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Improving sunlight-driven photocatalytic activity of ZnO nanostructures upon decoration with Fe(III) cocatalyst
Materials Characterization 127 (2017) 179–184
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Materials Characterization journal homepage: www.elsevier.com/locate/matchar
Improving sunlight-driven photocatalytic activity of ZnO nanostructures upon decoration with Fe(III) cocatalyst Lixiong Yin a,⁎, Dongdong Zhang a, Jia Wang b, Jianfeng Huang a,⁎, Xingang Kong a, Jiameng Fang a, Feng Zhang a a b
School of Materials Science and Engineering, Shaanxi University of Science and Technology, Xi'an, Shaanxi 710021, China Shaanxi Research Design Institute of Petroleum and Chemical Industry, Xi'an, Shaanxi 710021, China
a r t i c l e
i n f o
Article history: Received 23 December 2016 Received in revised form 2 March 2017 Accepted 5 March 2017 Available online 09 March 2017 Keywords: ZnO Photocatalysis Cocatalyst Fe(III) nanocluster
a b s t r a c t In the study, the Fe(III) cocatalysts were grafted successfully on the surface of ZnO nanostructures to form Fe(III)/ ZnO photocatalysts by a simple impregnation technique. It was found that the morphologies and crystal structures of ZnO remain unchanged after modification of the Fe(III) clusters. And the photocatalytic activity and the stability for the degradation of Rhodamine B were also evaluated. Under simulated sunlight illumination for 210 min, an almost complete degradation was achieved over Fe(III)/ZnO nanocomposites. The mechanism of the enhancement is associated with a direct interfacial charge transfer process (IFCT). The Fe(III) clusters not only acted as electron sinks to enhance the separation of photoinduced electrons from holes, but also elevated the amount of the holes, leading to the production of more hydroxyl radical with high oxidizability. © 2017 Elsevier Inc. All rights reserved.
1. Introduction Following the discovery of the water splitting on the semiconductorbased photochemical electrode in 1972 [1], it was soon recognized that the photocatalytic process of organic pollutants has created great interest in the past many years. Of the well-known photocatalysts, ZnO has attracted much attention because of its non-toxicity, long-term stability and the high electron mobility of ~115–155 cm−2 V−1 s−1 [2–4]. These features can be exploited to solve several current global challenges involving environmental issues and energy crisis. However, it should be note the ZnO semiconductor with a wide band-gap (3.37 eV) still cannot be widely used in practical photocatalysis applications owing to limited visible-light absorption [5,6]. Thus, it is imperative to develop efficient strategies to modify the ZnO photocatalysts sensitive to visible light. In fact, over the last few decades, numerous attempts have been extensively used to extend to the photocatalytic activity of ZnO in the visible light based on the band-gap adjustment [7–11]. For instance, Khanchandani and co-workers [7] synthesized successfully the ZnO/ CdS core/shell nanorod arrays that can efficiently improve photocatalytic efficiency. Deng et al. [9] found that the rate of degradation of the ZnO after deposition of Ag are more than nearly 5.6 times faster than that of bare ZnO under the solar light irradiation. Kuriakose et al. further extend the photocatalytic activity of ZnO nanostructures in the visible region by inducing impurity levels in the band gap [10]. Recently, the surface modification, such as semiconductors photocatalysts grafted with ⁎ Corresponding authors. E-mail addresses: [email protected] (L. Yin), [email protected] (J. Huang).
http://dx.doi.org/10.1016/j.matchar.2017.03.004 1044-5803/© 2017 Elsevier Inc. All rights reserved.
Fe(III) or Cu(II) nanoclusters, has been demonstrated as an efficient way to realize the visible photocatalytic capability by photoinduced interfacial charge transfer (IFCT) process [12–18]. Specifically, Miyauchi reported ZnO-based photocatalysts grafted with Cu2+ co-catalyst and demonstrated the strategy is a promising approach for applying ZnObased photocatalysts for indoor applications [19,20]. It is desirable and meaningful to seek the development of efficient ZnO based materials through IFCT process for photocatalysis under visible light illumination. In the current work, the Fe(III) -modified ZnO was successfully synthesized and well clarified according to various characterization methods. It is found that Fe(III)/ZnO nanostructures exhibit significantly enhanced photocatalytic efficiency under simulated sunlight illumination.
2. Experimental 2.1. Sample Preparation All the reagents were of analytical grade and used without further purification. The ZnO reagent purchased from Sinopharm Chemical Reagent Co. Ltd. was used as the source of ZnO. The Fe(III)/ZnO photocatalysts were prepared by an impregnation technique. In the typical preparation, 1 g ZnO powder was dispersed in the 10 mL FeCl3·6H2O solution under stirring and the weight fraction of Fe relative to ZnO was 3.0%. The suspension was heated at 90 °C, stirred for 1 h in the vital reactor, and then filtered and washed with a sufficient amount of deionized water. The resulting residues were dried at 110 °C for 12 h and labeled as Fe(III)/ZnO.
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2.2. Characterizations The crystalline phases of the samples were identified by powder Xray diffraction (XRD, Rigaku D/max-2000) with CuKα radiation (λ = 0.15406 nm). The morphologies of the samples were observed by field emission scanning electron microscopy (FE-SEM, Hitachi S-4800). The specific surface areas of the samples were measured by nitrogen adsorption method using an American Quantachrome NOVA-2200E instrument and an Agilent Cary 5000 UV–vis spectrophotometer was used to record the UV–vis diffuse reflectance spectra (DRS). The X-ray photoelectron spectroscopy (XPS) measurement was performed in a Surface Science Instruments Spectrometer with monochromatic Al Ka radiation 1486.6 eV. The binding energy (BE) was calibrated using the C1s signal located at 284.6 eV. 2.3. Photocatalytic Activity The photocatalytic activity were measured by the decomposition of RhB solution under simulated sunlight irradiation using a 1000 W xenon lamp (λ N 350 nm). In a typical photocatalytic experiment, 50 mg of the photocatalysts were added to 50 mL of RhB (10 mM) solution, and it was agitated for 1 h in the dark to achieve an absorption-desorption equilibrium between the photocatalysts and RhB molecules. After that, the suspension was continuously stirred and exposed to light irradiation. 4 mL of the solution were taken out every 30 min and some photocatalyts were separated by centrifugation. The RhB concentration in each sample was monitored by UV–vis spectroscopy (Unico UV-2600) at a wavelength of 553 nm. To probe the active species involved in the photocatalysis, 1 mM potassium iodide, isopropanol and sufficient nitrogen were introduced to the RhB solution, respectively. 2.4. Analysis of Hydroxyl Radicals The formation of hydroxyl radicals (•OH) on the surface of simulated sunlight illuminated Fe(Ш)/ZnO was detected by PL using terephthalic acid as a probe molecule. Terephthalic acid readily reacts with •OH to produce a highly fluorescent product, 2-hydroxyterephthalic acid. The amount of •OH radicals produced in water is proportional to the intensity of the PL signal at a wavelength of 425 nm. The typical experimental procedure was similar to the measurement of photocatalytic activity except that the RhB solution was replaced by the mixed solution of 5 × 10−4 M terephthalic acid and 2 × 10−3 M NaOH. The reaction solution was analyzed after centrifugation on an Edinburgh F35 fluorescence spectrophotometer. 3. Results and Discussion 3.1. Phase Structure and Morphology Fig. 1 gives the XRD patterns of the ZnO particles before and after surface modification by Fe(Ш) cocatalyst. It is seen that all of the diffraction peaks of two samples can be assigned to the hexagonal type of ZnO (JCPDS No. 36-1451). After modification by Fe(III) in FeCl3 aqueous solution, the Fe(III)/ZnO show a consistent diffraction peak as the bare ZnO and no related diffraction peaks of Fe(Ш) compounds are observed. In addition, the three most intense peaks does not show shifting toward higher 2θ value though the foreign Fe(Ш) ion with a smaller radius (0.064 nm) substitutes for the host Zn2 + ion with a larger radius (0.074 nm) in the ZnO lattice (inset of Fig. 1) [21]. These results indicate the Fe(Ш) compounds exists only on the surface of the ZnO. The FESEM images of bare ZnO and Fe(Ш)/ZnO are showed in the Fig. 2. It is clear that the bare ZnO exhibited irregular morphology with wide particle-size distribution (0.1–0.5 μm) and relatively smooth surface (Fig. 2A). As for Fe(Ш)/ZnO photocatalyst, no apparent changes in morphology are observed owing to a low temperature modification process. However, it can be clearly seen that some nanoparticles with
Fig. 1. X-ray diffraction patterns of (a) bare ZnO and (b) Fe(Ш)/ZnO samples. The inset is the diffraction peak positionsin the range of 2θ = 30 – 38°.
the size of about 2–4 nm are dispersed on the surface of the ZnO substrates. Further, the EDX spectrum was investigated to detect the element content of the composites and the results are showed in Fig. 2C. It is apparent that the peaks of Zn, O and Fe existed in the sample and the content of Fe is close to theoretical values, indicating Fe(Ш) compounds are grafted successfully on the surface of ZnO nanostructures. The TEM/HRTEM images in Fig. 3 corroborate the observation in the FESEM images. It is easily seen that some nanoparticles are anchored on the surfaces of ZnO nanocrystals, and no separate particles is observed (Fig. 3A), indicate the strong interaction between the nanoparticles and ZnO though fairly long period of ultrasonication time. In the HRTEM as depicted in Fig. 3B, the clear lattice fringes demonstrate that the substrate is highly crystallizing ZnO, and the interface of ZnO and particles can be observed clearly. It is interesting that a diffused halo ring is observed in the SEAD inset, which suggests an amorphous phase. However, the (110) plane of Fe2O3 confirm existence of nanocrystalline. To further confirm the definite composition and the existence of Fe compounds, the elemental mapping were displayed in Fig. 3C–F. These data show that Fe compounds are formed as tiny particles on the ZnO nanocrycstals surface, affording intimate contact between the two materials. 3.2. XPS Analysis and Optical Property To investigate the chemical compositions and states of ZnO before and after loading of Fe(Ш) cocatalyst, XPS, a surface sensitive technique, was further carried out. The survey spectrum of Fe(Ш)/ZnO resembles that of bare ZnO, indicating the corresponding photoelectron peaks of Zn and O elements respectively appear (Fig. 4A). Compared with the bare ZnO, a new weak peak of Fe element are observed in the Fe(Ш)/ ZnO samples at binding energy of ~ 711.3–723.9 eV. Fig. 4B gives the Zn 2p core-level spectra of the samples. It is obvious that two intensive peaks at 1021.4 and 1044.5 eV are observed, which are ascribed to Zn2p3/2 and Zn2p1/2 levels of Zn2 + in ZnO [22]. Notably, after Fe(Ш) clusters modification, the peaks position does not show critical shift, indicating the Fe(Ш) clusters were just fixed on the surface of ZnO and did not affect the surface microstructures of ZnO phase. Not surprisingly, after Fe(Ш) clusters modification, weak Fe2p signals are identified and showed in Fig. 4C. The binding energy of Fe2p3/2 and Fe 2p1/2 is located at approximately 711.1 and 723.9 eV, respectively, corresponding well to the binding energy of Fe3 + [13,14]. The low intensity of the Fe2p peaks suggests that the incorporated Fe3+ ions do not accumulate at the microsphere surface layers but uniformly distribute in the ZnO matrix [23]. It is quite interesting and important to determine the details of Fe(Ш) state on the ZnO matrix. However, take account into the
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Fig. 2. Typical SEM images of bare ZnO (A), and Fe(Ш)/ZnO (B), EDX analysis spectrum of Fe(Ш)/ZnO (C).
modification process and the related literatures [12–14], it is believed that the present Fe(Ш) cocatalyst could be existed in an amorphous FeOOH-like structures. The optical properties of the bare ZnO and Fe(Ш)/ZnO were further measured by the diffuse reflectance spectroscopy at room temperature. As shown in Fig. 5A, all samples reveal an intense absorption with the steep edge at wavelength near 400 nm, which corresponding to the intrinsic band to band transition of ZnO. For the Fe(Ш)/ZnO samples, it is clear that a new additional absorption appeared from 400 nm to 700 nm. The insets of Fig. 5A give the differences of UV–vis spectra of Fe(Ш)/ZnO and bare ZnO and their corresponding colors changing from white to brick red. The additional absorption in the visible region could be primarily attributed to the interface charge transfer (IFCT) from the valence band of ZnO to the surface Fe(Ш) nanoclusters [14– 16,24]. In addition, the d-d transition of Fe(Ш) in the FeOOH-like amorphous nanoclusters also possibly contributes to the enhanced visiblelight absorption [12–14]. As a direct band transition semiconductor, the relationship of band edge and optical absorption follows the equation: αhν = A(hν − Eg)1/2, where hν, α, Eg and A represent the absorption coefficient, light frequency, band-gap energy, and a constant, respectively. As
shown in Fig. 5B, the intercept of the tangent to the X-axis suggests that the band-gap energy of the ZnO samples are 3.11 eV before and after the surface of Fe(Ш) modification. After that, XPS valence band (VB) spectra were recorded to determine the band edges, as illustrated in Fig. 5C. The energy level is in alignment with the work function of the XPS instrument (4.10 eV, Fermi level) [25]. The Fe(Ш)/ZnO shows a VBM position of about 2.60 eV (2.40 V, vs NHE), which is consist of that of bare ZnO, indicating the valence band position of ZnO is still unchanged by modified Fe on surface. According to the band gap of ZnO (3.11 eV), the conduction band minimum for ZnO is estimated to be about − 0.51 V (vs NHE). In view of the band gap of Fe(Ш)/ZnO is almost as that of pure ZnO, it is found that the band structures of ZnO remain the same after modification of the Fe(III) clusters. 3.3. Photocatalytic Activity and Mechanism RhB is often used as a model pollutant to study the catalytic efficiency of photocatalysts. In this study, the photocatalytic activity of the bare ZnO and Fe(Ш)/ZnO were evaluated by the photodegradation of RhB under simulated sunlight irradiation, and the results are showed in Fig. 6A. Obviously, the blank test shows that the concentration of RhB
Fig. 3. Typical TEM (A), HRTEM (B) and elemental mapping images (C–F) images of Fe(Ш)/ZnO samples.
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Fig. 4. (A) XPS survey spectra and the high-resolution XPS spectra of (B) Zn 2p, (C) Fe 2p for various samples: (a) Bare ZnO, (b) Fe(Ш)/ZnO.
Fig. 5. UV–vis diffuse reflectance spectra (A), Plots of the (αhν)1/2 vs photo energy (hν) (B) and XPS valence band spectra (C) of various samples: (a) Bare ZnO, (b) Fe(Ш)/ZnO. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
changes little under simulated sunlight irradiation. More than 40% of the RhB was degraded after 210 min irradiation over bare ZnO photocatalysts. Quantitatively, an almost complete degradation are achieved in the presence of Fe(Ш)/ZnO catalysts, about three times as much as that decomposed by bare ZnO samples. The absorption spectra of RhB solution for different time in the presence of Fe(Ш)/ZnO are given in Fig. 6B. The intensity of the absorption peak is proportional to the concentration of RhB present in the solution. With increasing irradiation time, the intensity of the absorption peak decreases and almost completely disappears after about 210 min, indicating effective decomposition of RhB solution. Generally, electrons transfer to the CB from the VB of the semiconductor when the energy of incident photons matches or exceeds its band-gap, leaving an equal number of vacant sites (holes). Meanwhile, the photo-generated electrons and holes move to the surface, and react with surface OH− groups, H2O or O2 to produce various radicals, such as h+, •OH or •O2– [26,27]. However, the role of photogenerated radical
species during photocatalytic process may change over different catalysts. To validate the function of h+, •OH, and •O2−, a series of controlled experiments with different radical scavengers have been performed. Here, KI and isopropyl were introduced to quench photogenerated holes (h+) and hydroxyl radical (·OH), respectively [28–30]. Pumping nitrogen every 30 min was used to scavenge O2 in the solution, prohibiting the production of superoxide radicals [30,31]. As shown in Fig. 7A, the photodegradation efficiency of Fe(Ш)/ZnO is concurrently inhibited in the presence of all these three scavengers, but there are great differences in the extent of inhibition. Specifically, pumping nitrogen had minimal impact on the photodegradation of RhB over the Fe(Ш)/ZnO, suggesting that the·O2– can be part of radical species, but only be a small part. However, compared to the reaction without radical scavengers, the photoactivity is remarkably prohibited with the addition of KI and isopropyl and the inhibition effect of isopropyl is slightly lower than that of KI, indicating that the hydroxyl radical (•OH) play much more important role for the degradation of RhB. Thus, it can be
Fig. 6. (A) Absorption spectra of the solution of RhB exposed to irradiation for different time in the presence of Fe(Ш)/ZnO as photocatalyst. (B) Photocatalytic performances of different samples under simulated sunlight irradiation.
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Fig. 7. (A) Photocatalytic degradation of RhB in the presence of KI, isopropyl and nitrogen over Fe(Ш)/ZnO; (B)PL spectra changes observed during simulated sunlight illumination of the Fe(Ш)/ZnO in a 5 × 10−4 M basic solution of terephthalic acid (excitation at 315 nm).
concluded that the h+ may not be directly involved in degradation of dye pollutant, in other words, they activate principally H2O and OH− in the aqueous solution to produce a large amount of hydroxyl radicals. To further understand the active species involved in the photocatalytic process, the PL technique was employed to detect hydroxyl radicals (•OH) on the surface of light illuminated Fe(Ш)/ZnO using terephthalic acid as a probe molecule [32]. Fig. 7B shows the changes in the PL spectra for the 5 × 10−4 M terephthalic acid solution with irradiation time in the presence of Fe(Ш)/ZnO. A gradual increase in PL intensity at about 425 nm is observed with increasing irradiation time, indicating that the free ·OH radical may likely be a main active species. This is consistent with the results of the controlled experiments with different radical scavengers. Besides the photocatalysts activity, the stability and reusability are significant to practical applications. The recycle experiments of the Fe(Ш)/ZnO photocatalysts were performed under consistent conditions and the results are showed in Fig. 8. The photocatalytic efficiency of the Fe(Ш)/ZnO decreases only 9% after 3 cycles, indicating good stability of this catalysis for the photodegradation of pollutant molecules. The above results suggest that under simulated sunlight irradiation, the Fe(III) clusters have more contribution to the photocatalytic oxidation of RhB solution. In view of the trapping experiments, the feasible schematic diagram for the higher activity of Fe(III)/ZnO under simulated sunlight is proposed and showed in Fig. 9. As the above discussion, the ZnO semiconductor can only be excited by lower ultraviolet, thus, a small amount of superoxide radical are generated for the removing of pollutant molecules. On the other hand, under visible light irradiation, photo-induced electrons are generated and transferred into Fe(III) clusters via the IFCT process. And the Fe(III) ion transfers into Fe(II) after
accepting a photogenerated electron. As a result of the instability of Fe(II), the Fe(III) can be well recovered via a muti-electron redox process of Fe(II) (4Fe2 ++O2 + 4H+ → 4Fe3 ++2H2O or 4Fe2 + + O2 + 2H2O → 4Fe3++4OH−) [16,18]. Thus, the Fe(Ш) clusters could act as electron sinks for photo-induced electrons and suppress recombination of electron-hole pairs effectively. Meanwhile, the holes left in the valence band of ZnO could migrate quickly to the surface and then scavenged by hydroxyl and water molecules to yield highly oxidative hydroxyl radical (•OH), which can mineralize organic substrates effectively. Based on the mechanism study, XPS were performed to investigate the effect of Fe(III) cocatalyst on the photoinduced stability of ZnO photocatalysts after third-cycles degradation. Fig. 10 shows the XPS Fe spectra of Fe(Ш)/ZnO before and after three-cycle photocatalytic test. It is found that there is no changes in the location of two peaks corresponding to the binding energies of Fe 2p3/2 and Fe 2p1/2. The amount of Fe in the Fe(Ш)/ZnO photocatalysts before and after three-cycle photocatalytic tests can be calculated to 2.41 and 2.28 at.%, respectively, based on the XPS results. It is clear that the subtle changes could be not the main reason of performance degradation. Amazingly, a weak peak at 709.5 eV (marked with a red ring) can be observed after third cycle test, which can be attributed to the Fe(II) irons in FeO [14]. The presence of Fe(II) irons could signify the reducing of the grafted Fe(Ш) acting as electron sinks, which could cause the performance degradation. However, comprehensive and in-depth study needs to be investigated to understand underlying reasons in the next work. 4. Conclusion In summary, simulated sunlight-driven Fe(Ш)/ZnO composite photocatalysts were successfully fabricated via a facile impregnation
Fig. 8. Cyclic tests of photodegradation of RhB over Fe(III)/ZnO samples.
Fig. 9. Schematic illustration of the Fe(Ш)/ZnO photocatalytic reaction process under simulated sunlight irradiation.
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Fig. 10. XPS spectra for Fe 2p of Fe(Ш)/ZnO photocatalysts before and after 3 cycles photocatalytic tests.
method. Fe(III) clusters are well dispersed and deposited on the surfaces of ZnO nanocrystals, and the shapes and crystal structures does not show any significant variation. The Fe(Ш)/ZnO exhibits significantly enhanced photocatalytic efficiency, which is 2.5 times as high as that of bare ZnO under the simulated sunlight irradiation. There are could be attributed to the IFCT process derived from the coupling of Fe(III) cocatalyst, which can effectively improve charge separation of ZnO nanocrystals and enhance the mobility of holes. Moreover, the trapping experiments demonstrate ·OH plays a dominant role for the degradation of RhB during photocatalytic process. Acknowledgments This work was supported by the National Natural Science Foundation of China (no. 51541204); China Postdoctoral Science Foundation (no. 2016M592737); Industrial science and technology research project in Shaanxi province (no. 2016GY-199); State Key Laboratory of Silicate Materials for Architectures (Wuhan University of Technology) (no. SYSJJ2015-8). References [1] A. Fujishima, K. Honda, Electrochemical Photolysis of Water at a Semiconductor Electrode, 238, 1972 37–38. [2] W. Wu, S.F. Zhang, X.H. Xiao, J. Zhou, F. Ren, L.L. Sun, C.Z. Jiang, Controllable synthesis, magnetic properties, and enhanced photocatalytic activity of spindlelike mesoporous α-Fe2O3/ZnO core−shell heterostructures, ACS Appl. Mater. Interfaces 4 (2012) 3602–3609. [3] S.W. Duo, Y.Y. Li, H. Zhang, T.Z. Liu, K. Wu, Z.Q. Li, A facile salicylic acid assisted hydrothermal synthesis of different flower-like ZnO hierarchical architectures with optical and concentration-dependent photocatalytic properties, Mater. Charact. 114 (2016) 185–196. [4] Q.P. Luo, X.Y. Yu, B.X. Lei, H.Y. Chen, D.B. Kuang, C.Y. Su, Reduced graphene oxide-hierarchical ZnO hollow sphere composites with enhanced photocurrent and photocatalytic activity, J. Phys. Chem. C 116 (2012) 8111–8117. [5] S. Martha, K.H. Reddy, K.M. Parida, Fabrication of In2O3 modified ZnO for enhancing stability, optical behaviour, electronic properties and photocatalytic activity for hydrogen production under visible light, J. Mater. Chem. A 2 (2014) 3621–3631. [6] N. Kannadasan, N. Shanmugam, S. Cholan, K. Sathishkumar, G. Viruthagiri, R. Poonguzhali, The effect of Ce4+ incorporation on structural, morphological and photocatalytic characters of ZnO nanoparticles, Mater. Charact. 97 (2014) 37–46. [7] S. Khanchandani, S. Kundu, A. Patra, A.K. Ganguli, Shell thickness dependent photocatalytic properties of ZnO/CdS core−shell nanorods, J. Phys. Chem. C 116 (2012) 23653–23662.
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Materials Characterization journal homepage: www.elsevier.com/locate/matchar
Improving sunlight-driven photocatalytic activity of ZnO nanostructures upon decoration with Fe(III) cocatalyst Lixiong Yin a,⁎, Dongdong Zhang a, Jia Wang b, Jianfeng Huang a,⁎, Xingang Kong a, Jiameng Fang a, Feng Zhang a a b
School of Materials Science and Engineering, Shaanxi University of Science and Technology, Xi'an, Shaanxi 710021, China Shaanxi Research Design Institute of Petroleum and Chemical Industry, Xi'an, Shaanxi 710021, China
a r t i c l e
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Article history: Received 23 December 2016 Received in revised form 2 March 2017 Accepted 5 March 2017 Available online 09 March 2017 Keywords: ZnO Photocatalysis Cocatalyst Fe(III) nanocluster
a b s t r a c t In the study, the Fe(III) cocatalysts were grafted successfully on the surface of ZnO nanostructures to form Fe(III)/ ZnO photocatalysts by a simple impregnation technique. It was found that the morphologies and crystal structures of ZnO remain unchanged after modification of the Fe(III) clusters. And the photocatalytic activity and the stability for the degradation of Rhodamine B were also evaluated. Under simulated sunlight illumination for 210 min, an almost complete degradation was achieved over Fe(III)/ZnO nanocomposites. The mechanism of the enhancement is associated with a direct interfacial charge transfer process (IFCT). The Fe(III) clusters not only acted as electron sinks to enhance the separation of photoinduced electrons from holes, but also elevated the amount of the holes, leading to the production of more hydroxyl radical with high oxidizability. © 2017 Elsevier Inc. All rights reserved.
1. Introduction Following the discovery of the water splitting on the semiconductorbased photochemical electrode in 1972 [1], it was soon recognized that the photocatalytic process of organic pollutants has created great interest in the past many years. Of the well-known photocatalysts, ZnO has attracted much attention because of its non-toxicity, long-term stability and the high electron mobility of ~115–155 cm−2 V−1 s−1 [2–4]. These features can be exploited to solve several current global challenges involving environmental issues and energy crisis. However, it should be note the ZnO semiconductor with a wide band-gap (3.37 eV) still cannot be widely used in practical photocatalysis applications owing to limited visible-light absorption [5,6]. Thus, it is imperative to develop efficient strategies to modify the ZnO photocatalysts sensitive to visible light. In fact, over the last few decades, numerous attempts have been extensively used to extend to the photocatalytic activity of ZnO in the visible light based on the band-gap adjustment [7–11]. For instance, Khanchandani and co-workers [7] synthesized successfully the ZnO/ CdS core/shell nanorod arrays that can efficiently improve photocatalytic efficiency. Deng et al. [9] found that the rate of degradation of the ZnO after deposition of Ag are more than nearly 5.6 times faster than that of bare ZnO under the solar light irradiation. Kuriakose et al. further extend the photocatalytic activity of ZnO nanostructures in the visible region by inducing impurity levels in the band gap [10]. Recently, the surface modification, such as semiconductors photocatalysts grafted with ⁎ Corresponding authors. E-mail addresses: [email protected] (L. Yin), [email protected] (J. Huang).
http://dx.doi.org/10.1016/j.matchar.2017.03.004 1044-5803/© 2017 Elsevier Inc. All rights reserved.
Fe(III) or Cu(II) nanoclusters, has been demonstrated as an efficient way to realize the visible photocatalytic capability by photoinduced interfacial charge transfer (IFCT) process [12–18]. Specifically, Miyauchi reported ZnO-based photocatalysts grafted with Cu2+ co-catalyst and demonstrated the strategy is a promising approach for applying ZnObased photocatalysts for indoor applications [19,20]. It is desirable and meaningful to seek the development of efficient ZnO based materials through IFCT process for photocatalysis under visible light illumination. In the current work, the Fe(III) -modified ZnO was successfully synthesized and well clarified according to various characterization methods. It is found that Fe(III)/ZnO nanostructures exhibit significantly enhanced photocatalytic efficiency under simulated sunlight illumination.
2. Experimental 2.1. Sample Preparation All the reagents were of analytical grade and used without further purification. The ZnO reagent purchased from Sinopharm Chemical Reagent Co. Ltd. was used as the source of ZnO. The Fe(III)/ZnO photocatalysts were prepared by an impregnation technique. In the typical preparation, 1 g ZnO powder was dispersed in the 10 mL FeCl3·6H2O solution under stirring and the weight fraction of Fe relative to ZnO was 3.0%. The suspension was heated at 90 °C, stirred for 1 h in the vital reactor, and then filtered and washed with a sufficient amount of deionized water. The resulting residues were dried at 110 °C for 12 h and labeled as Fe(III)/ZnO.
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2.2. Characterizations The crystalline phases of the samples were identified by powder Xray diffraction (XRD, Rigaku D/max-2000) with CuKα radiation (λ = 0.15406 nm). The morphologies of the samples were observed by field emission scanning electron microscopy (FE-SEM, Hitachi S-4800). The specific surface areas of the samples were measured by nitrogen adsorption method using an American Quantachrome NOVA-2200E instrument and an Agilent Cary 5000 UV–vis spectrophotometer was used to record the UV–vis diffuse reflectance spectra (DRS). The X-ray photoelectron spectroscopy (XPS) measurement was performed in a Surface Science Instruments Spectrometer with monochromatic Al Ka radiation 1486.6 eV. The binding energy (BE) was calibrated using the C1s signal located at 284.6 eV. 2.3. Photocatalytic Activity The photocatalytic activity were measured by the decomposition of RhB solution under simulated sunlight irradiation using a 1000 W xenon lamp (λ N 350 nm). In a typical photocatalytic experiment, 50 mg of the photocatalysts were added to 50 mL of RhB (10 mM) solution, and it was agitated for 1 h in the dark to achieve an absorption-desorption equilibrium between the photocatalysts and RhB molecules. After that, the suspension was continuously stirred and exposed to light irradiation. 4 mL of the solution were taken out every 30 min and some photocatalyts were separated by centrifugation. The RhB concentration in each sample was monitored by UV–vis spectroscopy (Unico UV-2600) at a wavelength of 553 nm. To probe the active species involved in the photocatalysis, 1 mM potassium iodide, isopropanol and sufficient nitrogen were introduced to the RhB solution, respectively. 2.4. Analysis of Hydroxyl Radicals The formation of hydroxyl radicals (•OH) on the surface of simulated sunlight illuminated Fe(Ш)/ZnO was detected by PL using terephthalic acid as a probe molecule. Terephthalic acid readily reacts with •OH to produce a highly fluorescent product, 2-hydroxyterephthalic acid. The amount of •OH radicals produced in water is proportional to the intensity of the PL signal at a wavelength of 425 nm. The typical experimental procedure was similar to the measurement of photocatalytic activity except that the RhB solution was replaced by the mixed solution of 5 × 10−4 M terephthalic acid and 2 × 10−3 M NaOH. The reaction solution was analyzed after centrifugation on an Edinburgh F35 fluorescence spectrophotometer. 3. Results and Discussion 3.1. Phase Structure and Morphology Fig. 1 gives the XRD patterns of the ZnO particles before and after surface modification by Fe(Ш) cocatalyst. It is seen that all of the diffraction peaks of two samples can be assigned to the hexagonal type of ZnO (JCPDS No. 36-1451). After modification by Fe(III) in FeCl3 aqueous solution, the Fe(III)/ZnO show a consistent diffraction peak as the bare ZnO and no related diffraction peaks of Fe(Ш) compounds are observed. In addition, the three most intense peaks does not show shifting toward higher 2θ value though the foreign Fe(Ш) ion with a smaller radius (0.064 nm) substitutes for the host Zn2 + ion with a larger radius (0.074 nm) in the ZnO lattice (inset of Fig. 1) [21]. These results indicate the Fe(Ш) compounds exists only on the surface of the ZnO. The FESEM images of bare ZnO and Fe(Ш)/ZnO are showed in the Fig. 2. It is clear that the bare ZnO exhibited irregular morphology with wide particle-size distribution (0.1–0.5 μm) and relatively smooth surface (Fig. 2A). As for Fe(Ш)/ZnO photocatalyst, no apparent changes in morphology are observed owing to a low temperature modification process. However, it can be clearly seen that some nanoparticles with
Fig. 1. X-ray diffraction patterns of (a) bare ZnO and (b) Fe(Ш)/ZnO samples. The inset is the diffraction peak positionsin the range of 2θ = 30 – 38°.
the size of about 2–4 nm are dispersed on the surface of the ZnO substrates. Further, the EDX spectrum was investigated to detect the element content of the composites and the results are showed in Fig. 2C. It is apparent that the peaks of Zn, O and Fe existed in the sample and the content of Fe is close to theoretical values, indicating Fe(Ш) compounds are grafted successfully on the surface of ZnO nanostructures. The TEM/HRTEM images in Fig. 3 corroborate the observation in the FESEM images. It is easily seen that some nanoparticles are anchored on the surfaces of ZnO nanocrystals, and no separate particles is observed (Fig. 3A), indicate the strong interaction between the nanoparticles and ZnO though fairly long period of ultrasonication time. In the HRTEM as depicted in Fig. 3B, the clear lattice fringes demonstrate that the substrate is highly crystallizing ZnO, and the interface of ZnO and particles can be observed clearly. It is interesting that a diffused halo ring is observed in the SEAD inset, which suggests an amorphous phase. However, the (110) plane of Fe2O3 confirm existence of nanocrystalline. To further confirm the definite composition and the existence of Fe compounds, the elemental mapping were displayed in Fig. 3C–F. These data show that Fe compounds are formed as tiny particles on the ZnO nanocrycstals surface, affording intimate contact between the two materials. 3.2. XPS Analysis and Optical Property To investigate the chemical compositions and states of ZnO before and after loading of Fe(Ш) cocatalyst, XPS, a surface sensitive technique, was further carried out. The survey spectrum of Fe(Ш)/ZnO resembles that of bare ZnO, indicating the corresponding photoelectron peaks of Zn and O elements respectively appear (Fig. 4A). Compared with the bare ZnO, a new weak peak of Fe element are observed in the Fe(Ш)/ ZnO samples at binding energy of ~ 711.3–723.9 eV. Fig. 4B gives the Zn 2p core-level spectra of the samples. It is obvious that two intensive peaks at 1021.4 and 1044.5 eV are observed, which are ascribed to Zn2p3/2 and Zn2p1/2 levels of Zn2 + in ZnO [22]. Notably, after Fe(Ш) clusters modification, the peaks position does not show critical shift, indicating the Fe(Ш) clusters were just fixed on the surface of ZnO and did not affect the surface microstructures of ZnO phase. Not surprisingly, after Fe(Ш) clusters modification, weak Fe2p signals are identified and showed in Fig. 4C. The binding energy of Fe2p3/2 and Fe 2p1/2 is located at approximately 711.1 and 723.9 eV, respectively, corresponding well to the binding energy of Fe3 + [13,14]. The low intensity of the Fe2p peaks suggests that the incorporated Fe3+ ions do not accumulate at the microsphere surface layers but uniformly distribute in the ZnO matrix [23]. It is quite interesting and important to determine the details of Fe(Ш) state on the ZnO matrix. However, take account into the
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Fig. 2. Typical SEM images of bare ZnO (A), and Fe(Ш)/ZnO (B), EDX analysis spectrum of Fe(Ш)/ZnO (C).
modification process and the related literatures [12–14], it is believed that the present Fe(Ш) cocatalyst could be existed in an amorphous FeOOH-like structures. The optical properties of the bare ZnO and Fe(Ш)/ZnO were further measured by the diffuse reflectance spectroscopy at room temperature. As shown in Fig. 5A, all samples reveal an intense absorption with the steep edge at wavelength near 400 nm, which corresponding to the intrinsic band to band transition of ZnO. For the Fe(Ш)/ZnO samples, it is clear that a new additional absorption appeared from 400 nm to 700 nm. The insets of Fig. 5A give the differences of UV–vis spectra of Fe(Ш)/ZnO and bare ZnO and their corresponding colors changing from white to brick red. The additional absorption in the visible region could be primarily attributed to the interface charge transfer (IFCT) from the valence band of ZnO to the surface Fe(Ш) nanoclusters [14– 16,24]. In addition, the d-d transition of Fe(Ш) in the FeOOH-like amorphous nanoclusters also possibly contributes to the enhanced visiblelight absorption [12–14]. As a direct band transition semiconductor, the relationship of band edge and optical absorption follows the equation: αhν = A(hν − Eg)1/2, where hν, α, Eg and A represent the absorption coefficient, light frequency, band-gap energy, and a constant, respectively. As
shown in Fig. 5B, the intercept of the tangent to the X-axis suggests that the band-gap energy of the ZnO samples are 3.11 eV before and after the surface of Fe(Ш) modification. After that, XPS valence band (VB) spectra were recorded to determine the band edges, as illustrated in Fig. 5C. The energy level is in alignment with the work function of the XPS instrument (4.10 eV, Fermi level) [25]. The Fe(Ш)/ZnO shows a VBM position of about 2.60 eV (2.40 V, vs NHE), which is consist of that of bare ZnO, indicating the valence band position of ZnO is still unchanged by modified Fe on surface. According to the band gap of ZnO (3.11 eV), the conduction band minimum for ZnO is estimated to be about − 0.51 V (vs NHE). In view of the band gap of Fe(Ш)/ZnO is almost as that of pure ZnO, it is found that the band structures of ZnO remain the same after modification of the Fe(III) clusters. 3.3. Photocatalytic Activity and Mechanism RhB is often used as a model pollutant to study the catalytic efficiency of photocatalysts. In this study, the photocatalytic activity of the bare ZnO and Fe(Ш)/ZnO were evaluated by the photodegradation of RhB under simulated sunlight irradiation, and the results are showed in Fig. 6A. Obviously, the blank test shows that the concentration of RhB
Fig. 3. Typical TEM (A), HRTEM (B) and elemental mapping images (C–F) images of Fe(Ш)/ZnO samples.
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Fig. 4. (A) XPS survey spectra and the high-resolution XPS spectra of (B) Zn 2p, (C) Fe 2p for various samples: (a) Bare ZnO, (b) Fe(Ш)/ZnO.
Fig. 5. UV–vis diffuse reflectance spectra (A), Plots of the (αhν)1/2 vs photo energy (hν) (B) and XPS valence band spectra (C) of various samples: (a) Bare ZnO, (b) Fe(Ш)/ZnO. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
changes little under simulated sunlight irradiation. More than 40% of the RhB was degraded after 210 min irradiation over bare ZnO photocatalysts. Quantitatively, an almost complete degradation are achieved in the presence of Fe(Ш)/ZnO catalysts, about three times as much as that decomposed by bare ZnO samples. The absorption spectra of RhB solution for different time in the presence of Fe(Ш)/ZnO are given in Fig. 6B. The intensity of the absorption peak is proportional to the concentration of RhB present in the solution. With increasing irradiation time, the intensity of the absorption peak decreases and almost completely disappears after about 210 min, indicating effective decomposition of RhB solution. Generally, electrons transfer to the CB from the VB of the semiconductor when the energy of incident photons matches or exceeds its band-gap, leaving an equal number of vacant sites (holes). Meanwhile, the photo-generated electrons and holes move to the surface, and react with surface OH− groups, H2O or O2 to produce various radicals, such as h+, •OH or •O2– [26,27]. However, the role of photogenerated radical
species during photocatalytic process may change over different catalysts. To validate the function of h+, •OH, and •O2−, a series of controlled experiments with different radical scavengers have been performed. Here, KI and isopropyl were introduced to quench photogenerated holes (h+) and hydroxyl radical (·OH), respectively [28–30]. Pumping nitrogen every 30 min was used to scavenge O2 in the solution, prohibiting the production of superoxide radicals [30,31]. As shown in Fig. 7A, the photodegradation efficiency of Fe(Ш)/ZnO is concurrently inhibited in the presence of all these three scavengers, but there are great differences in the extent of inhibition. Specifically, pumping nitrogen had minimal impact on the photodegradation of RhB over the Fe(Ш)/ZnO, suggesting that the·O2– can be part of radical species, but only be a small part. However, compared to the reaction without radical scavengers, the photoactivity is remarkably prohibited with the addition of KI and isopropyl and the inhibition effect of isopropyl is slightly lower than that of KI, indicating that the hydroxyl radical (•OH) play much more important role for the degradation of RhB. Thus, it can be
Fig. 6. (A) Absorption spectra of the solution of RhB exposed to irradiation for different time in the presence of Fe(Ш)/ZnO as photocatalyst. (B) Photocatalytic performances of different samples under simulated sunlight irradiation.
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Fig. 7. (A) Photocatalytic degradation of RhB in the presence of KI, isopropyl and nitrogen over Fe(Ш)/ZnO; (B)PL spectra changes observed during simulated sunlight illumination of the Fe(Ш)/ZnO in a 5 × 10−4 M basic solution of terephthalic acid (excitation at 315 nm).
concluded that the h+ may not be directly involved in degradation of dye pollutant, in other words, they activate principally H2O and OH− in the aqueous solution to produce a large amount of hydroxyl radicals. To further understand the active species involved in the photocatalytic process, the PL technique was employed to detect hydroxyl radicals (•OH) on the surface of light illuminated Fe(Ш)/ZnO using terephthalic acid as a probe molecule [32]. Fig. 7B shows the changes in the PL spectra for the 5 × 10−4 M terephthalic acid solution with irradiation time in the presence of Fe(Ш)/ZnO. A gradual increase in PL intensity at about 425 nm is observed with increasing irradiation time, indicating that the free ·OH radical may likely be a main active species. This is consistent with the results of the controlled experiments with different radical scavengers. Besides the photocatalysts activity, the stability and reusability are significant to practical applications. The recycle experiments of the Fe(Ш)/ZnO photocatalysts were performed under consistent conditions and the results are showed in Fig. 8. The photocatalytic efficiency of the Fe(Ш)/ZnO decreases only 9% after 3 cycles, indicating good stability of this catalysis for the photodegradation of pollutant molecules. The above results suggest that under simulated sunlight irradiation, the Fe(III) clusters have more contribution to the photocatalytic oxidation of RhB solution. In view of the trapping experiments, the feasible schematic diagram for the higher activity of Fe(III)/ZnO under simulated sunlight is proposed and showed in Fig. 9. As the above discussion, the ZnO semiconductor can only be excited by lower ultraviolet, thus, a small amount of superoxide radical are generated for the removing of pollutant molecules. On the other hand, under visible light irradiation, photo-induced electrons are generated and transferred into Fe(III) clusters via the IFCT process. And the Fe(III) ion transfers into Fe(II) after
accepting a photogenerated electron. As a result of the instability of Fe(II), the Fe(III) can be well recovered via a muti-electron redox process of Fe(II) (4Fe2 ++O2 + 4H+ → 4Fe3 ++2H2O or 4Fe2 + + O2 + 2H2O → 4Fe3++4OH−) [16,18]. Thus, the Fe(Ш) clusters could act as electron sinks for photo-induced electrons and suppress recombination of electron-hole pairs effectively. Meanwhile, the holes left in the valence band of ZnO could migrate quickly to the surface and then scavenged by hydroxyl and water molecules to yield highly oxidative hydroxyl radical (•OH), which can mineralize organic substrates effectively. Based on the mechanism study, XPS were performed to investigate the effect of Fe(III) cocatalyst on the photoinduced stability of ZnO photocatalysts after third-cycles degradation. Fig. 10 shows the XPS Fe spectra of Fe(Ш)/ZnO before and after three-cycle photocatalytic test. It is found that there is no changes in the location of two peaks corresponding to the binding energies of Fe 2p3/2 and Fe 2p1/2. The amount of Fe in the Fe(Ш)/ZnO photocatalysts before and after three-cycle photocatalytic tests can be calculated to 2.41 and 2.28 at.%, respectively, based on the XPS results. It is clear that the subtle changes could be not the main reason of performance degradation. Amazingly, a weak peak at 709.5 eV (marked with a red ring) can be observed after third cycle test, which can be attributed to the Fe(II) irons in FeO [14]. The presence of Fe(II) irons could signify the reducing of the grafted Fe(Ш) acting as electron sinks, which could cause the performance degradation. However, comprehensive and in-depth study needs to be investigated to understand underlying reasons in the next work. 4. Conclusion In summary, simulated sunlight-driven Fe(Ш)/ZnO composite photocatalysts were successfully fabricated via a facile impregnation
Fig. 8. Cyclic tests of photodegradation of RhB over Fe(III)/ZnO samples.
Fig. 9. Schematic illustration of the Fe(Ш)/ZnO photocatalytic reaction process under simulated sunlight irradiation.
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Fig. 10. XPS spectra for Fe 2p of Fe(Ш)/ZnO photocatalysts before and after 3 cycles photocatalytic tests.
method. Fe(III) clusters are well dispersed and deposited on the surfaces of ZnO nanocrystals, and the shapes and crystal structures does not show any significant variation. The Fe(Ш)/ZnO exhibits significantly enhanced photocatalytic efficiency, which is 2.5 times as high as that of bare ZnO under the simulated sunlight irradiation. There are could be attributed to the IFCT process derived from the coupling of Fe(III) cocatalyst, which can effectively improve charge separation of ZnO nanocrystals and enhance the mobility of holes. Moreover, the trapping experiments demonstrate ·OH plays a dominant role for the degradation of RhB during photocatalytic process. Acknowledgments This work was supported by the National Natural Science Foundation of China (no. 51541204); China Postdoctoral Science Foundation (no. 2016M592737); Industrial science and technology research project in Shaanxi province (no. 2016GY-199); State Key Laboratory of Silicate Materials for Architectures (Wuhan University of Technology) (no. SYSJJ2015-8). References [1] A. Fujishima, K. Honda, Electrochemical Photolysis of Water at a Semiconductor Electrode, 238, 1972 37–38. [2] W. Wu, S.F. Zhang, X.H. Xiao, J. Zhou, F. Ren, L.L. Sun, C.Z. Jiang, Controllable synthesis, magnetic properties, and enhanced photocatalytic activity of spindlelike mesoporous α-Fe2O3/ZnO core−shell heterostructures, ACS Appl. Mater. Interfaces 4 (2012) 3602–3609. [3] S.W. Duo, Y.Y. Li, H. Zhang, T.Z. Liu, K. Wu, Z.Q. Li, A facile salicylic acid assisted hydrothermal synthesis of different flower-like ZnO hierarchical architectures with optical and concentration-dependent photocatalytic properties, Mater. Charact. 114 (2016) 185–196. [4] Q.P. Luo, X.Y. Yu, B.X. Lei, H.Y. Chen, D.B. Kuang, C.Y. Su, Reduced graphene oxide-hierarchical ZnO hollow sphere composites with enhanced photocurrent and photocatalytic activity, J. Phys. Chem. C 116 (2012) 8111–8117. [5] S. Martha, K.H. Reddy, K.M. Parida, Fabrication of In2O3 modified ZnO for enhancing stability, optical behaviour, electronic properties and photocatalytic activity for hydrogen production under visible light, J. Mater. Chem. A 2 (2014) 3621–3631. [6] N. Kannadasan, N. Shanmugam, S. Cholan, K. Sathishkumar, G. Viruthagiri, R. Poonguzhali, The effect of Ce4+ incorporation on structural, morphological and photocatalytic characters of ZnO nanoparticles, Mater. Charact. 97 (2014) 37–46. [7] S. Khanchandani, S. Kundu, A. Patra, A.K. Ganguli, Shell thickness dependent photocatalytic properties of ZnO/CdS core−shell nanorods, J. Phys. Chem. C 116 (2012) 23653–23662.
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