe. Raman spectroscopy of the composite was collected on a Dilor Labram-1B microspectrometer with 632.8 nm laser excitation. The core electron configuration of samples was determined by X-ray photoelectron spectroscopy (XPS, PHI-5500). The photoluminescence (PL) spectra of the sample were recorded at room temperature using a fluorescence spectrophotometer (Dong Woo Optron, South Korea) with an attached He-Cd laser at an excitation wavelength of 325 nm. The UVevisible diffuse reflectance spectra of samples, recorded in the range of 200e800 nm, were obtained using an ultravioletevisible (UVeVis, TU-1901) spectrophotometer.
2. Experimental procedure 2.5. Photocatalytic degradation of methyl orange 2.1. Preparation of ZnO A mixture of 10 mmol Zn(Ac)2$2H2O with Tris (2-Hydroxyethyl) amine was mixed thoroughly under stirring before the mixture was transferred into a Teflon-lined reactor and heated at 90 C for 1 h. The ZnO was then filtered, washed with DI and ethanol, and finally dried in an oven at 60 C until it is ready to be used. 2.2. Preparation of Fe3O4/ZnO As illustrated in Scheme 1, 2.7 g FeCl3$6H2O was dissolved in 100 mL of ethylene glycol before the mixture of 7.2 g NaAC and 2 g polyethyleneglycol was added under a nitrogen atmosphere at
The photocatalytic activity of Fe3O4/ZnO-GO nanocomposite material was evaluated by the degradation of Methyl Orange (1.0 105 M, 100 mL) under a 300 W Xe lamp with 420 nm light filter (custom-made) and monitored via UVeVis spectrophotometer. Particularly, 20 mg Fe3O4/ZnO-GO nanocomposite material was dispersed in the Methyl Orange dye solution with stirring in the dark for 1 h to allow absorption-desorption equilibrium before light irradiation. Then the dye solution containing the dispersed catalyst was illuminated by the custom-made Xe lamp while being continuously stirred. After given time intervals (30 min), 3 mL solution was withdrawn, centrifuged, and analyzed by a UVeVis spectrophotometer. The photocatalytic kinetic rate was calculated
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Scheme 1. Illustration for the preparation of Fe3O4/ZnO-GO nanocomposites.
based on the Langmuir-Hinshelwood kinetic model using the empirical Equation:
Ct ¼ C0 ekt Through this first order kinetic model, we can correlate UVeVis spectra with the change in concentration of the Methyl Orange dye before (C0) and after light irradiation (Ct), as well as derive the energy band gap. As a comparison, the photocatalytic activity of ZnO and Fe3O4/ZnO composite material were evaluated under the same condition. 3. Results and discussion 3.1. Morphology and surface area analysis Due to the abundance of oxygen-containing functional groups (hydroxyl, epoxide and carboxylic groups) and vast p-orbitals pdelocalization of carbon atoms on the basal planes of GO nanosheets, the GO sheets have high electron density on its surface and prone to local polarization. This unique electronic property enables GO sheets easily attract Fe3O4/ZnO nanoparticles on its surface. This can be identified and characterized through the morphology characterization tools such as scanning electron microscopy (SEM) as shown in Fig. 1(a). This SEM image of GO sheet itself demonstrates relatively flat layer structure and contains a unique fold
structure. For comparison, Fig. 1(b) shows the Raman spectra of between graphite (blue) and GO (red). Fig. 1(b) showed that graphite has a unique sharp and strong absorption peak (G peak) at 1576 cm1 and a relatively strong peak (2D band or G band) 2711 cm1, indicating a neat structure of sp2 graphite. These bands correspond to the stretching of the C]C bond, which is typical of all graphitic materials [42]. However, the Raman spectra of GO is quite different from that of graphite. For example, not only did 2D band intensity of GO diminish dramatically and the G peak (1578 cm1) broadened, but also there is a drastic increase the intensity of absorption peak (D band) at 1345 cm1. This difference is because when the graphite was oxidized, the sp2 hybrid carbon atoms in graphite were transformed into the sp3 hybrid structure, thus destroys the C]C double bond in the graphite layer. Raman spectroscopy is sensitive enough to characterize such distortion and shows such as D band [43]. In addition, the ratio of the G band with the D band also corresponds to the sp2/sp3 carbon atom ratio [44]. The transmission electron microscopy (TEM) images of prepared Fe3O4 and ZnO are shown in Fig. 2(a) and (b), respectively. We can find that the Fe3O4 particles have regular and loose spherical structure, and the sizes range from 160 to 220 nm. The ZnO particles show an irregular geometry and the size distribution has a main concentration of 20e50 nm, which is significantly smaller than that of Fe3O4. As a comparison, the SEM and TEM images of the Fe3O4/ZnO composite nanoparticles are shown in Fig. 2(c) and (d),
Fig. 1. SEM image (a) and Raman spectra (b) of GO.
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Fig. 2. TEM image of Fe3O4 (a) and ZnO (b), SEM image (c) and TEM image (d) of Fe3O4/ZnO, SEM image (e) of Fe3O4/ZnO-GO and the corresponding EDS image (f).
respectively. Compared to the Fe3O4 images in Fig. 2(a), the Fe3O4/ ZnO composite images in Fig. 2(c) and (d) exhibited small ZnO crystals on top of spherical Fe3O4. After the ZnO modification, structure of Fe3O4 becomes irregular, and the sizes of Fe3O4/ZnO have slightly increased according to Fig. 2(d). These images confirm the distribution and attachment of ZnO particles onto the Fe3O4 surface. To illustrate the distribution of Fe3O4/ZnO composite nanoparticle onto the GO sheets, the SEM image is illustrated in Fig. 2(e). The elemental analysis of the Fe3O4/ZnO-GO composites is confirmed using spot energy dispersive X-ray spectroscopy (EDS) in Fig. 2(f). The EDS spectra show Fe, Zn and O peaks, which was consistent with Graphene, ZnO and Fe3O4 composition of the Fe3O4/ZnO-GO nanostructure. After the hybridization process, the specific surface area of Fe3O4/ZnO-GO was investigated through N2 adsorptionedesorption isotherm (Fig. 3), which displays a typical type IV isotherm according to IUPAC, indicating the presence of mesopores. The BET surface area is 96.8 m2/g, and the pore sizes of the Fe3O4/ ZnO-GO are mainly focus on 2.7 nm, which might be attributed to the loose Fe3O4 particles or interspaces between the layers of GO sheets [45]. The relatively large surface area should be better for mass diffusion and transmission, which improves the
Fig. 3. N2 adsorptionedesorption isotherms of Fe3O4/ZnO-GO with the inset containing pore size distribution curves for Fe3O4/ZnO-GO.
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photocatalytic property of Fe3O4/ZnO-GO.
covered nature, the peak intensity of Fe3O4 nanocomposite materials is relatively lower than those of ZnO [49].
3.2. XRD
3.3. XPS characterization
The X-ray diffraction (XRD) is employed to study the crystalline phase and structure features. Fig. 4 shows the XRD of (a) GO powders, (b) ZnO, (c) Fe3O4, (d) Fe3O4/ZnO, and (e) and Fe3O4/ZnOGO. Through the improved Hummers synthesis procedure, the XRD pattern of the GO in Fig. 4(a) displays a clean diffraction peak at 2q angle of 11.2 . This corresponds to a d-spacing of 0.83 nm in the lamellar structure of the GO. The XRD spectra of ZnO are shown in Fig. 4(b), which shows eight diffraction peaks of (100), (002), (101), (102), (110), (103), (112) and (203). These peaks correspond to the wurtzite phase of ZnO(JCPDS 36-1451). The XRD spectra of Fe3O4 in Fig. 4(c) shows (220), (311), (400), (422), (511) and (440), and (533) peaks that can be attributed to magnetite Fe3O4(JDPDS 65-7) and previous research results [46]. The XRD spectra of Fe3O4/ZnO and Fe3O4/ ZnO-GO are shown in Fig. 4(d) and (e), respectively. These two spectra 4(e) and 4(d) are strikingly alike, both figures demonstrate two sets of diffraction peaks, one set for hexagonal wurtzite ZnO and the other set for magnetite Fe3O4with no obvious graphene peaks observed. We believe a few factors contribute to this phenomenon: 1) the sensitization of graphene did not affect the orientation and structure of the core-shell Fe3O4/ZnO; 2) the GO content/weight is relative low in the overall Fe3O4/ZnO-GO composite material, and 3) XRD peaks of GO are much lower in intensities compared with that for the Fe3O4/ZnO peak. Similar observations have been reported in the literature [47,48]. Furthermore, the major XRD diffraction peaks of Fe3O4/ZnO samples (Fig. 4(d)) resemble more of ZnO sample (Fig. 4(b)), which is consistent with SEM and TEM observation of ZnO-covered Fe3O4 nanoparticles. The noise-to-signal ratio is relatively low with ZnO sample and broadening in the 70-degree region. Because of ZnO-
The XPS spectrum is a great tool for identifying the materials composition and monitoring the surface chemistry evolution via core electron detection. Fig. 5(a) is the overall XPS spectrum of the nanocomposite material Fe3O4/ZnO-GO, which shows the peaks corresponding to Zn, Fe, O, and C. The close-up of Zn 2p orbital region (1015e1050 eV) is observed in Fig. 5(b): one at a binding energy of 1021.9 eV corresponds to Zn 2p3/2, and the other at 1045.1 eV corresponds to Zn 2p1/2. This is consistent with the zinc oxide formation [50]. The close-up of the three-peak structure of the Fe 2p core region (704e736 eV) was shown in Fig. 5(c). Two dominant peaks located at 710.6 and 724 eV are in good accordance with Fe 2p3/2 and Fe 2p1/2 spin orbit peaks accompanied by their satellite peaks between 718.2 and 732.4 eV, respectively. The results are consistent with the standard Fe3O4 XPS spectrum [51]. The close-up XPS spectra of C region (280e292 eV) in Fig. 5(d) illustrates a strong peak centered at 284.7 eV, which corresponded to the C]C group. Additional C ls peaks at 286.5eV and 288.7 eV correspond to C-O and C]O [52,53]. This close-upXPS region for oxygen (525e535 eV) demonstrates two O 1s peaks at 529.0eV, 530.8 eV as shown in Fig. 5(e). This morphology, XRD, and XPS spectrum characterizations successfully confirm the formation of the Fe3O4/ZnO-GO composite. To investigate the photocatalytic property, the degradation of methyl orange (MO) dye with ZnO, Fe3O4/ZnO, and Fe3O4/ZnO-GO were performed through monitoring UVeVis absorption spectra.
Fig. 4. XRD patterns of GO (a), ZnO (b), Fe3O4 (c), Fe3O4/ZnO (d), Fe3O4/ZnO-GO (e).
3.4. Band gap and UVevis absorption To determine the band gap of these nanocomposite materials, the well-established Tauc's plot is used by utilizing the UVevis absorption spectra [54]. Here we employed the UVevis spectra of methyl orange (MO) dye to monitor the photocatalytic activity and its effectiveness of various photocatalysts such as ZnO, Fe3O4/ZnO, and Fe3O4/ZnO-GO composite materials. The UVevis absorption spectra of MO dye with the corresponding photocatalysts ZnO, Fe3O4/ZnO or Fe3O4/ZnO-GO were recorded after 30 min reaction time and shown in Fig. 6(a). One can see that UVeVis spectrum of ZnO had strong optical absorption only in the 200e400 nm regions, which suggests the pure ZnO effectively response to UV light. On the contrary, UV spectra of both Fe3O4/ZnO catalyst (red) and Fe3O4/ ZnO-GO (black) had a strong absorption around 500 nm, which suggests both composite materials have broader absorption in visible light region. This increased and enhanced absorption in the visible region can be attributed to the multilayer core-shell structure of these nanocomposite materials. For example, the lattice parameters of ZnO are ɑ, b ¼ 3.2498 Å, c ¼ 5.2066 Å and the lattice parameters of Fe3O4 are ɑ, b, c ¼ 8.384 Å [55]. These crystal structure differences cause the stress at the interface where two materials are to form a multilayer structure, namely the tensile strain. This stress-strain effect eventually leads to a red-shift of the band gap of the composite complex, which explains the broader absorption to visible light of Fe3O4/ZnO and Fe3O4/ZnO-GO [56]. Compared to the UV spectrum of Fe3O4/ZnO (red), one can see that the further enhancement and broader absorption of the UV spectrum of Fe3O4/ZnO-GO nanocomposite. This feature suggests that the Fe3O4/ZnO-GO photocatalyst would be the most effective photocatalyst among three in degradation of MO dye. To quantitatively derive the band gap of these three photo 2 catalysts, the formula Ahv ¼ hv Eg [57] is employed, where A K
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Fig. 5. XPS spectrum of Fe3O4/ZnO-GO (a), high-resolution spectrum of the Zn2p region (b), high-resolution spectrum of the Fe2p region (c), high-resolution spectrum of the C 1s region (d), and high-resolution spectrum of the O 1s region (e).
represents the absorption, K is the absorption constant, and Eg corresponds to the band gap energy. By plotting the (Ahn/K) 2 vs. the photon energy (hn) as shown in Fig. 6(b),(c) and (d), one could calculate the band gap of ZnO, Fe3O4/ZnO, and Fe3O4/ZnO-GO, respectively, by extrapolating the slope of the linear region to the xaxis intersection. The results show that the band gap of ZnO, Fe3O4/ ZnO and Fe3O4/ZnO-GO is 3.38 eV, 2.88 eV, and 2.07 eV, respectively. The continuous red-shift of these band gaps from pure ZnO to Fe3O4/ZnO and to Fe3O4/ZnO-GO is consistent with the strong response to visible light greater than 400 nm, as evidenced in the UVevis spectra.
3.5. Photoluminescence The photoluminescence (PL) spectrum can be used to characterize the transfer behavior of the photon-generated electronsholes pair, which reflects the separation and recombination of photon-generated charge carriers. The fluorescence quenching spectra of ZnO and Fe3O4/ZnO were shown in Fig. 7. By creating the multilayer structure of Fe3O4/ZnO nanocomposite, its spectra demonstrated drastic quenching effect where the peaks at lower energy region (525 nm, 550 nm, 575 nm and 600 nm) show a much lower intensity, while the high-energy region (around 400 nm)
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Fig. 6. UVeVis absorption spectra of ZnO,Fe3O4/ZnO and Fe3O4/ZnO-GO nanocomposites (a), Tauc's plots for ZnO (b), Fe3O4/ZnO (c) and Fe3O4/ZnO-GO (d).
thus directly weakening the photoluminescence intensity of ZnO. This is consistent with the photocatalytic improvement observed in the next section.
3.6. Photocatalytic performance
Fig. 7. PL spectra of ZnO, Fe3O4/ZnO, and Fe3O4/ZnO-GO.
shows little impact. However, once this multiplayer structure combined with GO surface, a significant fluorescence quenching can be observed on the spectrum for Fe3O4/ZnO-GO. This quenching feature suggests that the combination of ZnO, Fe3O4 and GO can reduce the recombination probability of photogenerated electronhole pairs [58]. By prolonging the lifetime of electron-hole pairs, electrons cannot return from the excited state to the ground state,
The photocatalytic effectiveness of various photocatalysts was measured through the photodegradation performance of methyl orange (MO) under visible illumination. The photodegradation performance was monitored through the UV spectra as described in experiment section 2.5. For comparison, a blank MO solution without photocatalyst was evaluated under the exact experimental conditions and procedure, and its UV spectrum was shown in Fig. 8(a). One can see that the spectrum showed little change before and after the illumination which indicates the light itself has no effect on the degradation of the dye. However, once the pure ZnO samples were introduced to the MO solution, the spectrum started to show the degradation of MO solution upon illumination. The most effective degradation of the MO molecules was demonstrated when Fe3O4/ZnO and Fe3O4/ZnO-GO composites were introduced. The most degradation occurred in the first 30-min of the introduction of composite materials. Compared to the ZnO material, these two composite structures double its catalytic performance and platoon after 120 min. The pseudo-first-order reaction can be best demonstrated through the kinetics model by plotting ln (C0/C) vs. time as shown in Fig. 8(b). Through this model, one can calculate the reaction rate constant k for the pure ZnO photocatalyst is 4.84 103 min1, for Fe3O4/ZnO composite is 1.564 102 min1and for Fe3O4/ZnO-GO
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Fig. 8. Photodegradation of MO with no catalyst, ZnO, Fe3O4/ZnO and Fe3O4/ZnO-GO nanocomposites under visible-light irradiation (a), and the corresponding degradation rate constants (b).
composite is 5.558 102 min1. One can see that the rate constant for graphene composite Fe3O4/ZnO-GO was 3.6 times larger than that for the Fe3O4/ZnO and 11.5 times larger than that of ZnO sample. Because the photodegradation performance mainly depends on photoelectron separation before recombination and rapid transport, the improved catalytic performance found in Fe3O4/ZnOGO composite most likely is due to their effective separation and transport character such as photoluminescence quenching effect, increased optical absorbance, and increased surface area and conductivity upon binding with graphene oxide sheet. 3.7. Magnetic properties and reusability of Fe3O4/ZnO-GO To investigate the separation of the as-prepared Fe3O4/ZnO/GO nanocomposites, the magnetic hysteresis loop of the Fe3O4/ZnO/GO nanocomposites is shown in Fig. 9(a), measured at 300 K under an applied magnetic field sweeping from 8.0 to 8.0 kOe. The existence of hysteresis loop in the M-H curve shows the ferromagnetic behavior of the Fe3O4@ZnO/GO nanocomposites. The saturation magnetization (Ms), coercivity (Hc), and remnant magnetization (Mr) of the sample are 60.5 emu/g, 456 Oe, and 18.3 emu/g, respectively. The relatively low saturation magnetization is most likely attributed to the existence of nonmagnetic ZnO and GO. However, compared with previous research results [59], it is still large enough for effectively magnetic separation. To study the reusability of the Fe3O4/ZnO/GO photocatalyst, cycling photocatalytic experiments were carried out over four cycles under visible light, applying a magnet to remove the photocatalysts from solution after each cycle. The reusability, stability, and retention of Fe3O4/ZnO-GO samples were evaluated through the UV spectra of methyl orange under illumination with the recycled photocatalyst as shown in Fig. 9(b) and (c). To be consistent, the comparison experiments were all run for 120 min. Although some loss of photocatalytic activity was observed in the fourth run (green), the Fe3O4/ZnO-GO composite maintained a relatively high photocatalytic activity throughout the recycling process. For example, the degradation efficiency was 92.8% for the first-turn test and 76.5% after fourth-turn. The decrease of the photodegradation of MO during the recycling reactions could be attributed to the loss of some of the catalysts (1) through withdrawn for testing, (2) transfer from one run to another and (3) consumption during the photocatalytic processes. Considering these possible loss venues, the reusability, stability, and retention of
Fe3O4/ZnO-GO sample are excellent. One can attribute this improved reusability and retention of Fe3O4/ZnO-GO structure to the magnetic component Fe3O4 [60]. 3.8. The proposed mechanism Based on the above observation and experimental results, an electron transport mechanism for the photodegradation of MO over Fe3O4/ZnO-GO composites is proposed. The ZnO species with wide band gap of ~3.38 eV, is difficult to reach excited states under visible light irradiation. In this work, the Fe3O4/ZnO hybrid materials has a band gap of ~2.88 eV, which can be partially excited by visible light with the energy of 1.6e3.2 eV. However, It is found that the addition of GO can sufficiently narrow the band gap of ZnO and Fe3O4/ZnO to the visible light region of ~2.07 eV. Thus, in this case, GO in the Fe3O4/ZnO-GO nanocomposites acts as photosensitizer, like the reduced graphene oxide (GR) in ZnS-GR nanocomposites in literature [61]. Moreover, the photo-generated electrons form the Fe3O4/ZnO semiconductor can be separated by GO, which thus further contributes to the semiconductor photoactivity enhancement. The GO with large surface area further enhances such reaction with the enhanced absorption of MO. With the effective separation of electrons and holes by Fe3O4/ZnO-GO composites, coupled with the superior conductivities and absorption abilities of GO sheet, these properties concertedly contribute to the light absorbance enhancement and the improved photocatalytic performance of Fe3O4/ZnO-GO composites. 4. Conclusions Magnetically separable Fe3O4/ZnO-GO photocatalyst was successfully fabricated through a low-temperature hydrothermal method. The prepared Fe3O4/ZnO-GO catalyst displayed significantly enhanced photocatalytic activity toward MO degradation under visible light irradiation compared to that of ZnO and Fe3O4/ ZnO catalysts. The Tauc's plot revealed the band gap of Fe3O4/ZnOGO sample decreases to 2.07 eV from 3.38 eV of ZnO. The introduction of GO and Fe3O4 broadened the visible light response range of the composite catalyst. The quenching of the visible emission peak in the room-temperature PL spectra suggests effective separation of photogenerated electron-hole pairs. The measured photocatalytic reaction rate of the pure ZnO photocatalyst is 4.84 103 min1, for Fe3O4/ZnO composite is 1.564 102 min1
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Fig. 9. Magnetic hysteresis loop of the as-prepared Fe3O4/ZnO-GO nanocomposites (a) and cyclic photocatalysis of the Fe3O4/ZnO-GO nanocomposite (b) and (c).
and for Fe3O4/ZnO-GO composite is 5.558 102 min1. Meanwhile, the composite can be easily separated by applying an external magnetic field due to the magnetism of the Fe3O4. The photocatalytic reactivity of Fe3O4/ZnO-GO nanocomposite structure reaches 92.8% efficiency on the first run and was at 75% efficiency after the fourth recovery. Thus, this enhanced photoelectron chemical performance could make this Fe3O4/ZnO-GO nanocomposite material a promising candidate for the removal of organic materials from wastewater. Acknowledgements Financial support of this work from the NSFC (51504117, 61764009, 51762043), Yunnan Applied Basic Research Project (Y0120150138), the Program for Innovative Research Team in University of Ministry of Education of China (No. IRT_17R48) and H.J. Fan would acknowledge the partial financial support from the U.S. Department of Energy, National Nuclear Security Administration grant (DE-NA 0001861 & DE-NA 0002630) References
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