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Fabrication and catalytic mechanism study of CeO2-Fe2O3-ZnO mixed oxides on double surfaces of polyimide substrate using ion-exchange technique
Materials Science in Semiconductor Processing 74 (2018) 154–164
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Fabrication and catalytic mechanism study of CeO2-Fe2O3-ZnO mixed oxides on double surfaces of polyimide substrate using ion-exchange technique☆
MARK
⁎
Yonglin Lei , Jichuan Huo, Huiwei Liao Engineering Research Center of Biomass Materials, Ministry of Education, School of Materials Science and Engineering, Southwest University of Science and Technology, Mianyang 621010, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Ion-exchange Polyimide CeO2-Fe2O3-ZnO Photo-thermal catalysis Microwave catalysis
A series of CeO2-Fe2O3-ZnO mixed oxides layers on double surfaces of polyimide film have been fabricated by direct ion exchange technique. The obtained products were thoroughly characterized by various techniques including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), ultraviolet-visible (UV–VIS) absorption spectra and thermogravimetric (TG). The results confirmed that the CeO2-Fe2O3-ZnO mixed oxides were successful formed and dispersed uniformly on surfaces of polyimide film. The PI/CeO2-Fe2O3-ZnO films displayed a wide visiblelight absorption in the range of 400–710 nm and low energy gap at 1.55–1.75 eV. The initial decomposition temperatures of PI/CeO2-Fe2O3-ZnO films were all larger than 550 °C. Moreover, the PI/CeO2-Fe2O3-ZnO films possessed outstanding photothermocatalytic and microwave catalytic activity compared with PI/ZnO nanocomposite film. The removal efficiency of methyl orange with the PIZNNIFE-3 film under UV–VIS–80 °C-H2O2 condition and microwave (MW)-H2O2 condition in 12.0 min reached respectively up to 92.7% and 98.7%. The improved photothermocatalytic activity could be mainly attributed to the electrons and holes transfer and the thermally oxidation of Ce3+ ions. The high microwave catalytic activity was due to the effect of MW “hot spots”, microwave thermally induced electron/hole pairs and high thermal oxidation of CeO2. This study introduces a new class of promising sunlight-driven photocatalytic nanocomposites with outstanding comprehensive performance.
1. Introduction Semiconductor-based metal oxides are widely used in treatment of wastewater [1], especially dyes, due to their low cost, low toxicity, recyclability, and the ability to facilitate multi-electron transfer processes. However, single-component metal oxide has some drawbacks, such as the rapid recombination of photo generated charge carriers of ZnO [2–4], the low spectral responsive range of TiO2 [5–7], the poor thermal and chemical stability of CeO2 [8,9], et al. Mixed metal oxide has attracted increasing interests because its unique physical and chemical properties. Due to the synergistic effect of individual metal oxide, the drawbacks of individual metal oxide can be overcome and the catalytic activity can be improved. Owing to the high value of exciting binding energy and suitable band gap (3.37 eV), ZnO exhibits high photocatalytic efficiency in degradation of organic pollutants. One major drawback of ZnO is the low quantum efficiency, which is due to very fast recombination of photo
generated electron-hole pairs [10–12]. Similarly, CeO2 has a band gap (3.2 eV) similar to TiO2 and holds promise as a suitable photocatalyst for the degradation of organic pollutants. More importantly, CeO2 is one of the efficient thermocatalysts, owing to its remarkable Ce4+/ Ce3+ redox properties giving rise to oxygen vacancies [13]. However, the use of pure CeO2 in a heterogeneous catalytic reaction is still limited due to its poor thermal stability. The mixed metal oxides of ZnO and CeO2 have been proposed as photocatalysts and redox mediators for organic pollutant detection and degradation. Fine ZnO/CeO2-based particles with very small sizes exhibit unique UV-absorbing abilities, excellent catalytic activities [14–17]. Recently, a synergetic effect of photothermocatalysis was reported by several groups [18,19]. And, perhaps most impressively, with the synergetic effect of photocatalysis and thermocatalysis, significant enhancement in the catalytic activity of ZnO/CeO2 mixed oxides was achieved under hot environments. However, a relative fast recombination of photo generated electron-hole pairs and a relative low thermal stability still limited the application of
☆ This work was supported by National Science and Technology support program (2014BAB15B02) and Engineering research center of biomass materials, Ministry of Education, China (Grant No13zxbk05). ⁎ Correspondence to: School of Material Science and Engineering, Southwest University of Science and Technology, Mianyang 621010, China. E-mail address: [email protected] (Y. Lei).
http://dx.doi.org/10.1016/j.mssp.2017.10.032 Received 25 August 2017; Received in revised form 15 September 2017; Accepted 22 October 2017 1369-8001/ © 2017 Elsevier Ltd. All rights reserved.
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the ZnO/CeO2 mixed oxides. Therefore, it is necessary to find novel ways how to combine other semiconductors to improve the catalytic efficiency of ZnO/CeO2 mixed oxides. It has been proved that combining some semiconductors with different band gaps to form heterojunctions in photocatalytic systems could drive the photogenerated electrons and holes to the opposite directions, thereby minimizing their recombination [20,21]. In addition, a general way to overcome thermally induced negative effects was to introduce transition metal ion into the CeO2 lattice to form the solid solution [22]. Fe2O3 is a narrow band gap (1.9–2.2 eV) semiconductor which is suitable to be coupled with ZnO to enhance the separation of photo-generated electron–hole pairs [23]. Fe2O3 is also a possible donor which can be used as a sensitizer under visible irradiation because it can be transfer electrons to wide band gap semiconductors such as TiO2 and ZnO. Recently, some combinations of ZnO with Fe2O3 showed the increase photocatalytic activity by increasing the charge separation and extend visible response [24]. Also, as reported in other studies, by combining Fe2O3 and CeO2 can improve the thermal stability and long-term catalytic stability of the pure CeO2 [25–27]. Thus, we expect that the combination of Fe2O3 with ZnO/CeO2 mixed oxides can be the viable option to overcome the aforementioned problems. To the best of our knowledge, there are few reports on CeO2-Fe2O3-ZnO mixed oxides for the degradation of organic pollutants. Additionally, Fe2O3 and CeO2 have a certain microwave absorption capacity. The conversion of microwave energy from the absorption energy into heat promotes the thermal catalytic efficiency of CeO2. Thus, we also expect that the CeO2-Fe2O3-ZnO mixed oxides have high microwave-induced catalytic oxidation activities. It is very indispensable to enhance the desired properties of the CeO2-Fe2O3-ZnO mixed oxides by dispersing them over a thermally stable support material. Polymeric materials are especially preferred due to excellent flexibility and easy machining according to the specific application shape [28,29]. Among them, aromatic polyimide (PI) has been considered to be one of the most important support materials, because of its excellent mechanical properties, thermal stability and chemical resistance [30,31]. Meanwhile, the PI supported mixed metal oxides has been demonstrated for enhancing photocatalytic efficiency, as a result of mixed metal oxides modifying by large conjugated aromatic structure of PI [32]. However, owing to the generated charges fast transfer from mixed metal oxides to PI, the energy efficiency of the PI supported mixed metal oxides is still low. The combination of Fe2O3 with ZnO/CeO2 mixed oxides can increase the lifetime of excited electrons and holes and improve the interfacial charge transfer efficiency to adsorbed substrate. Therefore, it can be presumed that the CeO2-Fe2O3-ZnO mixed oxides supported by PI are very useful for the catalytic oxidation of organic pollutants. On the other hanid, immobilization of CeO2-Fe2O3-ZnO mixed oxides on PI surface is an effective route to overcome disadvantages of being difficult to reclaim CeO2-Fe2O3-ZnO mixed oxides nanoparticles from the reaction medium. Diverse approaches were developed to prepare functional inorganic oxide nanoparticles [33–37]. Among them, the ion-exchange technique to prepare PI/metal oxide nanocomposites had its unique advantages [38–42]. The major advantage of ion-exchange technique lies in better dispersion that the nano-layer metal oxide can be dispersed uniformly on surfaces of polyimide film by the controlling of the size, thickness and distribution of the metal oxide nanoparticle. Another advantage is that altering distribution of the mixed metal ion in vertical and
horizontal PI matrix via ion exchange reaction can easily obtain heterostructured metal oxides on the surface of PI. Thus, we also expect that the fabrication of CeO2-Fe2O3-ZnO nanoparticles onto polyimide surfaces can decrease the tendency of nanoparticle aggregation and improve the dispersion of CeO2-Fe2O3-ZnO nanoparticles. With this background, the aim of the present work was to study the effect of the PI supported CeO2-Fe2O3-ZnO mixed oxides on catalytic oxidation of organic pollutants. Accordingly, the PI supported CeO2Fe2O3-ZnO mixed oxides were, for the first time, prepared by ion-exchange technique. The effects of initial Ce, Fe and Zn ion loading in different proportions on microstructure, thermal properties, and catalyzed properties of the final PI supported CeO2-Fe2O3-ZnO mixed oxides were also studied. 2. Experimental 2.1. Materials Polyimide (a-BPDA, s-BPDA, 4,4′-ODA and PDA) films with thickness of 50 µm were prepared by our laboratory. The preparation procedure for polyimide films is referred to our previously work [30]. The co-poly (amic acid)s (PAAs) were synthesized by adding solid of aBPDA (0.02 mol) and s-BPDA (0.08 mol) to a stirred mixture solution of 4,4′-ODA (0.01 mol) and PDA(0.09 mol) in DMAC in a nitrogen atmosphere at 0 ℃. The reaction mixture was stirred for 0.5 h at this 0 ℃, after that it was heated to room temperature and stirred for 48 h. The obtained PAAs solutions were coated onto clean, dry plate-glass and then dried at 60 ℃ for 2 h in an air-convection oven. The coatings were cured by heating under mild thermal conditions in follow stages: at 100 ℃ for 40 min, 125 ℃ for 30 min, 145 ℃ for 20 min, 185 ℃ for 10 min, 220 ℃ for 20 min, 260 ℃ for 20 min, 300 ℃ for 40 min, 330 ℃ for 20 min, 350 ℃ for 20 min. The PI films were rinsed by ultrasonic cleaning for 10 min in deionized water and dried in ambient environment prior to use. Zinc nitrate (Zn(NO3)2·6H2O), Cerium nitrate (Ce (NO3)3·6H2O), Ferrous nitrate (Fe(NO3)2·6H2O) and Potassium hydroxide(KOH) was purchased from Shanghai Chemical Reagent Plant. All chemicals were analytical grade and used without further purification. 2.2. Preparation of composite films The procedures for preparing PI/CeO2-Fe2O3-ZnO films are illustrated in Scheme 1. The determination of the preparation conditions is based on the relevant literature [39,41] and previous experiments. First, PI films were treated by 2 mol/L aqueous KOH solution at 50 °C for 5 h to perform alkaline-induced hydrolysis resulting in the cleavage of the imide rings and the formation of carboxylates, and then washed with deionized water and blown dry with air. The surface modified films were next immersed into Zn(NO3)2, Ce(NO3)3 and Fe(NO3)2 mixed solution (0.1 mol/L Zn(NO3)2, Zn:Ce:Fe = 1:0:0, 2:1:1, 4:1:1,6:1:1 and 8:1:1, molar ratio) at room temperature for 3 h under the batch ultrasonic condition to incorporate Zn, Ce and Fe ions into the films through an ion-exchange reaction between potassium and Zn, Ce and Fe ions. After ion exchange, the films were rinsed by deionized water to remove the surface residual mixed solution and dried in ambient atmosphere. Finally, the precursor films were reimidized by slowly heating to 410 °C within 3 h in a forced air oven and were kept at 410 °C for 4.5 h, in which the hydrolyzed polyimide surface was reimidized coupled, the Scheme 1. Schematic process for the PI supported CeO2-Fe2O3ZnO mixed oxides preparation.
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metal cations in the surface of the PI matrix were oxidized to metal oxide particles and the metal ions are mutually bonded to form CeO2Fe2O3-ZnO mixed oxides solid solution by inserting oxygen atom. According to this method, the PI supported CeO2-Fe2O3-ZnO mixed oxides with different molar ratios of 1:0:0, 8:1:1, 6:1:1,4:1:1 and 2:1:1 have been synthesized and named as PIZNCEFE-0,PIZNCEFE-1, PIZNCEFE-2, PIZNCEFE-3 and PIZNCEFE-4 (All are called PIZNCEFE-x). 2.3. Characterization The crystal structure and composition phase were analyzed by X-ray diffraction (XRD, Cu Ka, 40 kV, 100 mA, Xpert MPD Pro). Chemical structure information of polyimide films and nanocomposite films was collected using infrared measurements (Version BM Spectrometer). The surface morphologies of nanocomposite films were determined by a field emission scanning electron microscopy (FE-SEM,Ultra 55). X-ray photoelectron spectroscopy (XPS) data were also obtained to analysis surface chemical composition using an XSAM-800 spectrometer with Al Kα radiation (hν= 1486.60 eV, Kratos). Thermo gravimetric (TG) analyses of the samples were performed with a TA Instrument SDT Q600 in a nitrogen atmosphere at a heating rate of 10 ℃/min. UV–VIS absorption spectra were recorded on a UV-3150 spectrophotometer equipped with an integrating sphere attachment.
Fig. 1. The XRD patterns of pure PI and PIZNCEFE-x (a) 5–80° and 30–60°.
respectively. It can be seen from Fig. 1, compared to the XRD curve of pure PI, the additional diffraction peaks in PIZNCEFE-0 can be indexed to ZnO and the additional diffraction peaks in PIZNCEFE-1, PIZNCEFE2, PIZNCEFE-3 and PIZNCEFE-4 can be indexed to ZnO, CeO2 and Fe2O3. Therefore, for PIZNNIFE-0 sample, the XRD results indicate the formation of ZnO on the surface of PI, and for PIZNCEFE-1, PIZNCEFE2, PIZNCEFE-3 and PIZNCEFE-4 samples, the XRD results indicate the coexistence of ZnO, CeO2 and Fe2O3 on the surface of PI. When comparing the XRD peaks of PIZNCEFE-0 sample to other PIZNCEFE-x samples, the XRD peaks of ZnO become broader and shift slightly to higher angles with increasing of Fe and Ce content. This phenomenon confirms that the formation of CeO2-Fe2O3-ZnO mixed oxides solid solution. When comparing the XRD peaks of PIZNCEFE-1, PIZNCEFE-2, PIZNCEFE-3 and PIZNCEFE-4, the intensity of XRD peaks of CeO and Fe2O3 increases with increasing of Fe and Ce content, which indicate that the PI supported CeO2-Fe2O3-ZnO mixed oxides of higher contents of Fe and Ni have more free CeO2 and Fe2O3 phase.
2.4. Elimination of methyl orange Photocatalytic degradation experiments were carried out in a customized reactor with a cooling-water-cycle system, and the reaction temperature of the aqueous solution was maintained at 25 ℃ and 80 ℃. A 300-W Xe lamp was employed as the exciting light source for simulated solar light irradiation. The photocatalytic activities were estimated by the degradation of methyl orange (5 × 10−5 M) solution. In each experiment, the pure PI and the PIZNCEFE-x hybrid photocatalysts (300 mg) were added into 50 mL dye aqueous solution. Prior to irradiation, the dye aqueous solution was stirred in the dark for 60 min to ensure the establishment of an adsorption/desorption equilibrium. During the reaction process under light illumination, 3 mL of solution was sampled at given time intervals, which was analyzed on a UV–VIS spectrophotometer to record the concentration changes of dye solutions. To investigate the impacts of H2O2 on elimination of methyl orange, 1.5 mg H2O2 was added into this photocatalytic degradation system, and the system was acidified to 2–3 pH. The microwave degradation behaviors of the PIZNCEFE-x samples were also studied by measuring the degradation of methyl orange in aqueous solution under microwave environment. Methyl orange (50 mL, 5 × 10−5 mol /L) and the PIZNCEFE-x catalyst (300 mg) were mixed by ultrasonic wave in the dark for 30 min to establish adsorption/desorption equilibrium, then adding 1.5 mg H2O2 into the system, and the system was acidified to 2–3 pH. Above system was placed into a microwave reactor under a microwave power of 750 W at 80 ℃. The methyl orange concentrations after microwave treatment were measured immediately by UV–VIS spectrophotometer at a wavelength of 515 nm, corresponding to the maximum absorption wavelength of acidified methyl orange. The degradation efficiency (%) has been calculated as:
Degradation efficiency(%) =
3.2. Morphology analysis Fig. 2 shows SEM images of pure PI, PIZNCEFE-x, respectively. It can be seen that, the reference pure PI is smooth without the detection of any particles as shown in Fig. 2a, while the metal oxide/PI nanocomposite films show interesting morphologies of metal oxide particles stacking, as seen in Figs. 2b-f. It is presented in Fig. 2b that the surface of PIZNCEFE-0 is homogeneously covered with regular ZnO nanoparticles (near 20 nm), and these nanoparticles are loosely arranged together, a clear boundary between neighboring particles can be observed. As clearly observed in the SEM images (Figs. 2c-f), the incorporation of the CeO2 and Fe2O3 into ZnO has resulted in a decrease in grain size (near 15 nm) and also has resulted in improved grain boundaries. Furthermore, in cases when the Zn:Ce:Fe is greater than 6:1:1 (PIZNCEFE-3, Fig. 2e and PIZNCEFE-4, Fig. 2f), the CeO2-Fe2O3ZnO mixed oxides consist of spherically shaped nanoparticles with some agglomeration. Moreover, the nanoparticles agglomerations intent to become more and bigger, and a large amount of nanoparticles are encapsulated by outer nanoparticles (seen in Fig. 2f).
C0 − C1 × 100 C0
Where C0 is the initial methyl orange concentration and C1 is the concentration of methyl orange after adsorption and degradation.
3.3. The XPS spectra analysis The XPS measurement was performed to verify the elemental states and determine the relative proportion of elements on the surface of catalysts. Fig. 3a exhibits the wide survey XPS spectra of PIZNCEFE-x. From Fig. 3a, it can be clearly observed that the peaks of Zn, Fe, Ce, C and O exist in sample of PIZNCEFE-1, PIZNCEFE-2, PIZNCEFE-3 and PIZNCEFE-4, and only characteristic signals of Zn, C,N and O appear in
3. Results and discussion 3.1. XRD characterization Fig. 1 displays the XRD patterns of pure PI and PIZNCEFE-x, 156
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Fig. 2. The SEM images of pure PI and PIZNCEFE-x: (a) pure PI, (b) PIZNCEFE −0, (c) PIZNCEFE −1, (d) PIZNCEFE −2, (e) PIZNCEFE −3 and (f) PIZNCEFE −4.
respectively 8:0.94:0.86, 6:1.05:0.99, 4: 1.13:1.04 and 2:1.14:0.96, which are close to the prepared molar ratio of Zn:Ce:Fe. It can also be found from Table 1, the carbon concentration decreases with the increase of Zn, Ce and Fe proportion, which confirms the initial findings
PIZNCEFE-0. The XPS surface atomic compositions for the pure PI and PIZNNIFE-x were displayed in Table 1. As indicated, the calculated surface atomic ratios of Zn:Ce:Fe for the PIZNCEFE-1, PIZNCEFE-2, PIZNCEFE-3 and PIZNCEFE-4 are
Fig. 3. The XPS spectra of PIZNCEFE-x including (a) a wide survey and (b)–(d) high-resolution spectra of Zn2p, Fe2p and Ce3d, respectively.
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907 and 916 eV, respectively, and the characteristic peaks of Ce3+ ions around 884 and 903 eV, respectively [46]. It can be seen from Fig. 3d that Ce4+ and Ce3+ ions are observed over all samples. It is proposed that the presence of Ce3+ is the symbol of the produce of oxygen vacancy [47]. 3.4. The FTIR spectra analysis Fig. 4 exhibits the FTIR spectra of PIZNCEFE-1, PIZNCEFE-2, PIZNCEFE-3 and PIZNCEFE-4, respectively. As shown in Fig. 4, the bands observed in all samples near 3440 cm−1 and 1610 cm−1 correspond to the O–H stretching and bending vibration of residual H2O molecules absorbed from the environment. The band observed in all samples near 2350 cm−1 is assigned to CO2 mode which may exist due to atmospheric CO2 by higher surface-to-volume ratio of the nanoparticles. Moreover, the bands observed around 1390 and 1110 cm−1 may assign to the C˭O and C-O groups of PI, respectively. Besides, the two broad absorption bands ～400–820 cm−1 in PIZNCEFE-1, PIZNCEFE-2, PIZNCEFE-3 and PIZNCEFE-4 are in response to the metaloxygen modes and confirm the formation of metal oxide on the surface of PI. According to the literature studies on the FTIR spectra analysis of pure nano ZnO, It was a well-known fact that nanoparticles of ZnO with sizes in the range of 20 nm could show a broad IR band around 430 cm−1[48]. However, as seen from Fig. 4, The disappearance of the characteristic absorption peak of Zn-O bond stretching vibrations and the existence of shift and broadening of Zn-O absorption peak in four samples are ascertained to the formation of mixed oxide solution where Ce and Fe ions are dispersed in the lattice of ZnO [15]. The new characteristic absorption peak of Fe2O3 at 780 cm−1 in the FTIR spectra of PIZNCEFE-2, PIZNCEFE-3 and PIZNCEFE-4 indicate that Fe2O3 exists on the surface of PI.
Fig. 4. The FTIR spectra of PIZNCEFE-1, PIZNCEFE-2, PIZNCEFE-3 and PIZNCEFE-4.
Table 1 The surface atomic compositions for PIZNCEFE-x. Samples
PIZNCEFE−0 PIZNCEFE−1 PIZNCEFE−2 PIZNCEFE−3 PIZNCEFE−4
Atomic Concentrations % Zn(2p)
Ce(3d)
Fe(2p)
O(1 s)
C(1 s)
N(1 s)
9.81 8.17 9.25 10.09 10.62
0 0.96 1.62 2.85 6.04
0 0.88 1.54 2.63 5.12
39.44 42.76 45.03 60.12 63.21
50.65 47.17 42.53 24.30 15.01
0.1 0.06 0.03 0.01 0
of SEMs that those nanoparticle agglomerations are intent to become more and larger in the higher loading molar ratio of Zn, Ce and Fe, and resulting in covering the PI. Fig. 3b exhibits the Zn2p high resolution XPS spectra of PIZNCEFEx. It can be seen from Fig. 3b, the binding energies presented at 1021.5 and 1044.8 eV belong to Zn 2p3/2 and Zn 3p1/2, respectively, which agree with the reference value for Zn2+ and are identical to the reported of related ZnO systems [43]. Fig. 3c exhibits the Fe2p high resolution XPS spectra of PIZNCEFE-x. It can be seen from Fig. 3c, the peaks located at 711.2 and 724.9 eV are attributed to the Fe 2p3/2 and Fe 2p1/2, respectively [44]. Moreover, two satellite peaks at 718.9 and 733.1 eV are detected. The observed peaks at these banding energy positions correspond to Fe3+ of Fe2O3 [45]. Fig. 3d exhibits the Ce3d high resolution XPS spectra of PIZNCEFE-1, PIZNCEFE-2, PIZNCEFE-3 and PIZNCEFE-4. It is reported that the peaks of Ce3d can be denoted by the characteristic peaks of Ce4+ ions at around 882, 888, 898, 900,
3.5. The UV–visible absorption spectra analysis Fig. 5a depicts the UV–VIS absorption spectra of ZnO, pure PI and PIZNCEIFE-x, respectively. It can be seen from Fig. 5a, despite weak adsorption in the visible wavelength region (λ > 500 nm) of pure PI and pure ZnO, the PIZNCEFE-0 film shows very broad absorption bands covering the visible region of 400–600 nm as reported in other literature [49]. As compared to the PIZNCEFE-0 film, the PI/CeO2-Fe2O3ZnO films exhibit a red-shift in the optical absorption band toward the visible regions, which reveals the existence of a chemical interaction among CeO2-Fe2O3-ZnO mixed oxides. As seen from Fig. 5a, the visible absorption regions of the PI/CeO2-Fe2O3-ZnO films increase with the increase of the Ce and Fe contents (400–620 nm for PIZNCEFE-1, 400–650 nm for PIZNCEFE-2, 400–690 nm for PIZNCEFE-3 and 400–710 nm for PIZNCEFE-4 respectively). This wide visible-light photo absorption of CeO2-Fe2O3-ZnO may be ascribed to the vectorial
Fig. 5. (a) The UV–visible absorption spectra of ZnO, pure PI and PIZNCEIFE-x(b) plots of (αhν)1/2 vs hν for the band gap energies of ZnO, pure PI and PIZNCEIFE-x.
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activity, which might be attributed to the vectorial transfer of induced electron and hole among the CeO2-Fe2O3-ZnO mixed oxides hindering the recombination of photogenerated electron–hole pairs. In addition, the lower photocatalytic degradation efficiency of the PIZNNIFE-3 and the PIZNNIFE-4 compared with the PIZNNIFE-2 could be clarified by that more aggregations of particles on the PIZNNIFE-3 and the PIZNNIFE-4 films surface (seen in Fig. 2) led to a decrease of the effective photocatalytic surface area. The methyl orange photodegradation over pure PI film and the obtained composite films follows the first-order kinetics model (Fig. 7b). The degradation rate constants of methyl orange over pure PI, PIZNCEFE-0, PIZNCEFE-1, PIZNCEFE-2, PIZNCEFE-3 and PIZNCEFE-4 films are about 0.0017, 0.0067, 0.076, 0.0144, 0.104 and 0.0087 min−1, respectively. 3.8. Photothermocatalytic activities Fig. 6. The TG curves in air for pure PI and PIZNCEFE-x, respectively.
Fig. 8 shows the photocatalytic degradation of methyl orange with the pure PI film and the PIZNCEFE-x films at 80 °C under simulated solar light irradiation. It can be seen from Fig. 8a that the PIZNCEFE-x films exhibit the same photocatalytic regularity as degrading methyl orange at 25 °C (Fig. 8a). However, the methyl orange degradation efficiency and the rate constant for the PI/CeO2-Fe2O3-ZnO films in case of photothermocatalytic degradation experiment (PIZNCEFE-1, 87.5%, 0.0089 min−1; PIZNCEFE-2, 98.4%, 0.0183 min−1; PIZNCEFE-3, 95.1%, 0.0130 min−1; PIZNCEFE-4, 90.8%, 0.0110 min−1) were far larger than that in case of photocatalytic degradation experiment. These results might be attributed to (1) the thermal energy increasing the electron–hole pair production, the conductivity of carriers and reaction's activation energy, which making them available for reaction with methyl orange at the surface. (2) the thermal energy accelerating Ce4+ trapping of photogenarated electrons of the PI/CeO2-Fe2O3-ZnO films to change into Ce3+ ions and triggering the reaction of exterior Ce3+ ions with O2 molecule to produce the radical •O2− resulting in improving the photothermocatalytic activities. In order to enhance the photothermocatalytic activities, the adding H2O2 into these photothermocatalytic systems was also investigated (Fig. 9). It can be seen from Fig. 9 that the PI/CeO2-Fe2O3-ZnO films exhibited very high photothermocatalytic efficiency for removing methyl orange (PIZNCEFE-1, 61.2%, 0.1059 min−1; PIZNCEFE-2, 79.6%, 0.1687 min−1; PIZNCEFE-3, 92.7%, 0.2475 min−1; PIZNCEFE-4, 89.3%, 0.2043 min−1 after 12 min simulated solar light irradiation at 80 °C), which indicated that the adding H2O2 into these photothermocatalytic systems could reduce processing time from hours to minutes.
transfer of induced electron and hole among the three semiconductors [15], and the formation of some localized band gap states caused by oxygen vacancies of Ce3+ (XPS results). The band gap energies of photocatalysts can be estimated by the Kubelka–Munk theory. In this study, (αhν) n(n = 1/2) vs hν of the materials were plotted in Fig. 5b. We find that the calculated energy gap for ZnO, pure PI, PZNCEFE-0, PZNCEFE-1, PZNCEFE-2, PZNCEFE-3 and PZNCEFE-4 is about 3.23, 2.75, 1.75, 1.74,1.7,1.61 and 1.55 eV, respectively. Therefore, the UV–VIS absorption results confirm that the PI/ CeO2-Fe2O3-ZnO films can harvest visible light to 600 nm. 3.6. Thermal properties Fig. 6 displays the TG curves in air for pure PI and PIZNCEFE-x, respectively. As can be observed, the initial decomposition temperatures of PIZNCEFE-1, PIZNCEFE-2, PIZNCEFE-3 and PIZNCEFE-4 films are all larger than 550 °C, indicating that the excellent thermal stability of the PI/CeO2-Fe2O3-ZnO films is beneficial to photothermocatalysis and microwave-induced catalytic oxidation. 3.7. Photocatalytic activities Fig. 7 shows the photocatalytic degradation of methyl orange with pure PI film and the PIZNCEFE-x films under simulated solar light irradiation. Clearly, as displayed in Fig. 7a, the PIZNNIFE-x show greater photocatalytic degradation efficiency (PIZNCEFE-0, 80.1%; PIZNCEFE1, 81.2%; PIZNCEFE-2, 96.6%;PIZNCEFE-3, 89.2%; PIZNCEFE-4, 84.5% after 240 min simulated solar light irradiation) for methyl orange degradation than pure PI (28.5%). As compared to the PI/ZnO film, the PI/CeO2-Fe2O3-ZnO films exhibited higher photocatalytic
3.9. Microwave combining with H2O2 catalytic activities The methyl orange degradation efficiency of the pure PI film and the
Fig. 7. (a) Photodegradation curves of methyl orange by different photocatalysts under simulated solar light irradiation at 25 °C (b) Their photodegradation rate constant K.
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Fig. 8. (a) Photodegradation curves of methyl orange by different photocatalysts under simulated solar light irradiation at 80 °C (b) Their photodegradation rate constant K.
PIZNCEFE-x films under microwave irradiation in the presence of H2O2 are seen in Fig. 10. It can be seen from the Fig. 10 that the values of the methyl orange degradation efficiencies after 12 min all reach to 98% in the cases of the PI/CeO2-Fe2O3-ZnO films, which indicating the methyl orange can be degraded effectively within short treatment time under the conditions of microwave combining with H2O2 using the PI/CeO2Fe2O3-ZnO films. Moreover the methyl orange degradation rate constants K of MW experiment over PIZNCEFE-1 (0.3116 min−1), PIZNCEFE-2 (0.3451 min−1), PIZNCEFE-3 (0.4637 min−1) and PIZNCEFE-4 (0.4018 min−1) films were all far larger than that of photo-thermal experiment. It was worthy of note that the values of the methyl orange degradation efficiency for the PI/CeO2-Fe2O3-ZnO films increased with the increases of Ce and Fe contents, which was attributed to more microwave absorption under greater Ce and Fe contents conditions producing more “hot spots”. However, the value of the methyl orange degradation efficiency for PIZNCEFE-4 being more Ce and Fe contents was less than that for PIZNCEFE-3. This result might be explained by that in addition to the microwave hotspot, microwave thermally induced electron/hole pairs and high thermal oxidation of CeO2 was also responsible for the microwave degradation of methyl orange. Owing to the most aggregations of particles on the PIZNNIFE-4 film surface (seen in Fig. 2), the generated electron/hole pairs and CeO2 on the surface of PIZNNIFE-4 film was less, which led to reducing it the methyl orange degradation efficiency. Because the pH can influence the structure of the contaminants and microwave combining with H2O2 catalytic activities of catalyst, the degradation of methyl orange was observed at the pH 2.5, 5.0, 7.0 and 10.0 in 6 min using the microwave combining with H2O2 catalytic method with as-prepared PIZNCEFE-3 catalyst. The results presented in
Table 2. It demonstrates that the degradation of methyl orange is affected significantly using the microwave combining with H2O2 catalytic method. Higher methyl orange removal was obtained using acidic medium of pH 2.5 (95.83%) compared to basic medium at pH 10 (64.78%). The main factor affecting methyl orange removal was generation hydroxyl radicals and its structure. The quinone structure of methyl orange is more easily degraded at pH < 3.4 (pKa value of methyl orange at 25 °C is 3.4) than that of its azo structure at pH > 3.4. Besides, under acidic conditions, the Fenton reagent ( the internal heatinduced electrons (e-) may be trapped Fe3+ on the surface of PI/CeO2Fe2O3-ZnO mixed oxides films to change into Fe2+ ions) exhibits a relatively excellent treatment effect with a large amount of •OH generated. Thus the degradation efficiency of methyl orange decreased at high pH value. To investigate the stability of the catalyst in the treatment methyl orange using the microwave combining with H2O2 catalytic method, 300 mg of PIZNCEFE-3 catalyst was used for five experimental runs at the pH 2.5 in 6 min. At each run, the catalyst was centrifuged and fresh methyl orange solution was added together with 1.5 mg H2O2 and at 80 °C. Fig. 11 indicates the stability of the catalyst for five experimental runs. As shown in Fig. 11, we can clearly see that the photocatalytic degraded rate of methyl orange over the PIZNCEFE-3 is no obvious decline even after five cycles. The results demonstrate that the as-fabricated nanocomposite film is considerably stable. 3.10. Performance of different processes To further study the influence of heat, ultraviolet-visible light (UV–VIS), microwave (MW) and H2O2 on the catalytic degradation behavior of the PI/CeO2-Fe2O3-ZnO films, nine kinds of methods were
Fig. 9. (a) Photodegradation curves of methyl orange by different photocatalysts under simulated solar light irradiation at 80 °C, 1.5 mg H2O2 and 2–3 pH (b) Their photodegradation rate constant K.
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Fig. 10. (a) Degradation efficiency of methyl orange by different catalysts under microwave irradiation (750 W) at 1.5 mg H2O2 and 2–3 pH (b) Their photodegradation rate constant K.
Table 2 Degradation efficiency of methyl orange at the pH 2.5, 5.0, 7.0 and 10.0 in 6 min using the microwave combining with H2O2 catalytic method with as-prepared PIZNCEFE-3 catalyst. Degradation efficiency
C0 − C1 C0
× 100
pH 2.5
5.0
7.0
10.0
95.83
89.54
72.33.
64.78
Fig. 12. The effect of different processes on 10 min degradation efficiency of methyl orange.
were rise up to 98.6%. The reason for the results of MW-H2O2 process was the hot-spot formation on the surface of PIZNNIFE-3 film could accelerate decomposition of H2O2.
3.11. Possible degradation mechanisms To investigate active species generated and understand their catalytic process, K2S2O8, KI, L-histidine, Tert-butyl alcohol and Vitamin C as e-, h+, •O2−, •OH and all kinds of reactive oxygen species scavenger [50,51] were added in the reaction solutions, respectively. As shown in Fig. 13, for UV–VIS–80 °C-H2O2 process treatment process, the degradation efficiency of methyl orange was significantly restrained in the presence of K2S2O8 (27.8%) or tert-butyl alcohol (34.6%), and the methyl orange degradation efficiency was not significantly decreased after adding the scavengers of KI (65.4%) and L-histidine (69.8%). Above results illustrated that •OH and e- were the main active species in the photo-thermal combining with H2O2 catalytic degradation. According to some reports, the main •OH generation came from the reaction of h+ with absorbed H2O and OH-[50], the degradation efficiency of methyl orange in the presence of KI or tert-butyl alcohol should be similar, however, the degradation efficiency in the presence of tert-butyl alcohol was far less than that of KI, which indicated that partial •OH derived from the reaction of e- with H2O2 under this system. According to the degradation efficiency of methyl orange under microwave combining with H2O2 process in the presence of tert-butyl alcohol (40.3%), we could conclude that •OH generated in this reaction process was the main oxidant for methyl orange degradation. Based on previous analysis, the •OH mainly originated from the reaction of h+
Fig. 11. Stability of the PIZNCEFE-3 catalyst under microwave irradiation (750 W)at 80 °C, pH of 2.5, 300 mg of catalyst, 1.5 mg H2O2 and reaction time of 6 min.
conducted to compare methyl orange removal efficiency of the PIZNNIFE-3 film after 10 min including 80 °C process, 80 °C-H2O2 process, UV–VIS process, UV–VIS–H2O2 process, Blank UV–VIS–80 °CH2O2 process, UV–VIS–80 °C-H2O2 process, Blank MW-H2O2 process, MW process and MW-H2O2 process, The results are shown in Fig. 12. It can be seen from Fig. 12 that the methyl orange degradation efficiencies are very low with 80 °C process (4.9%), 80 °C-H2O2 process (7.3%), UV–VIS process (10.3%) and Blank UV-80 °C-H2O2 process (19.8%), and the values of methyl orange degradation efficiency for UV–VIS–H2O2 process and UV–VIS–80 °C-H2O2 process are rise up to 75.9% and 91.3%, respectively. These results suggest that the PI/CeO2-Fe2O3ZnO films can accelerate the decomposition of H2O2 into radicals under UV–VIS irradiation and thereby enhance the methyl orange removal efficiency. The methyl orange degradation efficiency in Blank MWH2O2 process was 12.1% and the methyl orange was removed about 45.8% for 10 min by PIZNNIFE-3 film in the absence of H2O2. However, the values of methyl orange removal efficiency for MW-H2O2 process 161
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mixed oxides can absorb and transfer microwave energy to heat, which may decrease the activation energy of chemical reaction. Thus, the organic pollutants around the hot spots can be decomposed in the presence of oxygen (O2) dissolved in water and CeO2 on the surface of PI/CeO2-Fe2O3-ZnO mixed oxides films, and H2O2 around the hot spots can also be resolved into •OH . In addition, the CeO2-Fe2O3-ZnO mixed oxides can be excited by high heat to generate the electrons and holes, which react with H2O, O2, and H2O2 respectively, to form •OH , •O2− and • HO2 in aqueous solution. Simultaneously, the internal heat-induced electrons (e-) may be trapped by Ce4+ and Fe3+ on the surface of PI/CeO2-Fe2O3-ZnO mixed oxides films to change into Ce3+ and Fe2+ ions. The Ce3+ ions can react with O2 to produce the radical •O2− and the Fe2+ ions could react with H2O2 to generate •OH . The generated •OH , •O2− and •HO2 in aqueous solution are highly reactive species that will oxidize the organic compounds. Fig. 13. Effect of adding radical scavengers on the methyl orange 10 min degradation efficiency under different treatment process.
4. Conclusion A series of CeO2-Fe2O3-ZnO mixed oxides layers on double surfaces of polyimide film have been fabricated by direct ion exchange technique. The XRD, XPS and FT-IR analyses confirmed the formation of the PI/CeO2-Fe2O3-ZnO mixed oxides films. The UV–visible absorption spectra analysis revealed that the PI/CeO2-Fe2O3-ZnO films displayed a wide visible-light absorption in the range of 400–710 nm. Especially, the ranges of visible-light absorption enlarge with the Ce and Fe content increase. The TG analyses indicated the obtained nanocomposite films had the excellent thermal stability, which was beneficial to photothermocatalysis and microwave-induced catalytic oxidation. The obtained nanocomposite films possessed outstanding photo-thermal combining with H2O2 catalytic activity and microwave combining with H2O2 catalytic activity. The degradation efficiency of methyl orange with the PIZNNIFE-3 film under UV–VIS–80 °C-H2O2 condition and MW-H2O2 condition in 12.0 min reached respectively up to 92.7% and 98.7%. The enhanced charge transfer and the thermally oxidation of Ce3+ ions were responsible for improvement in dye degradation efficiency of the PI/CeO2-Fe2O3-ZnO films in case of the UV–VIS–80 °CH2O2 process. The high removal efficiency of methyl orange for PI/ CeO2-Fe2O3-ZnO films in case of MW-H2O2 process was attributed to the effect of MW “hot spots”, microwave thermally induced electron/ hole pairs and high thermal oxidation of CeO2. This study introduces a potential new family of catalytic materials for degradation of organic pollution.
with H2O and e- with H2O2. However, the degradation efficiency of methyl orange was not significantly decreased after adding the scavengers of KI (83.1%) and K2S2O8 (69.6%), suggesting the •OH generated basically originated H2O2 decompose under the MW “hot spot”. Most impressively, the methyl orange could be also degraded in the absence of any oxidizing group after adding Vitamin C (22.7%), implying that MW “hot spots” of the catalyst and high thermal oxidation of CeO2 had moderate contribution to the degradation efficiency. According to the analysis above, the photo-thermal combining with H2O2 catalytic mechanism for PI/CeO2-Fe2O3-ZnO film is offered (Scheme 2). Under the UV–VIS irradiation, the electrons will transfer from valence band (VB) to conduction band (CB) of the Fe2O3 and ZnO and the holes left at VB of the Fe2O3 and ZnO. The photoinduced electrons (e-) in CB of ZnO can transfer to the CB of Fe2O3 and the generated holes (h+) can transfer from the VB of Fe2O3 to the VB of ZnO. The CB electrons of both Fe2O3 and ZnO can react with H2O2 or O2 to form •OH or •O2−, and the VB holes of both Fe2O3 and ZnO can react with H2O, OH− and H2O2 to form •OH and • HO2. Additionally, the electrons (e-) in CB of both Fe2O3 and ZnO can also react with Ce4+ ions on the surface of CeO2 to form Ce3+ ions. The Ce3+ ions react with O2 molecule to generate the radical •O2− and Ce4+ ions under heat. Accordingly, the produced •OH, • HO2 and •O2− decompose the dye to the final products. The microwave combining with H2O2 catalytic mechanism for PI/ CeO2-Fe2O3-ZnO film is listed in Scheme 3. In the process of microwave combining with H2O2, the hot-spots could be formed on the surface of the CeO2-Fe2O3-ZnO mixed oxides particles as results of CeO2-Fe2O3
Acknowledgments This work was supported by National Science and Technology Scheme 2. The possible photo-thermal combining with H2O2 catalytic mechanism for PI/CeO2-Fe2O3-ZnO mixed oxides film.
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Scheme 3. The possible microwave combining with H2O2 catalytic mechanism for PI/CeO2-Fe2O3-ZnO mixed oxides film.
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Fabrication and catalytic mechanism study of CeO2-Fe2O3-ZnO mixed oxides on double surfaces of polyimide substrate using ion-exchange technique☆
MARK
⁎
Yonglin Lei , Jichuan Huo, Huiwei Liao Engineering Research Center of Biomass Materials, Ministry of Education, School of Materials Science and Engineering, Southwest University of Science and Technology, Mianyang 621010, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Ion-exchange Polyimide CeO2-Fe2O3-ZnO Photo-thermal catalysis Microwave catalysis
A series of CeO2-Fe2O3-ZnO mixed oxides layers on double surfaces of polyimide film have been fabricated by direct ion exchange technique. The obtained products were thoroughly characterized by various techniques including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), ultraviolet-visible (UV–VIS) absorption spectra and thermogravimetric (TG). The results confirmed that the CeO2-Fe2O3-ZnO mixed oxides were successful formed and dispersed uniformly on surfaces of polyimide film. The PI/CeO2-Fe2O3-ZnO films displayed a wide visiblelight absorption in the range of 400–710 nm and low energy gap at 1.55–1.75 eV. The initial decomposition temperatures of PI/CeO2-Fe2O3-ZnO films were all larger than 550 °C. Moreover, the PI/CeO2-Fe2O3-ZnO films possessed outstanding photothermocatalytic and microwave catalytic activity compared with PI/ZnO nanocomposite film. The removal efficiency of methyl orange with the PIZNNIFE-3 film under UV–VIS–80 °C-H2O2 condition and microwave (MW)-H2O2 condition in 12.0 min reached respectively up to 92.7% and 98.7%. The improved photothermocatalytic activity could be mainly attributed to the electrons and holes transfer and the thermally oxidation of Ce3+ ions. The high microwave catalytic activity was due to the effect of MW “hot spots”, microwave thermally induced electron/hole pairs and high thermal oxidation of CeO2. This study introduces a new class of promising sunlight-driven photocatalytic nanocomposites with outstanding comprehensive performance.
1. Introduction Semiconductor-based metal oxides are widely used in treatment of wastewater [1], especially dyes, due to their low cost, low toxicity, recyclability, and the ability to facilitate multi-electron transfer processes. However, single-component metal oxide has some drawbacks, such as the rapid recombination of photo generated charge carriers of ZnO [2–4], the low spectral responsive range of TiO2 [5–7], the poor thermal and chemical stability of CeO2 [8,9], et al. Mixed metal oxide has attracted increasing interests because its unique physical and chemical properties. Due to the synergistic effect of individual metal oxide, the drawbacks of individual metal oxide can be overcome and the catalytic activity can be improved. Owing to the high value of exciting binding energy and suitable band gap (3.37 eV), ZnO exhibits high photocatalytic efficiency in degradation of organic pollutants. One major drawback of ZnO is the low quantum efficiency, which is due to very fast recombination of photo
generated electron-hole pairs [10–12]. Similarly, CeO2 has a band gap (3.2 eV) similar to TiO2 and holds promise as a suitable photocatalyst for the degradation of organic pollutants. More importantly, CeO2 is one of the efficient thermocatalysts, owing to its remarkable Ce4+/ Ce3+ redox properties giving rise to oxygen vacancies [13]. However, the use of pure CeO2 in a heterogeneous catalytic reaction is still limited due to its poor thermal stability. The mixed metal oxides of ZnO and CeO2 have been proposed as photocatalysts and redox mediators for organic pollutant detection and degradation. Fine ZnO/CeO2-based particles with very small sizes exhibit unique UV-absorbing abilities, excellent catalytic activities [14–17]. Recently, a synergetic effect of photothermocatalysis was reported by several groups [18,19]. And, perhaps most impressively, with the synergetic effect of photocatalysis and thermocatalysis, significant enhancement in the catalytic activity of ZnO/CeO2 mixed oxides was achieved under hot environments. However, a relative fast recombination of photo generated electron-hole pairs and a relative low thermal stability still limited the application of
☆ This work was supported by National Science and Technology support program (2014BAB15B02) and Engineering research center of biomass materials, Ministry of Education, China (Grant No13zxbk05). ⁎ Correspondence to: School of Material Science and Engineering, Southwest University of Science and Technology, Mianyang 621010, China. E-mail address: [email protected] (Y. Lei).
http://dx.doi.org/10.1016/j.mssp.2017.10.032 Received 25 August 2017; Received in revised form 15 September 2017; Accepted 22 October 2017 1369-8001/ © 2017 Elsevier Ltd. All rights reserved.
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the ZnO/CeO2 mixed oxides. Therefore, it is necessary to find novel ways how to combine other semiconductors to improve the catalytic efficiency of ZnO/CeO2 mixed oxides. It has been proved that combining some semiconductors with different band gaps to form heterojunctions in photocatalytic systems could drive the photogenerated electrons and holes to the opposite directions, thereby minimizing their recombination [20,21]. In addition, a general way to overcome thermally induced negative effects was to introduce transition metal ion into the CeO2 lattice to form the solid solution [22]. Fe2O3 is a narrow band gap (1.9–2.2 eV) semiconductor which is suitable to be coupled with ZnO to enhance the separation of photo-generated electron–hole pairs [23]. Fe2O3 is also a possible donor which can be used as a sensitizer under visible irradiation because it can be transfer electrons to wide band gap semiconductors such as TiO2 and ZnO. Recently, some combinations of ZnO with Fe2O3 showed the increase photocatalytic activity by increasing the charge separation and extend visible response [24]. Also, as reported in other studies, by combining Fe2O3 and CeO2 can improve the thermal stability and long-term catalytic stability of the pure CeO2 [25–27]. Thus, we expect that the combination of Fe2O3 with ZnO/CeO2 mixed oxides can be the viable option to overcome the aforementioned problems. To the best of our knowledge, there are few reports on CeO2-Fe2O3-ZnO mixed oxides for the degradation of organic pollutants. Additionally, Fe2O3 and CeO2 have a certain microwave absorption capacity. The conversion of microwave energy from the absorption energy into heat promotes the thermal catalytic efficiency of CeO2. Thus, we also expect that the CeO2-Fe2O3-ZnO mixed oxides have high microwave-induced catalytic oxidation activities. It is very indispensable to enhance the desired properties of the CeO2-Fe2O3-ZnO mixed oxides by dispersing them over a thermally stable support material. Polymeric materials are especially preferred due to excellent flexibility and easy machining according to the specific application shape [28,29]. Among them, aromatic polyimide (PI) has been considered to be one of the most important support materials, because of its excellent mechanical properties, thermal stability and chemical resistance [30,31]. Meanwhile, the PI supported mixed metal oxides has been demonstrated for enhancing photocatalytic efficiency, as a result of mixed metal oxides modifying by large conjugated aromatic structure of PI [32]. However, owing to the generated charges fast transfer from mixed metal oxides to PI, the energy efficiency of the PI supported mixed metal oxides is still low. The combination of Fe2O3 with ZnO/CeO2 mixed oxides can increase the lifetime of excited electrons and holes and improve the interfacial charge transfer efficiency to adsorbed substrate. Therefore, it can be presumed that the CeO2-Fe2O3-ZnO mixed oxides supported by PI are very useful for the catalytic oxidation of organic pollutants. On the other hanid, immobilization of CeO2-Fe2O3-ZnO mixed oxides on PI surface is an effective route to overcome disadvantages of being difficult to reclaim CeO2-Fe2O3-ZnO mixed oxides nanoparticles from the reaction medium. Diverse approaches were developed to prepare functional inorganic oxide nanoparticles [33–37]. Among them, the ion-exchange technique to prepare PI/metal oxide nanocomposites had its unique advantages [38–42]. The major advantage of ion-exchange technique lies in better dispersion that the nano-layer metal oxide can be dispersed uniformly on surfaces of polyimide film by the controlling of the size, thickness and distribution of the metal oxide nanoparticle. Another advantage is that altering distribution of the mixed metal ion in vertical and
horizontal PI matrix via ion exchange reaction can easily obtain heterostructured metal oxides on the surface of PI. Thus, we also expect that the fabrication of CeO2-Fe2O3-ZnO nanoparticles onto polyimide surfaces can decrease the tendency of nanoparticle aggregation and improve the dispersion of CeO2-Fe2O3-ZnO nanoparticles. With this background, the aim of the present work was to study the effect of the PI supported CeO2-Fe2O3-ZnO mixed oxides on catalytic oxidation of organic pollutants. Accordingly, the PI supported CeO2Fe2O3-ZnO mixed oxides were, for the first time, prepared by ion-exchange technique. The effects of initial Ce, Fe and Zn ion loading in different proportions on microstructure, thermal properties, and catalyzed properties of the final PI supported CeO2-Fe2O3-ZnO mixed oxides were also studied. 2. Experimental 2.1. Materials Polyimide (a-BPDA, s-BPDA, 4,4′-ODA and PDA) films with thickness of 50 µm were prepared by our laboratory. The preparation procedure for polyimide films is referred to our previously work [30]. The co-poly (amic acid)s (PAAs) were synthesized by adding solid of aBPDA (0.02 mol) and s-BPDA (0.08 mol) to a stirred mixture solution of 4,4′-ODA (0.01 mol) and PDA(0.09 mol) in DMAC in a nitrogen atmosphere at 0 ℃. The reaction mixture was stirred for 0.5 h at this 0 ℃, after that it was heated to room temperature and stirred for 48 h. The obtained PAAs solutions were coated onto clean, dry plate-glass and then dried at 60 ℃ for 2 h in an air-convection oven. The coatings were cured by heating under mild thermal conditions in follow stages: at 100 ℃ for 40 min, 125 ℃ for 30 min, 145 ℃ for 20 min, 185 ℃ for 10 min, 220 ℃ for 20 min, 260 ℃ for 20 min, 300 ℃ for 40 min, 330 ℃ for 20 min, 350 ℃ for 20 min. The PI films were rinsed by ultrasonic cleaning for 10 min in deionized water and dried in ambient environment prior to use. Zinc nitrate (Zn(NO3)2·6H2O), Cerium nitrate (Ce (NO3)3·6H2O), Ferrous nitrate (Fe(NO3)2·6H2O) and Potassium hydroxide(KOH) was purchased from Shanghai Chemical Reagent Plant. All chemicals were analytical grade and used without further purification. 2.2. Preparation of composite films The procedures for preparing PI/CeO2-Fe2O3-ZnO films are illustrated in Scheme 1. The determination of the preparation conditions is based on the relevant literature [39,41] and previous experiments. First, PI films were treated by 2 mol/L aqueous KOH solution at 50 °C for 5 h to perform alkaline-induced hydrolysis resulting in the cleavage of the imide rings and the formation of carboxylates, and then washed with deionized water and blown dry with air. The surface modified films were next immersed into Zn(NO3)2, Ce(NO3)3 and Fe(NO3)2 mixed solution (0.1 mol/L Zn(NO3)2, Zn:Ce:Fe = 1:0:0, 2:1:1, 4:1:1,6:1:1 and 8:1:1, molar ratio) at room temperature for 3 h under the batch ultrasonic condition to incorporate Zn, Ce and Fe ions into the films through an ion-exchange reaction between potassium and Zn, Ce and Fe ions. After ion exchange, the films were rinsed by deionized water to remove the surface residual mixed solution and dried in ambient atmosphere. Finally, the precursor films were reimidized by slowly heating to 410 °C within 3 h in a forced air oven and were kept at 410 °C for 4.5 h, in which the hydrolyzed polyimide surface was reimidized coupled, the Scheme 1. Schematic process for the PI supported CeO2-Fe2O3ZnO mixed oxides preparation.
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metal cations in the surface of the PI matrix were oxidized to metal oxide particles and the metal ions are mutually bonded to form CeO2Fe2O3-ZnO mixed oxides solid solution by inserting oxygen atom. According to this method, the PI supported CeO2-Fe2O3-ZnO mixed oxides with different molar ratios of 1:0:0, 8:1:1, 6:1:1,4:1:1 and 2:1:1 have been synthesized and named as PIZNCEFE-0,PIZNCEFE-1, PIZNCEFE-2, PIZNCEFE-3 and PIZNCEFE-4 (All are called PIZNCEFE-x). 2.3. Characterization The crystal structure and composition phase were analyzed by X-ray diffraction (XRD, Cu Ka, 40 kV, 100 mA, Xpert MPD Pro). Chemical structure information of polyimide films and nanocomposite films was collected using infrared measurements (Version BM Spectrometer). The surface morphologies of nanocomposite films were determined by a field emission scanning electron microscopy (FE-SEM,Ultra 55). X-ray photoelectron spectroscopy (XPS) data were also obtained to analysis surface chemical composition using an XSAM-800 spectrometer with Al Kα radiation (hν= 1486.60 eV, Kratos). Thermo gravimetric (TG) analyses of the samples were performed with a TA Instrument SDT Q600 in a nitrogen atmosphere at a heating rate of 10 ℃/min. UV–VIS absorption spectra were recorded on a UV-3150 spectrophotometer equipped with an integrating sphere attachment.
Fig. 1. The XRD patterns of pure PI and PIZNCEFE-x (a) 5–80° and 30–60°.
respectively. It can be seen from Fig. 1, compared to the XRD curve of pure PI, the additional diffraction peaks in PIZNCEFE-0 can be indexed to ZnO and the additional diffraction peaks in PIZNCEFE-1, PIZNCEFE2, PIZNCEFE-3 and PIZNCEFE-4 can be indexed to ZnO, CeO2 and Fe2O3. Therefore, for PIZNNIFE-0 sample, the XRD results indicate the formation of ZnO on the surface of PI, and for PIZNCEFE-1, PIZNCEFE2, PIZNCEFE-3 and PIZNCEFE-4 samples, the XRD results indicate the coexistence of ZnO, CeO2 and Fe2O3 on the surface of PI. When comparing the XRD peaks of PIZNCEFE-0 sample to other PIZNCEFE-x samples, the XRD peaks of ZnO become broader and shift slightly to higher angles with increasing of Fe and Ce content. This phenomenon confirms that the formation of CeO2-Fe2O3-ZnO mixed oxides solid solution. When comparing the XRD peaks of PIZNCEFE-1, PIZNCEFE-2, PIZNCEFE-3 and PIZNCEFE-4, the intensity of XRD peaks of CeO and Fe2O3 increases with increasing of Fe and Ce content, which indicate that the PI supported CeO2-Fe2O3-ZnO mixed oxides of higher contents of Fe and Ni have more free CeO2 and Fe2O3 phase.
2.4. Elimination of methyl orange Photocatalytic degradation experiments were carried out in a customized reactor with a cooling-water-cycle system, and the reaction temperature of the aqueous solution was maintained at 25 ℃ and 80 ℃. A 300-W Xe lamp was employed as the exciting light source for simulated solar light irradiation. The photocatalytic activities were estimated by the degradation of methyl orange (5 × 10−5 M) solution. In each experiment, the pure PI and the PIZNCEFE-x hybrid photocatalysts (300 mg) were added into 50 mL dye aqueous solution. Prior to irradiation, the dye aqueous solution was stirred in the dark for 60 min to ensure the establishment of an adsorption/desorption equilibrium. During the reaction process under light illumination, 3 mL of solution was sampled at given time intervals, which was analyzed on a UV–VIS spectrophotometer to record the concentration changes of dye solutions. To investigate the impacts of H2O2 on elimination of methyl orange, 1.5 mg H2O2 was added into this photocatalytic degradation system, and the system was acidified to 2–3 pH. The microwave degradation behaviors of the PIZNCEFE-x samples were also studied by measuring the degradation of methyl orange in aqueous solution under microwave environment. Methyl orange (50 mL, 5 × 10−5 mol /L) and the PIZNCEFE-x catalyst (300 mg) were mixed by ultrasonic wave in the dark for 30 min to establish adsorption/desorption equilibrium, then adding 1.5 mg H2O2 into the system, and the system was acidified to 2–3 pH. Above system was placed into a microwave reactor under a microwave power of 750 W at 80 ℃. The methyl orange concentrations after microwave treatment were measured immediately by UV–VIS spectrophotometer at a wavelength of 515 nm, corresponding to the maximum absorption wavelength of acidified methyl orange. The degradation efficiency (%) has been calculated as:
Degradation efficiency(%) =
3.2. Morphology analysis Fig. 2 shows SEM images of pure PI, PIZNCEFE-x, respectively. It can be seen that, the reference pure PI is smooth without the detection of any particles as shown in Fig. 2a, while the metal oxide/PI nanocomposite films show interesting morphologies of metal oxide particles stacking, as seen in Figs. 2b-f. It is presented in Fig. 2b that the surface of PIZNCEFE-0 is homogeneously covered with regular ZnO nanoparticles (near 20 nm), and these nanoparticles are loosely arranged together, a clear boundary between neighboring particles can be observed. As clearly observed in the SEM images (Figs. 2c-f), the incorporation of the CeO2 and Fe2O3 into ZnO has resulted in a decrease in grain size (near 15 nm) and also has resulted in improved grain boundaries. Furthermore, in cases when the Zn:Ce:Fe is greater than 6:1:1 (PIZNCEFE-3, Fig. 2e and PIZNCEFE-4, Fig. 2f), the CeO2-Fe2O3ZnO mixed oxides consist of spherically shaped nanoparticles with some agglomeration. Moreover, the nanoparticles agglomerations intent to become more and bigger, and a large amount of nanoparticles are encapsulated by outer nanoparticles (seen in Fig. 2f).
C0 − C1 × 100 C0
Where C0 is the initial methyl orange concentration and C1 is the concentration of methyl orange after adsorption and degradation.
3.3. The XPS spectra analysis The XPS measurement was performed to verify the elemental states and determine the relative proportion of elements on the surface of catalysts. Fig. 3a exhibits the wide survey XPS spectra of PIZNCEFE-x. From Fig. 3a, it can be clearly observed that the peaks of Zn, Fe, Ce, C and O exist in sample of PIZNCEFE-1, PIZNCEFE-2, PIZNCEFE-3 and PIZNCEFE-4, and only characteristic signals of Zn, C,N and O appear in
3. Results and discussion 3.1. XRD characterization Fig. 1 displays the XRD patterns of pure PI and PIZNCEFE-x, 156
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Fig. 2. The SEM images of pure PI and PIZNCEFE-x: (a) pure PI, (b) PIZNCEFE −0, (c) PIZNCEFE −1, (d) PIZNCEFE −2, (e) PIZNCEFE −3 and (f) PIZNCEFE −4.
respectively 8:0.94:0.86, 6:1.05:0.99, 4: 1.13:1.04 and 2:1.14:0.96, which are close to the prepared molar ratio of Zn:Ce:Fe. It can also be found from Table 1, the carbon concentration decreases with the increase of Zn, Ce and Fe proportion, which confirms the initial findings
PIZNCEFE-0. The XPS surface atomic compositions for the pure PI and PIZNNIFE-x were displayed in Table 1. As indicated, the calculated surface atomic ratios of Zn:Ce:Fe for the PIZNCEFE-1, PIZNCEFE-2, PIZNCEFE-3 and PIZNCEFE-4 are
Fig. 3. The XPS spectra of PIZNCEFE-x including (a) a wide survey and (b)–(d) high-resolution spectra of Zn2p, Fe2p and Ce3d, respectively.
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907 and 916 eV, respectively, and the characteristic peaks of Ce3+ ions around 884 and 903 eV, respectively [46]. It can be seen from Fig. 3d that Ce4+ and Ce3+ ions are observed over all samples. It is proposed that the presence of Ce3+ is the symbol of the produce of oxygen vacancy [47]. 3.4. The FTIR spectra analysis Fig. 4 exhibits the FTIR spectra of PIZNCEFE-1, PIZNCEFE-2, PIZNCEFE-3 and PIZNCEFE-4, respectively. As shown in Fig. 4, the bands observed in all samples near 3440 cm−1 and 1610 cm−1 correspond to the O–H stretching and bending vibration of residual H2O molecules absorbed from the environment. The band observed in all samples near 2350 cm−1 is assigned to CO2 mode which may exist due to atmospheric CO2 by higher surface-to-volume ratio of the nanoparticles. Moreover, the bands observed around 1390 and 1110 cm−1 may assign to the C˭O and C-O groups of PI, respectively. Besides, the two broad absorption bands ～400–820 cm−1 in PIZNCEFE-1, PIZNCEFE-2, PIZNCEFE-3 and PIZNCEFE-4 are in response to the metaloxygen modes and confirm the formation of metal oxide on the surface of PI. According to the literature studies on the FTIR spectra analysis of pure nano ZnO, It was a well-known fact that nanoparticles of ZnO with sizes in the range of 20 nm could show a broad IR band around 430 cm−1[48]. However, as seen from Fig. 4, The disappearance of the characteristic absorption peak of Zn-O bond stretching vibrations and the existence of shift and broadening of Zn-O absorption peak in four samples are ascertained to the formation of mixed oxide solution where Ce and Fe ions are dispersed in the lattice of ZnO [15]. The new characteristic absorption peak of Fe2O3 at 780 cm−1 in the FTIR spectra of PIZNCEFE-2, PIZNCEFE-3 and PIZNCEFE-4 indicate that Fe2O3 exists on the surface of PI.
Fig. 4. The FTIR spectra of PIZNCEFE-1, PIZNCEFE-2, PIZNCEFE-3 and PIZNCEFE-4.
Table 1 The surface atomic compositions for PIZNCEFE-x. Samples
PIZNCEFE−0 PIZNCEFE−1 PIZNCEFE−2 PIZNCEFE−3 PIZNCEFE−4
Atomic Concentrations % Zn(2p)
Ce(3d)
Fe(2p)
O(1 s)
C(1 s)
N(1 s)
9.81 8.17 9.25 10.09 10.62
0 0.96 1.62 2.85 6.04
0 0.88 1.54 2.63 5.12
39.44 42.76 45.03 60.12 63.21
50.65 47.17 42.53 24.30 15.01
0.1 0.06 0.03 0.01 0
of SEMs that those nanoparticle agglomerations are intent to become more and larger in the higher loading molar ratio of Zn, Ce and Fe, and resulting in covering the PI. Fig. 3b exhibits the Zn2p high resolution XPS spectra of PIZNCEFEx. It can be seen from Fig. 3b, the binding energies presented at 1021.5 and 1044.8 eV belong to Zn 2p3/2 and Zn 3p1/2, respectively, which agree with the reference value for Zn2+ and are identical to the reported of related ZnO systems [43]. Fig. 3c exhibits the Fe2p high resolution XPS spectra of PIZNCEFE-x. It can be seen from Fig. 3c, the peaks located at 711.2 and 724.9 eV are attributed to the Fe 2p3/2 and Fe 2p1/2, respectively [44]. Moreover, two satellite peaks at 718.9 and 733.1 eV are detected. The observed peaks at these banding energy positions correspond to Fe3+ of Fe2O3 [45]. Fig. 3d exhibits the Ce3d high resolution XPS spectra of PIZNCEFE-1, PIZNCEFE-2, PIZNCEFE-3 and PIZNCEFE-4. It is reported that the peaks of Ce3d can be denoted by the characteristic peaks of Ce4+ ions at around 882, 888, 898, 900,
3.5. The UV–visible absorption spectra analysis Fig. 5a depicts the UV–VIS absorption spectra of ZnO, pure PI and PIZNCEIFE-x, respectively. It can be seen from Fig. 5a, despite weak adsorption in the visible wavelength region (λ > 500 nm) of pure PI and pure ZnO, the PIZNCEFE-0 film shows very broad absorption bands covering the visible region of 400–600 nm as reported in other literature [49]. As compared to the PIZNCEFE-0 film, the PI/CeO2-Fe2O3ZnO films exhibit a red-shift in the optical absorption band toward the visible regions, which reveals the existence of a chemical interaction among CeO2-Fe2O3-ZnO mixed oxides. As seen from Fig. 5a, the visible absorption regions of the PI/CeO2-Fe2O3-ZnO films increase with the increase of the Ce and Fe contents (400–620 nm for PIZNCEFE-1, 400–650 nm for PIZNCEFE-2, 400–690 nm for PIZNCEFE-3 and 400–710 nm for PIZNCEFE-4 respectively). This wide visible-light photo absorption of CeO2-Fe2O3-ZnO may be ascribed to the vectorial
Fig. 5. (a) The UV–visible absorption spectra of ZnO, pure PI and PIZNCEIFE-x(b) plots of (αhν)1/2 vs hν for the band gap energies of ZnO, pure PI and PIZNCEIFE-x.
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activity, which might be attributed to the vectorial transfer of induced electron and hole among the CeO2-Fe2O3-ZnO mixed oxides hindering the recombination of photogenerated electron–hole pairs. In addition, the lower photocatalytic degradation efficiency of the PIZNNIFE-3 and the PIZNNIFE-4 compared with the PIZNNIFE-2 could be clarified by that more aggregations of particles on the PIZNNIFE-3 and the PIZNNIFE-4 films surface (seen in Fig. 2) led to a decrease of the effective photocatalytic surface area. The methyl orange photodegradation over pure PI film and the obtained composite films follows the first-order kinetics model (Fig. 7b). The degradation rate constants of methyl orange over pure PI, PIZNCEFE-0, PIZNCEFE-1, PIZNCEFE-2, PIZNCEFE-3 and PIZNCEFE-4 films are about 0.0017, 0.0067, 0.076, 0.0144, 0.104 and 0.0087 min−1, respectively. 3.8. Photothermocatalytic activities Fig. 6. The TG curves in air for pure PI and PIZNCEFE-x, respectively.
Fig. 8 shows the photocatalytic degradation of methyl orange with the pure PI film and the PIZNCEFE-x films at 80 °C under simulated solar light irradiation. It can be seen from Fig. 8a that the PIZNCEFE-x films exhibit the same photocatalytic regularity as degrading methyl orange at 25 °C (Fig. 8a). However, the methyl orange degradation efficiency and the rate constant for the PI/CeO2-Fe2O3-ZnO films in case of photothermocatalytic degradation experiment (PIZNCEFE-1, 87.5%, 0.0089 min−1; PIZNCEFE-2, 98.4%, 0.0183 min−1; PIZNCEFE-3, 95.1%, 0.0130 min−1; PIZNCEFE-4, 90.8%, 0.0110 min−1) were far larger than that in case of photocatalytic degradation experiment. These results might be attributed to (1) the thermal energy increasing the electron–hole pair production, the conductivity of carriers and reaction's activation energy, which making them available for reaction with methyl orange at the surface. (2) the thermal energy accelerating Ce4+ trapping of photogenarated electrons of the PI/CeO2-Fe2O3-ZnO films to change into Ce3+ ions and triggering the reaction of exterior Ce3+ ions with O2 molecule to produce the radical •O2− resulting in improving the photothermocatalytic activities. In order to enhance the photothermocatalytic activities, the adding H2O2 into these photothermocatalytic systems was also investigated (Fig. 9). It can be seen from Fig. 9 that the PI/CeO2-Fe2O3-ZnO films exhibited very high photothermocatalytic efficiency for removing methyl orange (PIZNCEFE-1, 61.2%, 0.1059 min−1; PIZNCEFE-2, 79.6%, 0.1687 min−1; PIZNCEFE-3, 92.7%, 0.2475 min−1; PIZNCEFE-4, 89.3%, 0.2043 min−1 after 12 min simulated solar light irradiation at 80 °C), which indicated that the adding H2O2 into these photothermocatalytic systems could reduce processing time from hours to minutes.
transfer of induced electron and hole among the three semiconductors [15], and the formation of some localized band gap states caused by oxygen vacancies of Ce3+ (XPS results). The band gap energies of photocatalysts can be estimated by the Kubelka–Munk theory. In this study, (αhν) n(n = 1/2) vs hν of the materials were plotted in Fig. 5b. We find that the calculated energy gap for ZnO, pure PI, PZNCEFE-0, PZNCEFE-1, PZNCEFE-2, PZNCEFE-3 and PZNCEFE-4 is about 3.23, 2.75, 1.75, 1.74,1.7,1.61 and 1.55 eV, respectively. Therefore, the UV–VIS absorption results confirm that the PI/ CeO2-Fe2O3-ZnO films can harvest visible light to 600 nm. 3.6. Thermal properties Fig. 6 displays the TG curves in air for pure PI and PIZNCEFE-x, respectively. As can be observed, the initial decomposition temperatures of PIZNCEFE-1, PIZNCEFE-2, PIZNCEFE-3 and PIZNCEFE-4 films are all larger than 550 °C, indicating that the excellent thermal stability of the PI/CeO2-Fe2O3-ZnO films is beneficial to photothermocatalysis and microwave-induced catalytic oxidation. 3.7. Photocatalytic activities Fig. 7 shows the photocatalytic degradation of methyl orange with pure PI film and the PIZNCEFE-x films under simulated solar light irradiation. Clearly, as displayed in Fig. 7a, the PIZNNIFE-x show greater photocatalytic degradation efficiency (PIZNCEFE-0, 80.1%; PIZNCEFE1, 81.2%; PIZNCEFE-2, 96.6%;PIZNCEFE-3, 89.2%; PIZNCEFE-4, 84.5% after 240 min simulated solar light irradiation) for methyl orange degradation than pure PI (28.5%). As compared to the PI/ZnO film, the PI/CeO2-Fe2O3-ZnO films exhibited higher photocatalytic
3.9. Microwave combining with H2O2 catalytic activities The methyl orange degradation efficiency of the pure PI film and the
Fig. 7. (a) Photodegradation curves of methyl orange by different photocatalysts under simulated solar light irradiation at 25 °C (b) Their photodegradation rate constant K.
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Fig. 8. (a) Photodegradation curves of methyl orange by different photocatalysts under simulated solar light irradiation at 80 °C (b) Their photodegradation rate constant K.
PIZNCEFE-x films under microwave irradiation in the presence of H2O2 are seen in Fig. 10. It can be seen from the Fig. 10 that the values of the methyl orange degradation efficiencies after 12 min all reach to 98% in the cases of the PI/CeO2-Fe2O3-ZnO films, which indicating the methyl orange can be degraded effectively within short treatment time under the conditions of microwave combining with H2O2 using the PI/CeO2Fe2O3-ZnO films. Moreover the methyl orange degradation rate constants K of MW experiment over PIZNCEFE-1 (0.3116 min−1), PIZNCEFE-2 (0.3451 min−1), PIZNCEFE-3 (0.4637 min−1) and PIZNCEFE-4 (0.4018 min−1) films were all far larger than that of photo-thermal experiment. It was worthy of note that the values of the methyl orange degradation efficiency for the PI/CeO2-Fe2O3-ZnO films increased with the increases of Ce and Fe contents, which was attributed to more microwave absorption under greater Ce and Fe contents conditions producing more “hot spots”. However, the value of the methyl orange degradation efficiency for PIZNCEFE-4 being more Ce and Fe contents was less than that for PIZNCEFE-3. This result might be explained by that in addition to the microwave hotspot, microwave thermally induced electron/hole pairs and high thermal oxidation of CeO2 was also responsible for the microwave degradation of methyl orange. Owing to the most aggregations of particles on the PIZNNIFE-4 film surface (seen in Fig. 2), the generated electron/hole pairs and CeO2 on the surface of PIZNNIFE-4 film was less, which led to reducing it the methyl orange degradation efficiency. Because the pH can influence the structure of the contaminants and microwave combining with H2O2 catalytic activities of catalyst, the degradation of methyl orange was observed at the pH 2.5, 5.0, 7.0 and 10.0 in 6 min using the microwave combining with H2O2 catalytic method with as-prepared PIZNCEFE-3 catalyst. The results presented in
Table 2. It demonstrates that the degradation of methyl orange is affected significantly using the microwave combining with H2O2 catalytic method. Higher methyl orange removal was obtained using acidic medium of pH 2.5 (95.83%) compared to basic medium at pH 10 (64.78%). The main factor affecting methyl orange removal was generation hydroxyl radicals and its structure. The quinone structure of methyl orange is more easily degraded at pH < 3.4 (pKa value of methyl orange at 25 °C is 3.4) than that of its azo structure at pH > 3.4. Besides, under acidic conditions, the Fenton reagent ( the internal heatinduced electrons (e-) may be trapped Fe3+ on the surface of PI/CeO2Fe2O3-ZnO mixed oxides films to change into Fe2+ ions) exhibits a relatively excellent treatment effect with a large amount of •OH generated. Thus the degradation efficiency of methyl orange decreased at high pH value. To investigate the stability of the catalyst in the treatment methyl orange using the microwave combining with H2O2 catalytic method, 300 mg of PIZNCEFE-3 catalyst was used for five experimental runs at the pH 2.5 in 6 min. At each run, the catalyst was centrifuged and fresh methyl orange solution was added together with 1.5 mg H2O2 and at 80 °C. Fig. 11 indicates the stability of the catalyst for five experimental runs. As shown in Fig. 11, we can clearly see that the photocatalytic degraded rate of methyl orange over the PIZNCEFE-3 is no obvious decline even after five cycles. The results demonstrate that the as-fabricated nanocomposite film is considerably stable. 3.10. Performance of different processes To further study the influence of heat, ultraviolet-visible light (UV–VIS), microwave (MW) and H2O2 on the catalytic degradation behavior of the PI/CeO2-Fe2O3-ZnO films, nine kinds of methods were
Fig. 9. (a) Photodegradation curves of methyl orange by different photocatalysts under simulated solar light irradiation at 80 °C, 1.5 mg H2O2 and 2–3 pH (b) Their photodegradation rate constant K.
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Fig. 10. (a) Degradation efficiency of methyl orange by different catalysts under microwave irradiation (750 W) at 1.5 mg H2O2 and 2–3 pH (b) Their photodegradation rate constant K.
Table 2 Degradation efficiency of methyl orange at the pH 2.5, 5.0, 7.0 and 10.0 in 6 min using the microwave combining with H2O2 catalytic method with as-prepared PIZNCEFE-3 catalyst. Degradation efficiency
C0 − C1 C0
× 100
pH 2.5
5.0
7.0
10.0
95.83
89.54
72.33.
64.78
Fig. 12. The effect of different processes on 10 min degradation efficiency of methyl orange.
were rise up to 98.6%. The reason for the results of MW-H2O2 process was the hot-spot formation on the surface of PIZNNIFE-3 film could accelerate decomposition of H2O2.
3.11. Possible degradation mechanisms To investigate active species generated and understand their catalytic process, K2S2O8, KI, L-histidine, Tert-butyl alcohol and Vitamin C as e-, h+, •O2−, •OH and all kinds of reactive oxygen species scavenger [50,51] were added in the reaction solutions, respectively. As shown in Fig. 13, for UV–VIS–80 °C-H2O2 process treatment process, the degradation efficiency of methyl orange was significantly restrained in the presence of K2S2O8 (27.8%) or tert-butyl alcohol (34.6%), and the methyl orange degradation efficiency was not significantly decreased after adding the scavengers of KI (65.4%) and L-histidine (69.8%). Above results illustrated that •OH and e- were the main active species in the photo-thermal combining with H2O2 catalytic degradation. According to some reports, the main •OH generation came from the reaction of h+ with absorbed H2O and OH-[50], the degradation efficiency of methyl orange in the presence of KI or tert-butyl alcohol should be similar, however, the degradation efficiency in the presence of tert-butyl alcohol was far less than that of KI, which indicated that partial •OH derived from the reaction of e- with H2O2 under this system. According to the degradation efficiency of methyl orange under microwave combining with H2O2 process in the presence of tert-butyl alcohol (40.3%), we could conclude that •OH generated in this reaction process was the main oxidant for methyl orange degradation. Based on previous analysis, the •OH mainly originated from the reaction of h+
Fig. 11. Stability of the PIZNCEFE-3 catalyst under microwave irradiation (750 W)at 80 °C, pH of 2.5, 300 mg of catalyst, 1.5 mg H2O2 and reaction time of 6 min.
conducted to compare methyl orange removal efficiency of the PIZNNIFE-3 film after 10 min including 80 °C process, 80 °C-H2O2 process, UV–VIS process, UV–VIS–H2O2 process, Blank UV–VIS–80 °CH2O2 process, UV–VIS–80 °C-H2O2 process, Blank MW-H2O2 process, MW process and MW-H2O2 process, The results are shown in Fig. 12. It can be seen from Fig. 12 that the methyl orange degradation efficiencies are very low with 80 °C process (4.9%), 80 °C-H2O2 process (7.3%), UV–VIS process (10.3%) and Blank UV-80 °C-H2O2 process (19.8%), and the values of methyl orange degradation efficiency for UV–VIS–H2O2 process and UV–VIS–80 °C-H2O2 process are rise up to 75.9% and 91.3%, respectively. These results suggest that the PI/CeO2-Fe2O3ZnO films can accelerate the decomposition of H2O2 into radicals under UV–VIS irradiation and thereby enhance the methyl orange removal efficiency. The methyl orange degradation efficiency in Blank MWH2O2 process was 12.1% and the methyl orange was removed about 45.8% for 10 min by PIZNNIFE-3 film in the absence of H2O2. However, the values of methyl orange removal efficiency for MW-H2O2 process 161
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mixed oxides can absorb and transfer microwave energy to heat, which may decrease the activation energy of chemical reaction. Thus, the organic pollutants around the hot spots can be decomposed in the presence of oxygen (O2) dissolved in water and CeO2 on the surface of PI/CeO2-Fe2O3-ZnO mixed oxides films, and H2O2 around the hot spots can also be resolved into •OH . In addition, the CeO2-Fe2O3-ZnO mixed oxides can be excited by high heat to generate the electrons and holes, which react with H2O, O2, and H2O2 respectively, to form •OH , •O2− and • HO2 in aqueous solution. Simultaneously, the internal heat-induced electrons (e-) may be trapped by Ce4+ and Fe3+ on the surface of PI/CeO2-Fe2O3-ZnO mixed oxides films to change into Ce3+ and Fe2+ ions. The Ce3+ ions can react with O2 to produce the radical •O2− and the Fe2+ ions could react with H2O2 to generate •OH . The generated •OH , •O2− and •HO2 in aqueous solution are highly reactive species that will oxidize the organic compounds. Fig. 13. Effect of adding radical scavengers on the methyl orange 10 min degradation efficiency under different treatment process.
4. Conclusion A series of CeO2-Fe2O3-ZnO mixed oxides layers on double surfaces of polyimide film have been fabricated by direct ion exchange technique. The XRD, XPS and FT-IR analyses confirmed the formation of the PI/CeO2-Fe2O3-ZnO mixed oxides films. The UV–visible absorption spectra analysis revealed that the PI/CeO2-Fe2O3-ZnO films displayed a wide visible-light absorption in the range of 400–710 nm. Especially, the ranges of visible-light absorption enlarge with the Ce and Fe content increase. The TG analyses indicated the obtained nanocomposite films had the excellent thermal stability, which was beneficial to photothermocatalysis and microwave-induced catalytic oxidation. The obtained nanocomposite films possessed outstanding photo-thermal combining with H2O2 catalytic activity and microwave combining with H2O2 catalytic activity. The degradation efficiency of methyl orange with the PIZNNIFE-3 film under UV–VIS–80 °C-H2O2 condition and MW-H2O2 condition in 12.0 min reached respectively up to 92.7% and 98.7%. The enhanced charge transfer and the thermally oxidation of Ce3+ ions were responsible for improvement in dye degradation efficiency of the PI/CeO2-Fe2O3-ZnO films in case of the UV–VIS–80 °CH2O2 process. The high removal efficiency of methyl orange for PI/ CeO2-Fe2O3-ZnO films in case of MW-H2O2 process was attributed to the effect of MW “hot spots”, microwave thermally induced electron/ hole pairs and high thermal oxidation of CeO2. This study introduces a potential new family of catalytic materials for degradation of organic pollution.
with H2O and e- with H2O2. However, the degradation efficiency of methyl orange was not significantly decreased after adding the scavengers of KI (83.1%) and K2S2O8 (69.6%), suggesting the •OH generated basically originated H2O2 decompose under the MW “hot spot”. Most impressively, the methyl orange could be also degraded in the absence of any oxidizing group after adding Vitamin C (22.7%), implying that MW “hot spots” of the catalyst and high thermal oxidation of CeO2 had moderate contribution to the degradation efficiency. According to the analysis above, the photo-thermal combining with H2O2 catalytic mechanism for PI/CeO2-Fe2O3-ZnO film is offered (Scheme 2). Under the UV–VIS irradiation, the electrons will transfer from valence band (VB) to conduction band (CB) of the Fe2O3 and ZnO and the holes left at VB of the Fe2O3 and ZnO. The photoinduced electrons (e-) in CB of ZnO can transfer to the CB of Fe2O3 and the generated holes (h+) can transfer from the VB of Fe2O3 to the VB of ZnO. The CB electrons of both Fe2O3 and ZnO can react with H2O2 or O2 to form •OH or •O2−, and the VB holes of both Fe2O3 and ZnO can react with H2O, OH− and H2O2 to form •OH and • HO2. Additionally, the electrons (e-) in CB of both Fe2O3 and ZnO can also react with Ce4+ ions on the surface of CeO2 to form Ce3+ ions. The Ce3+ ions react with O2 molecule to generate the radical •O2− and Ce4+ ions under heat. Accordingly, the produced •OH, • HO2 and •O2− decompose the dye to the final products. The microwave combining with H2O2 catalytic mechanism for PI/ CeO2-Fe2O3-ZnO film is listed in Scheme 3. In the process of microwave combining with H2O2, the hot-spots could be formed on the surface of the CeO2-Fe2O3-ZnO mixed oxides particles as results of CeO2-Fe2O3
Acknowledgments This work was supported by National Science and Technology Scheme 2. The possible photo-thermal combining with H2O2 catalytic mechanism for PI/CeO2-Fe2O3-ZnO mixed oxides film.
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Scheme 3. The possible microwave combining with H2O2 catalytic mechanism for PI/CeO2-Fe2O3-ZnO mixed oxides film.
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