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C nanocomposite with enhanced adsorption capacity and photocatalytic performance under sunlight
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MOF derived ZnO/C nanocomposite with enhanced adsorption capacity and photocatalytic performance under sunlight Cuicui Hua, Xiaoxia Hub, Rong Lic, Yanjun Xinga,* a
Key Laboratory of Science and Technology of Eco-Textiles, Ministry of Education, College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, Shanghai 201620, China b Division of Science and Technology, Shanghai Urban Construction Vocational College, Shanghai 201415, China c National Engineering Research Center for Dyeing and Finishing of Textiles, Donghua University, Shanghai 201620, China
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Zinc-based MOF ZnO/C nanocomposite Adsorption Photocatalysis
In this work, ZnO/C nanocomposites were obtained by calcining the prepared metal-organic framework precursor under nitrogen. The crystallinity and structure of the prepared products were characterized by XRD, FTIR, XPS and EDS. The morphologies of samples before and after calcination were observed by FESEM. The photocatalytic performances of ZnO/C were evaluated by the degradation of methylene blue under sunlight irradiation. Combined with DRS, PL and BET, the influence of calcination temperature on photocatalytic activities of assynthesized zinc oxide were discussed as compared with commercial zinc oxide. The results indicated that ZnO/ C composite obtained at 600 °C and 700 °C exhibited the superior adsorption capacity and photocatalytic activity. The possible photocatalytic mechanism of ZnO/C nanocomposite for degradation of MB under sunlight irradiation was proposed.
1. Introduction In the past decades, the serious water pollution due to organic dyes and pigments affected the earth's environment, animal and human health. Photocatalysis based on semiconductor as a promising and ecofriendly treatment approach for organic contaminants which does not reproduce any secondary pollution has drawn more and more attention
⁎
(Hou et al., 2019). Among them, ZnO has received significant scientific interest due to its high electron mobility, high chemical stability, environmental friendliness and low cost (Ong et al., 2018; Sharma et al., 2019; Vaiano et al., 2018; Wang et al., 2019; Hu et al., 2018). Due to the low visible light utilization and low photocatalytic quantum efficiency on the applications for ZnO, ZnO composites, such as ZnO/C (Jiao and Zhang, 2018; Yan et al., 2019; Wang et al., 2018), have been
Corresponding author. E-mail address: [email protected] (Y. Xing).
https://doi.org/10.1016/j.jhazmat.2019.121599 Received 8 August 2019; Received in revised form 24 October 2019; Accepted 2 November 2019 0304-3894/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Cuicui Hu, et al., Journal of Hazardous Materials, https://doi.org/10.1016/j.jhazmat.2019.121599
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acid (H2BDC, Shanghai Titan Scientific Co., Ltd.), N,N’-dimethylformamide (DMF, Shanghai Lingfeng Chemical Reagent Co., Ltd., ≥99.5%), ethanol (Changshu Yangyuan Chemical Co., Ltd, ≥99.5%), commercial pure ZnO (denoted as Com-ZnO, Sinopharm Chemical Reagent Co., Ltd.), and methylene blue (MB, Shanghai Reagent Factory). All the reagents were of analytical grade and used without further modification.
used to improve the properties of ZnO. However, it is a problem for the poor dispersion and heterogeneity of the hybrid material (Hussain et al., 2018). Metal organic frameworks (MOFs) as hybrid organic-inorganic compounds, have drawn much attention in various applications such as sensing (Dong et al., 2019), gas sorption and separation (Lin et al., 2019) and catalysis (Joharian et al., 2018; Yi et al., 2019; Sun et al., 2018; Cui et al., 2019) due to high surface area and tunable structure (ZareKarizi et al., 2018). Recently, MOF-5 has been used as sacrificial precursor to obtain ZnO/C composites with large specific surface area and different porous nano/micro structures by pyrolysis or thermolysis method (Dekrafft et al., 2012; Yang et al., 2011; Jiang et al., 2011; Shi et al., 2017; Wu et al., 2019a,b). The ZnO nanoparticle in ZnO/C composites derived from MOFs was reported to have a homogeneous distribution in a highly porous carbon matrix (Hussain et al., 2019). Meanwhile, amorphous carbon formed from the carbonization of organic linker during pyrolysis process could also diffuse into the crystal lattice of ZnO, which can adjust the energy band gap of ZnO (Hussain et al., 2018; Pan et al., 2016). The form and content of carbon in ZnO/C composites generated during carbonization are related to the structure of the MOF precursor, which can greatly influence the application of ZnO/C (Hussain et al., 2018; Pan et al., 2016). MOF-5 has also been investigated to obtain different predictable nano/microstructures by controlling the species of surfactant, solvent and other synthesis condition (Li et al., 1999; Cheng et al., 2009; Lu et al., 2010; Tranchemontagne et al., 2008). The structure (Zhang and Hu, 2011; Hafizovic et al., 2007; Huang et al., 2003), pore sizes (Spokoyny et al., 2010) and synthesis methods (Huang et al., 2003; Yaghi et al., 2003) of MOF-5 (or Zn4O(BDC)3, BDC2− = 1,4-benzenedicarboxylate) should play a decisive role in the final shape and the pore size of the pyrolysis or thermolysis product (Yan et al., 2019; Gao et al., 2016; Li et al., 2018). These different factors of MOF-5 will be crucial for not only inorganic hybrid nanostructure features of the pyrolysis or thermolysis product, such as porosity, pore sizes and fascinating structures, but also their final promising applications (Hussain et al., 2019; Mai et al., 2017). To the best of our knowledge, there is few report about the photocatalytic activity and adsorption capacity of ZnO/C composite derived from cubic or octahedral MOF-5 (Hussain et al., 2018; Yang et al., 2011; Zhang et al., 2019a). The ZnO/C composite derived from other MOF-5 with different nanostructure caused by different synthesis condition (Zhang and Hu, 2011; Hafizovic et al., 2007; Huang et al., 2003), especially partially hydrolyzed MOF-5 (Huang et al., 2003), was also not been reported. Herein, a novel and simple strategy to prepare a hierarchical ZnO/C nanocomposite was reported by calcining partially hydrolyzed MOF-5 (Huang et al., 2003) at different temperatures under nitrogen. The influence of calcination temperature and calcination atmosphere on the morphology, adsorption and photocatalytic performance of prepared ZnO/C nanocomposite was investigated. The MOF-derived ZnO/C nanocomposite displayed excellent adsorption capacity comparable to the reported ZnO/C composites (Hussain et al., 2018; Yang et al., 2011) and ZnO/C prepared by calcining partially hydrolyzed MOF-5 under air. This method could be broadly applied to the synthesis of other metal oxide/carbon materials with low carbon content, which significantly extends its use to the energy and electronic related applications (Gao et al., 2016; Zhong et al., 2018).
2.2. Synthesis of ZnO/C nanocomposite In a typical synthesis, zinc nitrate hexahydrate (4 mmol), trisodium citrate dihydrate (2 mmol), and hexamethylenetetramine (2 mmol) were dissolved in distilled water (80 mL) and stirred continuously for 20 min to form a clear solution at room temperature. Terephthalic acid (1 mmol) in 20 mL of DMF was gradually added to the above solution under magnetic stirring. The resulted mixture was stirred vigorously at 85 °C for 1 h. The white product (named as Zn-MOF) was washed thoroughly with absolute ethanol and distilled water for several times and freeze dried after centrifugation at room temperature. Finally, the obtained white powder was calcined for 2 h under nitrogen at 350, 500, 600 and 700 °C, respectively. The heating ramp rate during the sintering process was kept at 5 °C min−1. The corresponding products were designated as Zn-MOF-350, ZnO/C-500, ZnO/C-600 and ZnO/C-700, respectively. The procedure was shown in Scheme 1. As a comparison, the obtained white powder was also calcined at 700 °C for 2 h in air. The corresponding product was designated as ZnO-700. 2.3. Characterization The crystallinity and structure of the prepared products were characterized by X-ray diffraction (XRD, Rigaku D/max-2550 PC, Japan) with Cu Kα radiation (λ = 1.5406 Å; scanning speed: 0.02° s−1). Thermogravimetric (TG) and differential thermal analysis (DTA) were performed by Germany NETZSCH TG209FI instrument with a rate of 10 °C min-1 under nitrogen condition. The chemical structures were studied using FT-IR spectrophotometer (Avatar 380, Thermo Scientific) in the range 4000–400 cm−1. X-ray photoelectron spectroscopy (XPS) was conducted on an ESCALAB 250Xi spectrometer (Thermo Fisher Scientific, USA) with Al Kα (1486.6 eV) X-ray source. The morphology, elemental composition and elemental mapping were observed by field emission scanning electron microscopy (FE-SEM, S-4800 HITACHI, Japan) equipped with energy dispersive X-ray spectroscopy (EDS). Elemental composition was studied by Vario EL III element analyzer (Germany, Elmentar). UV–vis diffusion reflection spectra (UV-DRS) were analyzed by using Shimadzu UV-3600 Plus UV-VIS-NIR spectrophotometer in the wavelength range from 200 to 800 nm. Photoluminescence (PL) spectra were taken on a fluorescence spectrometer (PTI QM, USA) at room temperature following an excitation wavelength of 325 nm. The special surface area and pore structure were calculated using the Brunauer–Emmett–Teller (BET) method with Micromeritics TriStar II 3020 Surface Area and Porosimetry Analyzer. 2.4. Adsorption experiment The adsorption of ZnO/C-600 and ZnO/C-700 were performed in dark condition at room temperature. 50 mL of MB aqueous solution with different initial concentrations in the range of 10–200 mg L−1 (2.67 × 10−5–5.35 × 10−4 mol L−1) were prepared. Equal mass of 20 mg of ZnO/C-600 (or ZnO/C-700) was added to the above solution and stirred in dark for 120 min to achieve equilibrium. At selected time intervals, 5 mL aliquot was taken out and centrifuged to remove the sample for analysis. The concentration of MB was measured using a Shimadzu UV-1800 UV–vis spectrophotometer at the maximum absorption wavelength of 664 nm. The adsorption amount of ZnO/C-600 (ZnO/C-700) at different time intervals, qt (mg g−1) was calculated according to the following
2. Experimental 2.1. Materials Zinc nitrate hexahydrate (Zn(NO3)2⋅6H2O, Sinopharm Chemical Reagent Co., Ltd.), trisodium citrate dihydrate (C6H5Na3O7⋅2H2O, Sinopharm Chemical Reagent Co., Ltd.), hexamethylenetetramine (C6H12N4, HMT, Sinopharm Chemical Reagent Co., Ltd.), terephthalic 2
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Scheme 1. Schematic diagram to show the fabrication procedure of the photocatalyst.
dark for 30 min to achieve the adsorption–desorption equilibrium. The aqueous solution was exposed to a 500 W Xenon lamp. At given time intervals, MB-adsorbed ZnO/C-700 was separated by centrifugation. MB adsorbed on ZnO/C-700 samples were eluted with 50 mL ethanol. The concentration of MB in ethanol eluent was measured using a Shimadzu UV-1800 UV–vis spectrophotometer.
equation: qt = (C0 – Ct)V/m
(1)
where C0 was the initial concentration and Ct was the concentration at time t of MB (mg L−1). V was the volume of MB solution (mL) and m was the mass of ZnO/C-600 (ZnO/C-700) (mg). The adsorption capacity of ZnO/C-600 (ZnO/C-700), qe (mg g−1) was calculated according to the following equation: qe = (C0 – Ce)V/m
3. Results and discussion
(2)
The key fabrication steps of the photocatalyst based on MOF-5 were shown in Scheme 1.
where C0 and Ce were the initial and equilibrium concentrations of MB (mg L−1), respectively. V was the volume of MB solution (mL) and m was the mass of ZnO/C-600 (ZnO/C-700) (mg).
3.1. XRD analysis Fig. 1a illustrated the XRD patterns of as-prepared precursor (ZnMOF), Zn-MOF-350, ZnO/C-500, ZnO/C-600, ZnO/C-700 and ZnO-700. As shown in Fig. 1a, the diffraction peaks of Zn-MOF and Zn-MOF-350 were in good agreement with the characteristic diffraction patterns of partially hydrolyzed MOF-5 (Huang et al., 2003; Lu et al., 2018). This suggested that the precursor didn’t decompose into ZnO when the calcination temperature was lower than 350 °C. For the XRD patterns of ZnO/C-500, ZnO/C-600, ZnO/C-700 and ZnO-700, all the peaks were assigned to standard hexagonal wurtzite ZnO crystal structure (JCPDS 36-1451). No other diffraction peaks of impurities were found. The expanded XRD patterns of ZnO/C-500, ZnO/C-600, ZnO/C-700 and ZnO-700 were exhibited in Fig. 1b. Compared with the three major diffraction peaks of the standard data (JCPDS, 36-1451), it can be seen that the peaks of ZnO/C-500, ZnO/C-600 and ZnO/C-700 shifted slightly toward lower angles. The lattice parameters calculated from XRD data were presented in Table 1. All results implied that carbon had been incorporated into the crystal lattice of ZnO (Hussain et al., 2019; Zhang et al., 2019a). Crystallite sizes of ZnO were calculated from the (002) diffraction peak using Scherrer formula, and were found to be 19.79, 20.52, 58.38 and 45.46 nm for ZnO/C-500, ZnO/C-600, ZnO/C700 and ZnO-700, respectively.
2.5. Photocatalysis experiment The photocatalysis experiment was performed in a BL-GHX-V photochemical reaction apparatus (Shanghai Bilon Instrument Co., Ltd.). A Xenon lamp of 500 W was used as simulated sunlight source. The reactor was provided with a circulating water system to maintain the temperature at 20 °C. In these experiments, 20 mg of the sample was dispersed into 50 mL of MB aqueous solution with concentration of 3.0 × 10−5 mol L−1 without adjusting pH. The suspension was magnetically stirred in dark for 30 min to achieve the adsorption–desorption equilibrium between the sample and the dye prior to sunlight irradiation. During the light irradiation, about 8 mL of the aqueous sample was taken out and centrifuged to remove the sample for analysis at given time intervals. The concentration of MB was measured using a Shimadzu UV-1800 UV–vis spectrophotometer at the maximum absorption wavelength of 664 nm. The decolorization efficiency (η) of sample to MB was calculated by the equation: η = (C0-Ct) / C0 × 100%
(3)
where C0 was the initial concentration and Ct was the concentration of dye at time t. Due to the outstanding adsorption performance of ZnO/C-700, an experiment was performed to further verify its photocatalytic performance. Equal mass of 20 mg of ZnO/C-700 was dispersed into 50 mL 3.0 × 10−5 mol L-1 MB aqueous solution and magnetically stirred in
3.2. TG-DTA analysis The thermal conversion of Zn-MOF was investigated by TG-DTG 3
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Fig. 3. FTIR spectra of Zn-MOF-350 (a), ZnO/C-500 (b), ZnO/C-600 (c), ZnO/ C-700 (d) and ZnO-700 (e).
(Fig. 2). The weight losses of Zn-MOF can be divided into three stages. The first weight loss (about 6.06%) below 150 °C with a DTG peak centered at about 64.7 °C could be attributed to the loss of surface water molecules. The second weight loss was from 250 to 390 °C and the weight loss was about 9.93%, which owing to the organic solvent (Kim et al., 2018). In the final stage, the significant weight loss began at about 390 °C, with a large DTG peak centered at about 467.9 °C. A weight loss of up to 36.95% was observed, which was due to the structural decomposition of Zn-MOF. The weight remained unchanged when the temperature was above 600 °C, which indicated that Zn-MOF had been decomposed completely into ZnO. The weight of the final residue was 47.06%, which showed extra 5.06% higher compared to the calculated 42% of final weight residue of ZnO (Zhang and Hu, 2010). This indicated that there were other components in the final residue, such as carbon residue (Zhang and Hu, 2010).
Fig. 1. (a) XRD patterns of as-prepared precursor and samples obtained under different calcination condition; (b) The expanded XRD patterns of ZnO/C-500, ZnO/C-600, ZnO/C-700 and ZnO-700 (the vertical lines from the standard ZnO datas (JCPDS, 36-1451)).
3.3. FT-IR analysis Table 1 The lattice parameters of ZnO/C-500, ZnO/C-600 and ZnO/C-700. Sample
JCPDS, 36-1451 ZnO/C-500 ZnO/C-600 ZnO/C-700
FTIR spectra of Zn-MOF-350, ZnO/C-500, ZnO/C-600, ZnO/C-700 and ZnO-700 were presented in Fig. 3. FTIR spectrum of Zn-MOF-350 (Fig. 3a) was consistent with that of the reported partially hydrolyzed MOF-5 (Huang et al., 2003; Lu et al., 2018). For ZnO/C-500 (Fig. 3b) and ZnO/C-600 (Fig. 3c), the broad absorption band at 3100–3700 cm−1 was due to the OeH stretching of water molecules adsorbed on the surface of ZnO. The observed peaks at 1568 and 1395 cm−1 (Fig. 3b–d) may correspond to the stretching vibration of the C]C bond and Zn-N bond (Kim et al., 2017; Lavand and Malghe, 2015). The broad peak centered at 1253 cm−1 (Fig. 3c) was associated with CeO stretching vibration (Sugiman et al., 2019). The broad peak centered at about 1120 cm−1 (Fig. 3d) was ascribed to CeC, CeO, and CeN stretching vibrations (Kim et al., 2017). This indicated the presences of C and N in the prepared ZnO samples. The bond at 876 cm−1 was assigned to the formation of tetrahedral coordination of Zn (Fig. 3b–e) (Jayarambabu et al., 2015). All the spectra of samples calcining above 500 °C showed strong absorption peaks below 600 cm−1, which corresponded to the characteristic absorption of ZneO bond in zinc oxide (Li et al., 2013).
Lattice parameters [Å] a=b
c
3.2498 3.2487 3.2496 3.2496
5.2066 5.2077 5.2060 5.2062
3.4. SEM analysis The morphologies of Zn-MOF, Zn-MOF-350, ZnO/C-500, ZnO/C600, ZnO/C-700 and ZnO-700 were revealed by FESEM (Fig. 4). The calcination temperature and atmosphere showed a great influence on the morphology and surface structure of Zn-MOF-350, ZnO/C-500,
Fig. 2. TG-DTG of the as-prepared Zn-MOF. 4
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Fig. 4. SEM images of Zn-MOF (a), Zn-MOF-350 (b), ZnO/C-500 (c, d), ZnO/C-600 (e, f), ZnO/C-700 (g, h) and ZnO-700 (i, j).
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respectively, while Zn 2p1/2 and Zn 2p3/2 peaks of ZnO/C-700 were located at 1044.5 eV and 1021.4 eV, respectively. The binding energy distances between Zn 2p1/2 and Zn 2p3/2 both were 23.1 eV for Zn-MOF and ZnO/C-700, which confirming the presence of Zn2+ ions (Hussain et al., 2018). O 1s spectra of Zn-MOF showed two different forms of oxygen (Fig. 6c). The peaks located at 531.8 eV and 533.1 eV can be ascribed to ZNneOeC bond and adsorbed hydroxyl or water and carbonate (CeO/ C]O) species, respectively (Hussain et al., 2019). O 1s spectra of ZnO/ C-700 can be deconvoluted into three peaks centered at 530.4 eV, 531.8 eV and 533.0 eV. The latter two peaks were at the same BE as Zn-MOF. The peak at 530.4 eV can be assigned to O2− ions in the ZneO bond of the wurtzite structure of ZnO (Hussain et al., 2019; Zhang et al., 2019b). In addition, the peak components of O and C were summarized in Table S2. The results showed that the peak area of ZneO bond in O 1s in ZnO/C-700 was the largest in three peaks. This confirmed that the main component of ZnO/C-700 was ZnO. The intensities of peaks related to carbon (531.8 eV and 533.0 eV) in ZnO/C-700 were reduced but not disappeared as compared with Zn-MOF, which indicated the presence of C in ZnO/C-700. C 1s spectra of Zn-MOF and ZnO/C-700 were depicted in Fig. 6d. C 1s spectra of Zn-MOF could be divided into three peaks at 284.8 eV, 286.5 eV and 288.7 eV, which were assigned to CeC bond, CeO bond and C]O bond, respectively (Liang et al., 2016). For ZnO/C-700, there were three peaks located at 283.9 eV, 285.1 eV and 288.2 eV. These peaks were attributed to ZneC bond, ZneOeC bond and C]O bond, respectively (Hussain et al., 2019; Liang et al., 2016). The dominant peak at 283.9 eV (Zn-C) in ZnO/C-700 indicated that C remained in the form of Zn-C in the residue after calcination at 700 °C. This may make the performance of ZnO/C-700 different from that of pure zinc oxide. The weak peaks of N 1s proved the presence of nitrogen and indicated that N was incorporated into the prepared ZnO/C-700 (Fig. 6e). N 1s spectra of ZnO/C-700 can be deconvoluted into three peaks centered at 397.8 eV, 399.9 eV and 401.7 eV, which were assigned to ZneN bond, OeZneN bond, and NeO bond, respectively (Zhang et al., 2019b).
ZnO/C-600, ZnO/C-700 and ZnO-700. As shown in Fig. 4a, Zn-MOF had a petaloid structure with willow-like aggregation, which also can be seen in magnified view (inset, in Fig. 4a). Although Zn-MOF-350, ZnO/ C-500, ZnO/C-600 and ZnO/C-700 all maintained the petaloid morphology, their surface structures varied (Fig. 4b–h), which structural representation had been described in Scheme 1. The surface of Zn-MOF350 showed a bit rough as compared with Zn-MOF. Small ZnO particles could be observed on the surface (inset in Fig. 4b). A layered porous structure was observed in the SEM image of ZnO/C-500 (Fig. 4c). The magnified view of ZnO/C-500 revealed that the layered porous structure was actually consisted of aggregates of little zinc oxide nanoparticles (< 120 nm) (Fig. 4d). The result indicated that the Zn-MOF began to break down when the calcination temperature was over 500 °C. The size of zinc oxide particle on the surface of petaloid morphological structure further increased with increasing calcination temperature. A combined amorphous and crystalline hierarchical structure could be found in the SEM images (Fig. 4e–h). The crystallined ZnO particles (0.5–2.5 μm) with polyhedral structure were observed in ZnO/C-700. As a comparison, the morphology of ZnO-700 (calcined in air) was also investigated using SEM (Fig. 4i, j). Although the petaloid morphology of Zn-MOF also remained in ZnO-700 like ZnO/C-700, the ZnO nanoparticles (40∼300 nm) in ZnO-700, in an irregular and agglomerated state, were smaller than those in ZnO/C700. The compositions of ZnO/C-600, ZnO/C-700 and ZnO-700 were investigated by energy dispersive spectroscopy (EDS) analysis. The results confirmed the presence of C, Zn and O in ZnO/C-600, ZnO/C-700 and ZnO-700 (Fig. S1). The elemental analysis for samples has also been carried out (Table S1). The results confirmed the composition of ZnMOF-350 didn't show significant change as compared with Zn-MOF. The obtain samples of thermal decomposition of Zn-MOF under nitrogen not less than 500 °C were ZnO/C composites. The C content in ZnO/C nanocomposite increased with the increase of calcination temperature under nitrogen (Table S1 and Fig. S1a, b). The sample ZnO700 obtained in air at 700 °C was mainly ZnO (Fig. S1c). The results showed that the calcination temperature and atmosphere had influence on the composition of samples. Combined with XRD and FESEM analyses, it can be concluded that carbon had been incorporated into the crystal lattice of zinc oxide during carbonization, and further changed the morphologies of samples. The EDS mapping (Fig. 5) also demonstrated the homogeneous distribution of Zn, O, C and N in ZnO/C-600 and ZnO/C-700.
3.6. UV–DRS analysis Fig. 7a showed the UV–vis diffuse reflectance spectra of Com-ZnO and samples obtained at different calcination temperatures. It can be seen that all the samples exhibited a broad absorption in UV region as well as Com-ZnO. Moreover, compared to Com-ZnO, Zn-MOF-350 still displayed an additional tailing absorption in the visible region. ZnO/C500 showed a distinct tailing absorption covering the whole visible region. ZnO/C-600 and ZnO/C-700 showed absorption in the whole range of 200∼800 nm. Such an enhanced absorption capacity will be beneficial for photocatalytic application. The direct band gap energies determined by extrapolating the linear part of plot of (αhν)2 versus the energy (hν) was depicted in Fig. 7b (Kumar et al., 2017). The calculated
3.5. XPS analysis The elemental compositions and chemical states of Zn-MOF and ZnO/C-700 were investigated by XPS (Fig. 6). The elemental survey (Fig. 6a) showed that Zn-MOF and ZnO/C-700 mainly consisted of Zn, C and O elements. In the Zn 2p spectra (Fig. 6b), the Zn 2p1/2 and Zn 2p3/ 2 peaks of Zn-MOF were located at 1045.3 eV and 1022.2 eV,
Fig. 5. (a)-(e) EDS mapping of ZnO/C-600; (f)-(j) EDS mapping of ZnO/C-700. 6
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Fig. 6. XPS spectra of Zn-MOF and ZnO/C-700: (a) elemental survey, (b) Zn 2p, (c) O 1s, (d) C 1s, and (e) N 1s of ZnO/C-700.
band gap values (Eg) were 3.21, 2.87, 3.18, 3.02 and 2.82 eV for ComZnO, Zn-MOF-350, ZnO/C-500, ZnO/C-600 and ZnO/C-700, respectively. As compared to Com-ZnO, band gaps of ZnO/C-500, ZnO/C-600 and ZnO/C-700 were reduced due to the doping of C and N. The results implied that carbon and nitrogen were incorporated into the crystal structure of ZnO than just covered on the surface of ZnO (Zhang et al., 2019a; Wu, 2014). In addition, the contribution of carbon and nitrogen to the valence band (VB) of prepared ZnO took the major role in enhancing light harvesting and utilizing visible light (Scheme 2) (Zhang et al., 2019a; Lavand and Malghe, 2015).
ZnO/C-500 located at ∼387 nm and ZnO/C-600 and ZnO/C-700 located at ∼378 nm were attributed to the excitonic emission. As compared with Zn-MOF-350, the quenching of the PL intensity of ZnO/C500, ZnO/C-600 and ZnO/C-700 were ascribed to the reduction in recombination of photogenerated electrons and holes (Rajbongshi et al., 2014). The reduction in recombination of photogenerated electrons and holes was beneficial to photocatalytic activity. This also suggested that ZnO/C-600 and ZnO/C-700 should have high photocatalytic activity.
3.7. PL analysis
The nitrogen adsorption/desorption isotherms of the obtained samples were exhibited in Fig. 8. The surface area, pore volume and pore size of the samples calculated from the adsorption isotherms were presented in Table 2. All the samples displayed the type IV curve
3.8. BET analysis
The PL spectra of the samples obtained at different calcination temperatures were shown in Fig. S2. The peaks of Zn-MOF-350 and 7
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could be due to the decomposition of the organic framework of Zn-MOF and the release of volatile gases such as CO2 and H2O. It can be seen that the BET surface area of Zn-MOF (1.10 m2⋅ g−1) was very small. This may be due to the fact that the prepared Zn-MOF was a partially hydrolyzed MOF-5 (Sabo et al., 2007). Both the calculated BET surface areas and the total pore volumes of Zn-MOF-350, ZnO/C-500, ZnO/C600 and ZnO/C-700 increased first and then decreased with the increase of calcination temperature. ZnO/C-600 showed the largest specific surface area and pore volume. This could be due to the collapse of the framework of Zn-MOF and the formation of ZnO with polyhedral structure over 600 °C. Moreover, the collapse rate of framework increased with the increase of calcination temperature, which further resulted in the reduction in the specific surface area of ZnO/C-700. 3.9. Adsorption capacity The adsorption capacities of ZnO/C-600 and ZnO/C-700 were measured using equilibrium adsorption isotherms with different initial concentrations of MB as shown in Fig. 9a. The as-obtained experimental data were analyzed using Langmuir (Fig. 9b), Freundlich (Fig. 9c) and Temkin (Fig. 9d) adsorption isotherm models (Malwal and Gopinath, 2017) and the results were listed in Table 3. The results indicated that the adsorption process for MB removal using ZnO/C-600 (ZnO/C-700) was fitted with Langmuir model than Freundlich and Temkin models. The theoretical maximum capacities of ZnO/C-600 and ZnO/C-700 from Langmuir adsorption model were 106.4 mg g−1 and 100.0 mg g−1, respectively. The kinetics of adsorption was studied to evaluate the adsorption process and efficiency of the adsorbent. Fig. 10a depicted the adsorption of MB using ZnO/C-600 and ZnO/C-700 with respect to time. It was performed by adding 20 mg of ZnO/C-600 (ZnO/C-700) in 50 mL of MB aqueous solution with a concentration of 150 mg⋅L−1. It can be seen that the adsorption increased fast initially and then reached the adsorption equilibrium slowly. To evaluate the adsorption efficiency of ZnO/C-600 (ZnO/C-700), the pseudo-second-order kinetics were chosen (Malwal and Gopinath, 2017).
Fig. 7. (a) UV–vis diffuse reflectance spectra and (b) plots of (αhν)2 versus the energy (hν) of the Com-ZnO and samples obtained at different calcination temperatures.
t 1 t = + qt qe k2 qe2
accompanied by a type H3 hysteresis loop, which was attributed to the predominance of mesopores according to the IUPAC classification (Kruk and Jaroniec, 2001). The pore size distributions were given in the inset of Fig. 8. The presence of micropores and mesopores in the samples
where qe (mg g−1) was the equilibrium adsorption capacity and qt was the amount of MB adsorbed onto ZnO/C-600 (ZnO/C-700) at a
Scheme 2. Possible schematic diagram for ZnO/C-700 photocatalytic degradation of MB under sunlight irradiation. 8
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Fig. 8. Nitrogen adsorption/desorption isotherms of obtained Zn-MOF (a), Zn-MOF-350 (b), ZnO/C-500 (c), ZnO/C-600 (d) and ZnO/C-700 (e). Inset: the corresponding BJH (Barret–Joyner–Halenda) pore size distribution.
particular time t, and k2 (g mg−1 h−1) was the pseudo-second-order rate constant of adsorption. The linear plot between t vs. t/qt was shown in Fig. 10b. The calculated values of k2 and qe were 0.0021 g mg−1 h−1 and 107.5 mg g−1 for ZnO/C-600 and 0.0025 g mg−1 h−1 and 100 mg g−1 for ZnO/C-700, respectively. The correlation coefficient (R2) values for pseudo-second-order kinetics were 0.998 and 0.999 for ZnO/C-600 and ZnO/C-700, respectively. It indicated that the pseudo-second-order kinetics were more suitable for the adsorption process.
Table 2 The surface area, pore volume and pore size of the samples. Sample
SBETa (m2 g−1)
Pore volumeb (cm3 g−1)
Pore sizec (nm)
Zn-MOF Zn-MOF-350 ZnO/C-500 ZnO/C-600 ZnO/C-700
1.10 1.80 63.41 197.21 134.79
0.007 0.008 0.146 0.324 0.239
23.99 17.35 9.22 6.57 7.08
a The BET surface area was calculated from the linear part of the BET plot (P/ P0 = 0.07–0.3). b Pore volume was determined by adsorption branch of the nitrogen isotherms at P/P0 = 0.97. c Average pore size was estimated using the adsorption branch of the nitrogen isotherms and the Barrett-Joyner-Halenda method.
3.10. Photocatalytic activity Methylene blue (MB) was used as a target pollutant to evaluate the photocatalytic activities of the as-synthesized samples under sunlight irradiation. As a contrast, the degradation of MB by Com-ZnO and the control (without catalyst) were also carried out. The UV–vis absorption spectra of MB aqueous solution after different time irradiation in the 9
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Table 3 Regression equations and calculated parameters for three adsorption isotherm models. Isotherm models
Sample
Regression equation acquired from the graph
Parameters
Langmuir
ZnO/C-600
Ce/qe = 0.0508 + 0.0094Ce
ZnO/C-700
Ce/qe = 0.0596 + 0.0100Ce
ZnO/C-600
ln qe = 3.7956 + 0.1700ln Ce
ZnO/C-700
ln qe = 3.6011 + 0.1996ln Ce
ZnO/C-600
qe = 55.227 + 9.142ln Ce
ZnO/C-700
qe = 43.230 + 10.357ln Ce
KL = 0.185 qm = 106.4 R2 = 0.992 KL = 0.168 qm = 100.0 R2 = 0.994 KF = 44.505 n = 5.882 R2 = 0.991 KF = 36.639 n = 5.010 R2 = 0.992 A = 420.321 B = 9.142 R2 = 0.935 A = 64.974 B = 10.357 R2 = 0.953
Freundlich
Temkin
Fig. 10. (a) Effect of time on the adsorption of MB onto ZnO/C-600 and ZnO/C700, (b) pseudo second-order kinetics plot for MB adsorption using ZnO/C-600 and ZnO/C-700.
presence of catalysts were depicted in Fig. 11a–f. It can be seen that the intensity of maximum absorption peak of MB at 664 nm decreases dramatically after stirring for 30 min in dark in the presence of ZnO/C500 (Fig. 11c), ZnO/C-600 (Fig. 11d) and ZnO/C-700 (Fig. 11e), especially for ZnO/C-600 and ZnO/C-700. It was due to the large specific surface area and the resulted higher adsorption capacity of ZnO/C-
Fig. 9. (a) Adsorption isotherms, (b) Langmuir plots, (c) Freundlich plots and (d) Temkin plots of MB adsorption on ZnO/C-600 and ZnO/C-700 for 120 min.
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Fig. 11. UV–vis absorption spectra of MB aqueous solution after different time irradiation in the presence of Com-ZnO (a), Zn-MOF-350 (b), ZnO/C-500 (c), ZnO/C600 (d), ZnO/C-700 (e) and ZnO-700 (f), and the insets were the corresponding photographs (d and e); (g) Relationship between C/C0 and reaction time for MB decomposition, where C and C0 were the actual concentration and the initial concentration of MB, respectively.
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Fig. 12. (a) Reusability experiment for the photocatalytic degradation of MB using the as-synthesized ZnO/C-700 sample; XRD patterns (b) and FTIR (c) of ZnO/C700 before and after five cycles; low (d) and high (e) magnification SEM of ZnO/C-700 after five cycles.
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4. Conclusions
500, ZnO/C-600 and ZnO/C-700. Moreover, MB aqueous solution was degraded in the presence of ZnO/C-600 or ZnO/C-700 from blue to colorless (inset photographs in Fig. 11d–e). However, for ZnO/C-500, the absorption of MB at 664 nm decreased slowly under sunlight irradiation (Fig. 11c). This different catalytic phenomenon was due to the higher crystallinity of ZnO/C-600 and ZnO/C-700, which reduced the recombination of electron-hole. The relationship between C/C0 (C0 and C were the initial concentration and the actual concentration of MB) and reaction time was also investigated (Fig. 11g). In the absence of catalyst, MB solution remained unchanged under sunlight irradiation, which meant MB was very stable and didn't degrade under sunlight irradiation. It can be clearly seen that Zn-MOF-350 had slight effect on degradation of MB. The direct adsorption of ZnO/C-500, ZnO/C-600 and ZnO/C-700 played a great role in the degradation of MB due to their larger specific surface area before sunlight irradiation. The final decolorization efficiencies of MB in the presence of Com-ZnO, Zn-MOF-350, ZnO/C-500, ZnO/C-600, ZnO/C-700 and ZnO-700 after 100 min of irradiation were 78.2%, 13.5%, 73.4%, ∼100%, ∼100% and 71%, respectively. Actually, the decolorization of MB from water using ZnO/C-700 was completed after 60 min of irradiation. Since ZnO/C-700 showed a high adsorption performance, the photocatalytic performance of ZnO/C-700 was further certified by the elution of MB with ethanol from ZnO/C-700 during photo-degradation. The results showed that the content of MB adsorbed on ZnO/C-700 gradually and quickly decreased with increasing of sunlight irradiation time (Fig. S3). This confirmed the photocatalytic activity of ZnO/C-700. To evaluate the photochemical stability of ZnO/C-700, the reusability experiment for the photocatalytic degradation of MB was performed (Fig. 12a). After each cycle, the photocatalyst was separated from the suspension by centrifugation, washed with absolute ethanol and deionized water, then dried for a new cycle. The results showed that the photocatalytic activity of ZnO/C-700 exhibited no significant decrease during the repeated five cycles. Furthermore, ZnO/C-700 after the fifth cycle was also characterized by XRD, FTIR and SEM (Fig. 12b–e). The XRD pattern, FTIR spectrum and morphology of the reused ZnO/C-700 showed no significant changes as compared with the new prepared ZnO/C-700. All the results indicated that ZnO/C-700 was highly stable and reusable, and beneficial for its application as a photocatalyst. The possible schematic diagram for ZnO/C-700 photocatalytic degradation of MB under sunlight irradiation was shown in Scheme 2. Similar to pure ZnO, the photocatalysis degradation of MB by ZnO/C700 also based on the photo-generated electron-hole (e−/h+) pairs, which generated free radicals to oxidize MB into neutral species (Hussain et al., 2018,). On this basis, the presence of carbon (including doped carbon and carbon matrix) in ZnO/C-700 composite further improved the degradation of MB. The black carbon matrix enhanced visible light absorption capacity and further improved the efficiency of photo-irradiation. As compared with pure ZnO, the band gap of ZnO in ZnO/C-700 was narrowed by the doped C (also N). This benefited the produce of photo-generated electron-hole (e−/h+) pairs under sunlight irradiation (Pan et al., 2016). The carbon matrix also reduced the recombination of photo-generated electron-hole by effectively transferring and receiving the photo-generated electrons from conduction band of ZnO (Hussain et al., 2018,). Moreover, the large specific surface area of ZnO/C-700 composite facilitated the reaction between photo-generated electrons (or holes) and dissolved molecular oxygen (or solvent H2O) to produce various reactive oxygen species. The large specific surface area of ZnO/C-700 could also increase the concentration of MB around the photocatalytic active sites in ZnO/C-700 by effective adsorption. As a result, the degradation efficiency of MB by ZnO/C-700 was improved.
In summary, ZnO/C nanocomposite had been prepared by annealing partially hydrolyzed MOF-5 at different temperatures in nitrogen. The obtained ZnO/C-600 and ZnO/C-700 retained the petaloid morphology with aggregates of ZnO nanoparticles and porous structure. The results indicated that ZnO/C-600 and ZnO/C-700 exhibited outstanding adsorption and photocatalytic performance on MB degradation under sunlight irradiation. The possible photocatalytic mechanism of ZnO/C700 for degradation of MB under sunlight irradiation was proposed. This work provided a new insight into the design and synthesis of highly efficient photocatalysts for organic dyes containing wastewater treatment in environmental applications. Declaration of Competing Interests None. Acknowledgements This work was supported by the Fundamental Research Funds for the Central Universities (No. 2232013A3-05) and the National Science and Technology Ministry (ID 2012BAK30B03). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jhazmat.2019.121599. References Cheng, S., Liu, S., Zhao, Q., Li, J., 2009. Improved synthesis and hydrogen storage of a microporous metal–organic framework material. Energy Convers. Manage. 50, 1314–1317. Cui, W.G., Zhang, G.Y., Hu, T.L., Bu, X.H., 2019. Metal-organic framework-based heterogeneous catalysts for the conversion of C1 chemistry: CO, CO2 and CH4. Coord. Chem. Rev. 387, 79–120. Dekrafft, K.E., Wang, C., Lin, W., 2012. Metal-organic framework templated synthesis of Fe2O3/TiO2 nanocomposite for hydrogen production. Adv. Mater. 24, 2014–2018. Dong, S., Zhang, D., Cui, H., Huang, T., 2019. ZnO/porous carbon composite from a mixed-ligand MOF for ultrasensitive electrochemical immunosensing of C-reactive protein. Sens. Actuators B Chem. 284, 354–361. Gao, S., Fan, R., Li, B., Qiang, L., Yang, Y., 2016. Porous carbon-coated ZnO nanoparticles derived from low carbon content formic acid-based Zn(II) metal-organic frameworks towards long cycle lithium-ion anode material. Electrochim. Acta 215, 171–178. Hafizovic, J., Bjørgen, M., Olsbye, U., Dietzel, P.D., Bordiga, S., Prestipino, C., Lamberti, C., Lillerud, K.P., 2007. The inconsistency in adsorption properties and powder XRD data of MOF-5 is rationalized by framework interpenetration and the presence of organic and inorganic species in the nanocavities. J. Am. Chem. Soc. 129, 3612–3620. Hou, X., Stanley, S.L., Zhao, M., Zhang, J., Zhou, H., Cai, Y., Huang, F., Wei, Q., 2019. MOF-based C-doped coupled TiO2/ZnO nanofibrous membrane with crossed network connection for enhanced photocatalytic activity. J. Alloys Compd. 777, 982–990. Hu, C., Lu, L., Zhu, Y., Li, R., Xing, Y., 2018. Morphological controlled preparation and photocatalytic activity of zinc oxide. Mater. Chem. Phys. 217, 182–191. Huang, L., Wang, H., Chen, J., Wang, Z., Sun, J., Zhao, D., Yan, Y., 2003. Synthesis, morphology control, and properties of porous metal–organic coordination polymers. Microporous Mesoporous Mater. 58, 105–114. Hussain, M.Z., Schneemann, A., Fischer, R.A., Zhu, Y., Xia, Y., 2018. MOF derived porous ZnO/C nanocomposites for efficient dye photodegradation. ACS Appl. Energy Mater. 1, 4695–4707. Hussain, M.Z., Pawar, G.S., Huang, Z., Tahir, A.A., Fischer, R.A., Zhu, Y., Xia, Y., 2019. Porous ZnO/carbon nanocomposites derived from metal organic frameworks for highly efficient photocatalytic applications: a correlational study. Carbon 146, 348–363. Jayarambabu, N., KumarI, B.S., Rao, K.V., Prabhu, Y., 2015. Beneficial role of zinc oxide nanoparticles on green crop production. Int. J. Multidiscip. Adv. Res. Trends 2, 273–282. Jiang, H.L., Liu, B., Lan, Y.Q., Kuratani, K., Akita, T., Shioyama, H., Zong, F., Xu, Q., 2011. From metal–organic framework to nanoporous carbon: toward a very high surface area and hydrogen uptake. J. Am. Chem. Soc. 133, 11854–11857. Jiao, W., Zhang, L., 2018. Fabrication of new C/ZnO/ZnO composite material and their enhanced gas sensing properties. Mater. Sci. Semicond. Process. 86, 63–68. Joharian, M., Morsali, A., Tehrani, A.A., Carlucci, L., Proserpio, D.M., 2018. Water-stable fluorinated metal–organic frameworks (F-MOFs) with hydrophobic properties as efficient and highly active heterogeneous catalysts in aqueous solution. Green Chem.
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MOF derived ZnO/C nanocomposite with enhanced adsorption capacity and photocatalytic performance under sunlight Cuicui Hua, Xiaoxia Hub, Rong Lic, Yanjun Xinga,* a
Key Laboratory of Science and Technology of Eco-Textiles, Ministry of Education, College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, Shanghai 201620, China b Division of Science and Technology, Shanghai Urban Construction Vocational College, Shanghai 201415, China c National Engineering Research Center for Dyeing and Finishing of Textiles, Donghua University, Shanghai 201620, China
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Zinc-based MOF ZnO/C nanocomposite Adsorption Photocatalysis
In this work, ZnO/C nanocomposites were obtained by calcining the prepared metal-organic framework precursor under nitrogen. The crystallinity and structure of the prepared products were characterized by XRD, FTIR, XPS and EDS. The morphologies of samples before and after calcination were observed by FESEM. The photocatalytic performances of ZnO/C were evaluated by the degradation of methylene blue under sunlight irradiation. Combined with DRS, PL and BET, the influence of calcination temperature on photocatalytic activities of assynthesized zinc oxide were discussed as compared with commercial zinc oxide. The results indicated that ZnO/ C composite obtained at 600 °C and 700 °C exhibited the superior adsorption capacity and photocatalytic activity. The possible photocatalytic mechanism of ZnO/C nanocomposite for degradation of MB under sunlight irradiation was proposed.
1. Introduction In the past decades, the serious water pollution due to organic dyes and pigments affected the earth's environment, animal and human health. Photocatalysis based on semiconductor as a promising and ecofriendly treatment approach for organic contaminants which does not reproduce any secondary pollution has drawn more and more attention
⁎
(Hou et al., 2019). Among them, ZnO has received significant scientific interest due to its high electron mobility, high chemical stability, environmental friendliness and low cost (Ong et al., 2018; Sharma et al., 2019; Vaiano et al., 2018; Wang et al., 2019; Hu et al., 2018). Due to the low visible light utilization and low photocatalytic quantum efficiency on the applications for ZnO, ZnO composites, such as ZnO/C (Jiao and Zhang, 2018; Yan et al., 2019; Wang et al., 2018), have been
Corresponding author. E-mail address: [email protected] (Y. Xing).
https://doi.org/10.1016/j.jhazmat.2019.121599 Received 8 August 2019; Received in revised form 24 October 2019; Accepted 2 November 2019 0304-3894/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Cuicui Hu, et al., Journal of Hazardous Materials, https://doi.org/10.1016/j.jhazmat.2019.121599
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acid (H2BDC, Shanghai Titan Scientific Co., Ltd.), N,N’-dimethylformamide (DMF, Shanghai Lingfeng Chemical Reagent Co., Ltd., ≥99.5%), ethanol (Changshu Yangyuan Chemical Co., Ltd, ≥99.5%), commercial pure ZnO (denoted as Com-ZnO, Sinopharm Chemical Reagent Co., Ltd.), and methylene blue (MB, Shanghai Reagent Factory). All the reagents were of analytical grade and used without further modification.
used to improve the properties of ZnO. However, it is a problem for the poor dispersion and heterogeneity of the hybrid material (Hussain et al., 2018). Metal organic frameworks (MOFs) as hybrid organic-inorganic compounds, have drawn much attention in various applications such as sensing (Dong et al., 2019), gas sorption and separation (Lin et al., 2019) and catalysis (Joharian et al., 2018; Yi et al., 2019; Sun et al., 2018; Cui et al., 2019) due to high surface area and tunable structure (ZareKarizi et al., 2018). Recently, MOF-5 has been used as sacrificial precursor to obtain ZnO/C composites with large specific surface area and different porous nano/micro structures by pyrolysis or thermolysis method (Dekrafft et al., 2012; Yang et al., 2011; Jiang et al., 2011; Shi et al., 2017; Wu et al., 2019a,b). The ZnO nanoparticle in ZnO/C composites derived from MOFs was reported to have a homogeneous distribution in a highly porous carbon matrix (Hussain et al., 2019). Meanwhile, amorphous carbon formed from the carbonization of organic linker during pyrolysis process could also diffuse into the crystal lattice of ZnO, which can adjust the energy band gap of ZnO (Hussain et al., 2018; Pan et al., 2016). The form and content of carbon in ZnO/C composites generated during carbonization are related to the structure of the MOF precursor, which can greatly influence the application of ZnO/C (Hussain et al., 2018; Pan et al., 2016). MOF-5 has also been investigated to obtain different predictable nano/microstructures by controlling the species of surfactant, solvent and other synthesis condition (Li et al., 1999; Cheng et al., 2009; Lu et al., 2010; Tranchemontagne et al., 2008). The structure (Zhang and Hu, 2011; Hafizovic et al., 2007; Huang et al., 2003), pore sizes (Spokoyny et al., 2010) and synthesis methods (Huang et al., 2003; Yaghi et al., 2003) of MOF-5 (or Zn4O(BDC)3, BDC2− = 1,4-benzenedicarboxylate) should play a decisive role in the final shape and the pore size of the pyrolysis or thermolysis product (Yan et al., 2019; Gao et al., 2016; Li et al., 2018). These different factors of MOF-5 will be crucial for not only inorganic hybrid nanostructure features of the pyrolysis or thermolysis product, such as porosity, pore sizes and fascinating structures, but also their final promising applications (Hussain et al., 2019; Mai et al., 2017). To the best of our knowledge, there is few report about the photocatalytic activity and adsorption capacity of ZnO/C composite derived from cubic or octahedral MOF-5 (Hussain et al., 2018; Yang et al., 2011; Zhang et al., 2019a). The ZnO/C composite derived from other MOF-5 with different nanostructure caused by different synthesis condition (Zhang and Hu, 2011; Hafizovic et al., 2007; Huang et al., 2003), especially partially hydrolyzed MOF-5 (Huang et al., 2003), was also not been reported. Herein, a novel and simple strategy to prepare a hierarchical ZnO/C nanocomposite was reported by calcining partially hydrolyzed MOF-5 (Huang et al., 2003) at different temperatures under nitrogen. The influence of calcination temperature and calcination atmosphere on the morphology, adsorption and photocatalytic performance of prepared ZnO/C nanocomposite was investigated. The MOF-derived ZnO/C nanocomposite displayed excellent adsorption capacity comparable to the reported ZnO/C composites (Hussain et al., 2018; Yang et al., 2011) and ZnO/C prepared by calcining partially hydrolyzed MOF-5 under air. This method could be broadly applied to the synthesis of other metal oxide/carbon materials with low carbon content, which significantly extends its use to the energy and electronic related applications (Gao et al., 2016; Zhong et al., 2018).
2.2. Synthesis of ZnO/C nanocomposite In a typical synthesis, zinc nitrate hexahydrate (4 mmol), trisodium citrate dihydrate (2 mmol), and hexamethylenetetramine (2 mmol) were dissolved in distilled water (80 mL) and stirred continuously for 20 min to form a clear solution at room temperature. Terephthalic acid (1 mmol) in 20 mL of DMF was gradually added to the above solution under magnetic stirring. The resulted mixture was stirred vigorously at 85 °C for 1 h. The white product (named as Zn-MOF) was washed thoroughly with absolute ethanol and distilled water for several times and freeze dried after centrifugation at room temperature. Finally, the obtained white powder was calcined for 2 h under nitrogen at 350, 500, 600 and 700 °C, respectively. The heating ramp rate during the sintering process was kept at 5 °C min−1. The corresponding products were designated as Zn-MOF-350, ZnO/C-500, ZnO/C-600 and ZnO/C-700, respectively. The procedure was shown in Scheme 1. As a comparison, the obtained white powder was also calcined at 700 °C for 2 h in air. The corresponding product was designated as ZnO-700. 2.3. Characterization The crystallinity and structure of the prepared products were characterized by X-ray diffraction (XRD, Rigaku D/max-2550 PC, Japan) with Cu Kα radiation (λ = 1.5406 Å; scanning speed: 0.02° s−1). Thermogravimetric (TG) and differential thermal analysis (DTA) were performed by Germany NETZSCH TG209FI instrument with a rate of 10 °C min-1 under nitrogen condition. The chemical structures were studied using FT-IR spectrophotometer (Avatar 380, Thermo Scientific) in the range 4000–400 cm−1. X-ray photoelectron spectroscopy (XPS) was conducted on an ESCALAB 250Xi spectrometer (Thermo Fisher Scientific, USA) with Al Kα (1486.6 eV) X-ray source. The morphology, elemental composition and elemental mapping were observed by field emission scanning electron microscopy (FE-SEM, S-4800 HITACHI, Japan) equipped with energy dispersive X-ray spectroscopy (EDS). Elemental composition was studied by Vario EL III element analyzer (Germany, Elmentar). UV–vis diffusion reflection spectra (UV-DRS) were analyzed by using Shimadzu UV-3600 Plus UV-VIS-NIR spectrophotometer in the wavelength range from 200 to 800 nm. Photoluminescence (PL) spectra were taken on a fluorescence spectrometer (PTI QM, USA) at room temperature following an excitation wavelength of 325 nm. The special surface area and pore structure were calculated using the Brunauer–Emmett–Teller (BET) method with Micromeritics TriStar II 3020 Surface Area and Porosimetry Analyzer. 2.4. Adsorption experiment The adsorption of ZnO/C-600 and ZnO/C-700 were performed in dark condition at room temperature. 50 mL of MB aqueous solution with different initial concentrations in the range of 10–200 mg L−1 (2.67 × 10−5–5.35 × 10−4 mol L−1) were prepared. Equal mass of 20 mg of ZnO/C-600 (or ZnO/C-700) was added to the above solution and stirred in dark for 120 min to achieve equilibrium. At selected time intervals, 5 mL aliquot was taken out and centrifuged to remove the sample for analysis. The concentration of MB was measured using a Shimadzu UV-1800 UV–vis spectrophotometer at the maximum absorption wavelength of 664 nm. The adsorption amount of ZnO/C-600 (ZnO/C-700) at different time intervals, qt (mg g−1) was calculated according to the following
2. Experimental 2.1. Materials Zinc nitrate hexahydrate (Zn(NO3)2⋅6H2O, Sinopharm Chemical Reagent Co., Ltd.), trisodium citrate dihydrate (C6H5Na3O7⋅2H2O, Sinopharm Chemical Reagent Co., Ltd.), hexamethylenetetramine (C6H12N4, HMT, Sinopharm Chemical Reagent Co., Ltd.), terephthalic 2
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Scheme 1. Schematic diagram to show the fabrication procedure of the photocatalyst.
dark for 30 min to achieve the adsorption–desorption equilibrium. The aqueous solution was exposed to a 500 W Xenon lamp. At given time intervals, MB-adsorbed ZnO/C-700 was separated by centrifugation. MB adsorbed on ZnO/C-700 samples were eluted with 50 mL ethanol. The concentration of MB in ethanol eluent was measured using a Shimadzu UV-1800 UV–vis spectrophotometer.
equation: qt = (C0 – Ct)V/m
(1)
where C0 was the initial concentration and Ct was the concentration at time t of MB (mg L−1). V was the volume of MB solution (mL) and m was the mass of ZnO/C-600 (ZnO/C-700) (mg). The adsorption capacity of ZnO/C-600 (ZnO/C-700), qe (mg g−1) was calculated according to the following equation: qe = (C0 – Ce)V/m
3. Results and discussion
(2)
The key fabrication steps of the photocatalyst based on MOF-5 were shown in Scheme 1.
where C0 and Ce were the initial and equilibrium concentrations of MB (mg L−1), respectively. V was the volume of MB solution (mL) and m was the mass of ZnO/C-600 (ZnO/C-700) (mg).
3.1. XRD analysis Fig. 1a illustrated the XRD patterns of as-prepared precursor (ZnMOF), Zn-MOF-350, ZnO/C-500, ZnO/C-600, ZnO/C-700 and ZnO-700. As shown in Fig. 1a, the diffraction peaks of Zn-MOF and Zn-MOF-350 were in good agreement with the characteristic diffraction patterns of partially hydrolyzed MOF-5 (Huang et al., 2003; Lu et al., 2018). This suggested that the precursor didn’t decompose into ZnO when the calcination temperature was lower than 350 °C. For the XRD patterns of ZnO/C-500, ZnO/C-600, ZnO/C-700 and ZnO-700, all the peaks were assigned to standard hexagonal wurtzite ZnO crystal structure (JCPDS 36-1451). No other diffraction peaks of impurities were found. The expanded XRD patterns of ZnO/C-500, ZnO/C-600, ZnO/C-700 and ZnO-700 were exhibited in Fig. 1b. Compared with the three major diffraction peaks of the standard data (JCPDS, 36-1451), it can be seen that the peaks of ZnO/C-500, ZnO/C-600 and ZnO/C-700 shifted slightly toward lower angles. The lattice parameters calculated from XRD data were presented in Table 1. All results implied that carbon had been incorporated into the crystal lattice of ZnO (Hussain et al., 2019; Zhang et al., 2019a). Crystallite sizes of ZnO were calculated from the (002) diffraction peak using Scherrer formula, and were found to be 19.79, 20.52, 58.38 and 45.46 nm for ZnO/C-500, ZnO/C-600, ZnO/C700 and ZnO-700, respectively.
2.5. Photocatalysis experiment The photocatalysis experiment was performed in a BL-GHX-V photochemical reaction apparatus (Shanghai Bilon Instrument Co., Ltd.). A Xenon lamp of 500 W was used as simulated sunlight source. The reactor was provided with a circulating water system to maintain the temperature at 20 °C. In these experiments, 20 mg of the sample was dispersed into 50 mL of MB aqueous solution with concentration of 3.0 × 10−5 mol L−1 without adjusting pH. The suspension was magnetically stirred in dark for 30 min to achieve the adsorption–desorption equilibrium between the sample and the dye prior to sunlight irradiation. During the light irradiation, about 8 mL of the aqueous sample was taken out and centrifuged to remove the sample for analysis at given time intervals. The concentration of MB was measured using a Shimadzu UV-1800 UV–vis spectrophotometer at the maximum absorption wavelength of 664 nm. The decolorization efficiency (η) of sample to MB was calculated by the equation: η = (C0-Ct) / C0 × 100%
(3)
where C0 was the initial concentration and Ct was the concentration of dye at time t. Due to the outstanding adsorption performance of ZnO/C-700, an experiment was performed to further verify its photocatalytic performance. Equal mass of 20 mg of ZnO/C-700 was dispersed into 50 mL 3.0 × 10−5 mol L-1 MB aqueous solution and magnetically stirred in
3.2. TG-DTA analysis The thermal conversion of Zn-MOF was investigated by TG-DTG 3
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Fig. 3. FTIR spectra of Zn-MOF-350 (a), ZnO/C-500 (b), ZnO/C-600 (c), ZnO/ C-700 (d) and ZnO-700 (e).
(Fig. 2). The weight losses of Zn-MOF can be divided into three stages. The first weight loss (about 6.06%) below 150 °C with a DTG peak centered at about 64.7 °C could be attributed to the loss of surface water molecules. The second weight loss was from 250 to 390 °C and the weight loss was about 9.93%, which owing to the organic solvent (Kim et al., 2018). In the final stage, the significant weight loss began at about 390 °C, with a large DTG peak centered at about 467.9 °C. A weight loss of up to 36.95% was observed, which was due to the structural decomposition of Zn-MOF. The weight remained unchanged when the temperature was above 600 °C, which indicated that Zn-MOF had been decomposed completely into ZnO. The weight of the final residue was 47.06%, which showed extra 5.06% higher compared to the calculated 42% of final weight residue of ZnO (Zhang and Hu, 2010). This indicated that there were other components in the final residue, such as carbon residue (Zhang and Hu, 2010).
Fig. 1. (a) XRD patterns of as-prepared precursor and samples obtained under different calcination condition; (b) The expanded XRD patterns of ZnO/C-500, ZnO/C-600, ZnO/C-700 and ZnO-700 (the vertical lines from the standard ZnO datas (JCPDS, 36-1451)).
3.3. FT-IR analysis Table 1 The lattice parameters of ZnO/C-500, ZnO/C-600 and ZnO/C-700. Sample
JCPDS, 36-1451 ZnO/C-500 ZnO/C-600 ZnO/C-700
FTIR spectra of Zn-MOF-350, ZnO/C-500, ZnO/C-600, ZnO/C-700 and ZnO-700 were presented in Fig. 3. FTIR spectrum of Zn-MOF-350 (Fig. 3a) was consistent with that of the reported partially hydrolyzed MOF-5 (Huang et al., 2003; Lu et al., 2018). For ZnO/C-500 (Fig. 3b) and ZnO/C-600 (Fig. 3c), the broad absorption band at 3100–3700 cm−1 was due to the OeH stretching of water molecules adsorbed on the surface of ZnO. The observed peaks at 1568 and 1395 cm−1 (Fig. 3b–d) may correspond to the stretching vibration of the C]C bond and Zn-N bond (Kim et al., 2017; Lavand and Malghe, 2015). The broad peak centered at 1253 cm−1 (Fig. 3c) was associated with CeO stretching vibration (Sugiman et al., 2019). The broad peak centered at about 1120 cm−1 (Fig. 3d) was ascribed to CeC, CeO, and CeN stretching vibrations (Kim et al., 2017). This indicated the presences of C and N in the prepared ZnO samples. The bond at 876 cm−1 was assigned to the formation of tetrahedral coordination of Zn (Fig. 3b–e) (Jayarambabu et al., 2015). All the spectra of samples calcining above 500 °C showed strong absorption peaks below 600 cm−1, which corresponded to the characteristic absorption of ZneO bond in zinc oxide (Li et al., 2013).
Lattice parameters [Å] a=b
c
3.2498 3.2487 3.2496 3.2496
5.2066 5.2077 5.2060 5.2062
3.4. SEM analysis The morphologies of Zn-MOF, Zn-MOF-350, ZnO/C-500, ZnO/C600, ZnO/C-700 and ZnO-700 were revealed by FESEM (Fig. 4). The calcination temperature and atmosphere showed a great influence on the morphology and surface structure of Zn-MOF-350, ZnO/C-500,
Fig. 2. TG-DTG of the as-prepared Zn-MOF. 4
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Fig. 4. SEM images of Zn-MOF (a), Zn-MOF-350 (b), ZnO/C-500 (c, d), ZnO/C-600 (e, f), ZnO/C-700 (g, h) and ZnO-700 (i, j).
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respectively, while Zn 2p1/2 and Zn 2p3/2 peaks of ZnO/C-700 were located at 1044.5 eV and 1021.4 eV, respectively. The binding energy distances between Zn 2p1/2 and Zn 2p3/2 both were 23.1 eV for Zn-MOF and ZnO/C-700, which confirming the presence of Zn2+ ions (Hussain et al., 2018). O 1s spectra of Zn-MOF showed two different forms of oxygen (Fig. 6c). The peaks located at 531.8 eV and 533.1 eV can be ascribed to ZNneOeC bond and adsorbed hydroxyl or water and carbonate (CeO/ C]O) species, respectively (Hussain et al., 2019). O 1s spectra of ZnO/ C-700 can be deconvoluted into three peaks centered at 530.4 eV, 531.8 eV and 533.0 eV. The latter two peaks were at the same BE as Zn-MOF. The peak at 530.4 eV can be assigned to O2− ions in the ZneO bond of the wurtzite structure of ZnO (Hussain et al., 2019; Zhang et al., 2019b). In addition, the peak components of O and C were summarized in Table S2. The results showed that the peak area of ZneO bond in O 1s in ZnO/C-700 was the largest in three peaks. This confirmed that the main component of ZnO/C-700 was ZnO. The intensities of peaks related to carbon (531.8 eV and 533.0 eV) in ZnO/C-700 were reduced but not disappeared as compared with Zn-MOF, which indicated the presence of C in ZnO/C-700. C 1s spectra of Zn-MOF and ZnO/C-700 were depicted in Fig. 6d. C 1s spectra of Zn-MOF could be divided into three peaks at 284.8 eV, 286.5 eV and 288.7 eV, which were assigned to CeC bond, CeO bond and C]O bond, respectively (Liang et al., 2016). For ZnO/C-700, there were three peaks located at 283.9 eV, 285.1 eV and 288.2 eV. These peaks were attributed to ZneC bond, ZneOeC bond and C]O bond, respectively (Hussain et al., 2019; Liang et al., 2016). The dominant peak at 283.9 eV (Zn-C) in ZnO/C-700 indicated that C remained in the form of Zn-C in the residue after calcination at 700 °C. This may make the performance of ZnO/C-700 different from that of pure zinc oxide. The weak peaks of N 1s proved the presence of nitrogen and indicated that N was incorporated into the prepared ZnO/C-700 (Fig. 6e). N 1s spectra of ZnO/C-700 can be deconvoluted into three peaks centered at 397.8 eV, 399.9 eV and 401.7 eV, which were assigned to ZneN bond, OeZneN bond, and NeO bond, respectively (Zhang et al., 2019b).
ZnO/C-600, ZnO/C-700 and ZnO-700. As shown in Fig. 4a, Zn-MOF had a petaloid structure with willow-like aggregation, which also can be seen in magnified view (inset, in Fig. 4a). Although Zn-MOF-350, ZnO/ C-500, ZnO/C-600 and ZnO/C-700 all maintained the petaloid morphology, their surface structures varied (Fig. 4b–h), which structural representation had been described in Scheme 1. The surface of Zn-MOF350 showed a bit rough as compared with Zn-MOF. Small ZnO particles could be observed on the surface (inset in Fig. 4b). A layered porous structure was observed in the SEM image of ZnO/C-500 (Fig. 4c). The magnified view of ZnO/C-500 revealed that the layered porous structure was actually consisted of aggregates of little zinc oxide nanoparticles (< 120 nm) (Fig. 4d). The result indicated that the Zn-MOF began to break down when the calcination temperature was over 500 °C. The size of zinc oxide particle on the surface of petaloid morphological structure further increased with increasing calcination temperature. A combined amorphous and crystalline hierarchical structure could be found in the SEM images (Fig. 4e–h). The crystallined ZnO particles (0.5–2.5 μm) with polyhedral structure were observed in ZnO/C-700. As a comparison, the morphology of ZnO-700 (calcined in air) was also investigated using SEM (Fig. 4i, j). Although the petaloid morphology of Zn-MOF also remained in ZnO-700 like ZnO/C-700, the ZnO nanoparticles (40∼300 nm) in ZnO-700, in an irregular and agglomerated state, were smaller than those in ZnO/C700. The compositions of ZnO/C-600, ZnO/C-700 and ZnO-700 were investigated by energy dispersive spectroscopy (EDS) analysis. The results confirmed the presence of C, Zn and O in ZnO/C-600, ZnO/C-700 and ZnO-700 (Fig. S1). The elemental analysis for samples has also been carried out (Table S1). The results confirmed the composition of ZnMOF-350 didn't show significant change as compared with Zn-MOF. The obtain samples of thermal decomposition of Zn-MOF under nitrogen not less than 500 °C were ZnO/C composites. The C content in ZnO/C nanocomposite increased with the increase of calcination temperature under nitrogen (Table S1 and Fig. S1a, b). The sample ZnO700 obtained in air at 700 °C was mainly ZnO (Fig. S1c). The results showed that the calcination temperature and atmosphere had influence on the composition of samples. Combined with XRD and FESEM analyses, it can be concluded that carbon had been incorporated into the crystal lattice of zinc oxide during carbonization, and further changed the morphologies of samples. The EDS mapping (Fig. 5) also demonstrated the homogeneous distribution of Zn, O, C and N in ZnO/C-600 and ZnO/C-700.
3.6. UV–DRS analysis Fig. 7a showed the UV–vis diffuse reflectance spectra of Com-ZnO and samples obtained at different calcination temperatures. It can be seen that all the samples exhibited a broad absorption in UV region as well as Com-ZnO. Moreover, compared to Com-ZnO, Zn-MOF-350 still displayed an additional tailing absorption in the visible region. ZnO/C500 showed a distinct tailing absorption covering the whole visible region. ZnO/C-600 and ZnO/C-700 showed absorption in the whole range of 200∼800 nm. Such an enhanced absorption capacity will be beneficial for photocatalytic application. The direct band gap energies determined by extrapolating the linear part of plot of (αhν)2 versus the energy (hν) was depicted in Fig. 7b (Kumar et al., 2017). The calculated
3.5. XPS analysis The elemental compositions and chemical states of Zn-MOF and ZnO/C-700 were investigated by XPS (Fig. 6). The elemental survey (Fig. 6a) showed that Zn-MOF and ZnO/C-700 mainly consisted of Zn, C and O elements. In the Zn 2p spectra (Fig. 6b), the Zn 2p1/2 and Zn 2p3/ 2 peaks of Zn-MOF were located at 1045.3 eV and 1022.2 eV,
Fig. 5. (a)-(e) EDS mapping of ZnO/C-600; (f)-(j) EDS mapping of ZnO/C-700. 6
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Fig. 6. XPS spectra of Zn-MOF and ZnO/C-700: (a) elemental survey, (b) Zn 2p, (c) O 1s, (d) C 1s, and (e) N 1s of ZnO/C-700.
band gap values (Eg) were 3.21, 2.87, 3.18, 3.02 and 2.82 eV for ComZnO, Zn-MOF-350, ZnO/C-500, ZnO/C-600 and ZnO/C-700, respectively. As compared to Com-ZnO, band gaps of ZnO/C-500, ZnO/C-600 and ZnO/C-700 were reduced due to the doping of C and N. The results implied that carbon and nitrogen were incorporated into the crystal structure of ZnO than just covered on the surface of ZnO (Zhang et al., 2019a; Wu, 2014). In addition, the contribution of carbon and nitrogen to the valence band (VB) of prepared ZnO took the major role in enhancing light harvesting and utilizing visible light (Scheme 2) (Zhang et al., 2019a; Lavand and Malghe, 2015).
ZnO/C-500 located at ∼387 nm and ZnO/C-600 and ZnO/C-700 located at ∼378 nm were attributed to the excitonic emission. As compared with Zn-MOF-350, the quenching of the PL intensity of ZnO/C500, ZnO/C-600 and ZnO/C-700 were ascribed to the reduction in recombination of photogenerated electrons and holes (Rajbongshi et al., 2014). The reduction in recombination of photogenerated electrons and holes was beneficial to photocatalytic activity. This also suggested that ZnO/C-600 and ZnO/C-700 should have high photocatalytic activity.
3.7. PL analysis
The nitrogen adsorption/desorption isotherms of the obtained samples were exhibited in Fig. 8. The surface area, pore volume and pore size of the samples calculated from the adsorption isotherms were presented in Table 2. All the samples displayed the type IV curve
3.8. BET analysis
The PL spectra of the samples obtained at different calcination temperatures were shown in Fig. S2. The peaks of Zn-MOF-350 and 7
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could be due to the decomposition of the organic framework of Zn-MOF and the release of volatile gases such as CO2 and H2O. It can be seen that the BET surface area of Zn-MOF (1.10 m2⋅ g−1) was very small. This may be due to the fact that the prepared Zn-MOF was a partially hydrolyzed MOF-5 (Sabo et al., 2007). Both the calculated BET surface areas and the total pore volumes of Zn-MOF-350, ZnO/C-500, ZnO/C600 and ZnO/C-700 increased first and then decreased with the increase of calcination temperature. ZnO/C-600 showed the largest specific surface area and pore volume. This could be due to the collapse of the framework of Zn-MOF and the formation of ZnO with polyhedral structure over 600 °C. Moreover, the collapse rate of framework increased with the increase of calcination temperature, which further resulted in the reduction in the specific surface area of ZnO/C-700. 3.9. Adsorption capacity The adsorption capacities of ZnO/C-600 and ZnO/C-700 were measured using equilibrium adsorption isotherms with different initial concentrations of MB as shown in Fig. 9a. The as-obtained experimental data were analyzed using Langmuir (Fig. 9b), Freundlich (Fig. 9c) and Temkin (Fig. 9d) adsorption isotherm models (Malwal and Gopinath, 2017) and the results were listed in Table 3. The results indicated that the adsorption process for MB removal using ZnO/C-600 (ZnO/C-700) was fitted with Langmuir model than Freundlich and Temkin models. The theoretical maximum capacities of ZnO/C-600 and ZnO/C-700 from Langmuir adsorption model were 106.4 mg g−1 and 100.0 mg g−1, respectively. The kinetics of adsorption was studied to evaluate the adsorption process and efficiency of the adsorbent. Fig. 10a depicted the adsorption of MB using ZnO/C-600 and ZnO/C-700 with respect to time. It was performed by adding 20 mg of ZnO/C-600 (ZnO/C-700) in 50 mL of MB aqueous solution with a concentration of 150 mg⋅L−1. It can be seen that the adsorption increased fast initially and then reached the adsorption equilibrium slowly. To evaluate the adsorption efficiency of ZnO/C-600 (ZnO/C-700), the pseudo-second-order kinetics were chosen (Malwal and Gopinath, 2017).
Fig. 7. (a) UV–vis diffuse reflectance spectra and (b) plots of (αhν)2 versus the energy (hν) of the Com-ZnO and samples obtained at different calcination temperatures.
t 1 t = + qt qe k2 qe2
accompanied by a type H3 hysteresis loop, which was attributed to the predominance of mesopores according to the IUPAC classification (Kruk and Jaroniec, 2001). The pore size distributions were given in the inset of Fig. 8. The presence of micropores and mesopores in the samples
where qe (mg g−1) was the equilibrium adsorption capacity and qt was the amount of MB adsorbed onto ZnO/C-600 (ZnO/C-700) at a
Scheme 2. Possible schematic diagram for ZnO/C-700 photocatalytic degradation of MB under sunlight irradiation. 8
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Fig. 8. Nitrogen adsorption/desorption isotherms of obtained Zn-MOF (a), Zn-MOF-350 (b), ZnO/C-500 (c), ZnO/C-600 (d) and ZnO/C-700 (e). Inset: the corresponding BJH (Barret–Joyner–Halenda) pore size distribution.
particular time t, and k2 (g mg−1 h−1) was the pseudo-second-order rate constant of adsorption. The linear plot between t vs. t/qt was shown in Fig. 10b. The calculated values of k2 and qe were 0.0021 g mg−1 h−1 and 107.5 mg g−1 for ZnO/C-600 and 0.0025 g mg−1 h−1 and 100 mg g−1 for ZnO/C-700, respectively. The correlation coefficient (R2) values for pseudo-second-order kinetics were 0.998 and 0.999 for ZnO/C-600 and ZnO/C-700, respectively. It indicated that the pseudo-second-order kinetics were more suitable for the adsorption process.
Table 2 The surface area, pore volume and pore size of the samples. Sample
SBETa (m2 g−1)
Pore volumeb (cm3 g−1)
Pore sizec (nm)
Zn-MOF Zn-MOF-350 ZnO/C-500 ZnO/C-600 ZnO/C-700
1.10 1.80 63.41 197.21 134.79
0.007 0.008 0.146 0.324 0.239
23.99 17.35 9.22 6.57 7.08
a The BET surface area was calculated from the linear part of the BET plot (P/ P0 = 0.07–0.3). b Pore volume was determined by adsorption branch of the nitrogen isotherms at P/P0 = 0.97. c Average pore size was estimated using the adsorption branch of the nitrogen isotherms and the Barrett-Joyner-Halenda method.
3.10. Photocatalytic activity Methylene blue (MB) was used as a target pollutant to evaluate the photocatalytic activities of the as-synthesized samples under sunlight irradiation. As a contrast, the degradation of MB by Com-ZnO and the control (without catalyst) were also carried out. The UV–vis absorption spectra of MB aqueous solution after different time irradiation in the 9
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Table 3 Regression equations and calculated parameters for three adsorption isotherm models. Isotherm models
Sample
Regression equation acquired from the graph
Parameters
Langmuir
ZnO/C-600
Ce/qe = 0.0508 + 0.0094Ce
ZnO/C-700
Ce/qe = 0.0596 + 0.0100Ce
ZnO/C-600
ln qe = 3.7956 + 0.1700ln Ce
ZnO/C-700
ln qe = 3.6011 + 0.1996ln Ce
ZnO/C-600
qe = 55.227 + 9.142ln Ce
ZnO/C-700
qe = 43.230 + 10.357ln Ce
KL = 0.185 qm = 106.4 R2 = 0.992 KL = 0.168 qm = 100.0 R2 = 0.994 KF = 44.505 n = 5.882 R2 = 0.991 KF = 36.639 n = 5.010 R2 = 0.992 A = 420.321 B = 9.142 R2 = 0.935 A = 64.974 B = 10.357 R2 = 0.953
Freundlich
Temkin
Fig. 10. (a) Effect of time on the adsorption of MB onto ZnO/C-600 and ZnO/C700, (b) pseudo second-order kinetics plot for MB adsorption using ZnO/C-600 and ZnO/C-700.
presence of catalysts were depicted in Fig. 11a–f. It can be seen that the intensity of maximum absorption peak of MB at 664 nm decreases dramatically after stirring for 30 min in dark in the presence of ZnO/C500 (Fig. 11c), ZnO/C-600 (Fig. 11d) and ZnO/C-700 (Fig. 11e), especially for ZnO/C-600 and ZnO/C-700. It was due to the large specific surface area and the resulted higher adsorption capacity of ZnO/C-
Fig. 9. (a) Adsorption isotherms, (b) Langmuir plots, (c) Freundlich plots and (d) Temkin plots of MB adsorption on ZnO/C-600 and ZnO/C-700 for 120 min.
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Fig. 11. UV–vis absorption spectra of MB aqueous solution after different time irradiation in the presence of Com-ZnO (a), Zn-MOF-350 (b), ZnO/C-500 (c), ZnO/C600 (d), ZnO/C-700 (e) and ZnO-700 (f), and the insets were the corresponding photographs (d and e); (g) Relationship between C/C0 and reaction time for MB decomposition, where C and C0 were the actual concentration and the initial concentration of MB, respectively.
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Fig. 12. (a) Reusability experiment for the photocatalytic degradation of MB using the as-synthesized ZnO/C-700 sample; XRD patterns (b) and FTIR (c) of ZnO/C700 before and after five cycles; low (d) and high (e) magnification SEM of ZnO/C-700 after five cycles.
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4. Conclusions
500, ZnO/C-600 and ZnO/C-700. Moreover, MB aqueous solution was degraded in the presence of ZnO/C-600 or ZnO/C-700 from blue to colorless (inset photographs in Fig. 11d–e). However, for ZnO/C-500, the absorption of MB at 664 nm decreased slowly under sunlight irradiation (Fig. 11c). This different catalytic phenomenon was due to the higher crystallinity of ZnO/C-600 and ZnO/C-700, which reduced the recombination of electron-hole. The relationship between C/C0 (C0 and C were the initial concentration and the actual concentration of MB) and reaction time was also investigated (Fig. 11g). In the absence of catalyst, MB solution remained unchanged under sunlight irradiation, which meant MB was very stable and didn't degrade under sunlight irradiation. It can be clearly seen that Zn-MOF-350 had slight effect on degradation of MB. The direct adsorption of ZnO/C-500, ZnO/C-600 and ZnO/C-700 played a great role in the degradation of MB due to their larger specific surface area before sunlight irradiation. The final decolorization efficiencies of MB in the presence of Com-ZnO, Zn-MOF-350, ZnO/C-500, ZnO/C-600, ZnO/C-700 and ZnO-700 after 100 min of irradiation were 78.2%, 13.5%, 73.4%, ∼100%, ∼100% and 71%, respectively. Actually, the decolorization of MB from water using ZnO/C-700 was completed after 60 min of irradiation. Since ZnO/C-700 showed a high adsorption performance, the photocatalytic performance of ZnO/C-700 was further certified by the elution of MB with ethanol from ZnO/C-700 during photo-degradation. The results showed that the content of MB adsorbed on ZnO/C-700 gradually and quickly decreased with increasing of sunlight irradiation time (Fig. S3). This confirmed the photocatalytic activity of ZnO/C-700. To evaluate the photochemical stability of ZnO/C-700, the reusability experiment for the photocatalytic degradation of MB was performed (Fig. 12a). After each cycle, the photocatalyst was separated from the suspension by centrifugation, washed with absolute ethanol and deionized water, then dried for a new cycle. The results showed that the photocatalytic activity of ZnO/C-700 exhibited no significant decrease during the repeated five cycles. Furthermore, ZnO/C-700 after the fifth cycle was also characterized by XRD, FTIR and SEM (Fig. 12b–e). The XRD pattern, FTIR spectrum and morphology of the reused ZnO/C-700 showed no significant changes as compared with the new prepared ZnO/C-700. All the results indicated that ZnO/C-700 was highly stable and reusable, and beneficial for its application as a photocatalyst. The possible schematic diagram for ZnO/C-700 photocatalytic degradation of MB under sunlight irradiation was shown in Scheme 2. Similar to pure ZnO, the photocatalysis degradation of MB by ZnO/C700 also based on the photo-generated electron-hole (e−/h+) pairs, which generated free radicals to oxidize MB into neutral species (Hussain et al., 2018,). On this basis, the presence of carbon (including doped carbon and carbon matrix) in ZnO/C-700 composite further improved the degradation of MB. The black carbon matrix enhanced visible light absorption capacity and further improved the efficiency of photo-irradiation. As compared with pure ZnO, the band gap of ZnO in ZnO/C-700 was narrowed by the doped C (also N). This benefited the produce of photo-generated electron-hole (e−/h+) pairs under sunlight irradiation (Pan et al., 2016). The carbon matrix also reduced the recombination of photo-generated electron-hole by effectively transferring and receiving the photo-generated electrons from conduction band of ZnO (Hussain et al., 2018,). Moreover, the large specific surface area of ZnO/C-700 composite facilitated the reaction between photo-generated electrons (or holes) and dissolved molecular oxygen (or solvent H2O) to produce various reactive oxygen species. The large specific surface area of ZnO/C-700 could also increase the concentration of MB around the photocatalytic active sites in ZnO/C-700 by effective adsorption. As a result, the degradation efficiency of MB by ZnO/C-700 was improved.
In summary, ZnO/C nanocomposite had been prepared by annealing partially hydrolyzed MOF-5 at different temperatures in nitrogen. The obtained ZnO/C-600 and ZnO/C-700 retained the petaloid morphology with aggregates of ZnO nanoparticles and porous structure. The results indicated that ZnO/C-600 and ZnO/C-700 exhibited outstanding adsorption and photocatalytic performance on MB degradation under sunlight irradiation. The possible photocatalytic mechanism of ZnO/C700 for degradation of MB under sunlight irradiation was proposed. This work provided a new insight into the design and synthesis of highly efficient photocatalysts for organic dyes containing wastewater treatment in environmental applications. Declaration of Competing Interests None. Acknowledgements This work was supported by the Fundamental Research Funds for the Central Universities (No. 2232013A3-05) and the National Science and Technology Ministry (ID 2012BAK30B03). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jhazmat.2019.121599. References Cheng, S., Liu, S., Zhao, Q., Li, J., 2009. Improved synthesis and hydrogen storage of a microporous metal–organic framework material. Energy Convers. Manage. 50, 1314–1317. Cui, W.G., Zhang, G.Y., Hu, T.L., Bu, X.H., 2019. Metal-organic framework-based heterogeneous catalysts for the conversion of C1 chemistry: CO, CO2 and CH4. Coord. Chem. Rev. 387, 79–120. Dekrafft, K.E., Wang, C., Lin, W., 2012. Metal-organic framework templated synthesis of Fe2O3/TiO2 nanocomposite for hydrogen production. Adv. Mater. 24, 2014–2018. Dong, S., Zhang, D., Cui, H., Huang, T., 2019. ZnO/porous carbon composite from a mixed-ligand MOF for ultrasensitive electrochemical immunosensing of C-reactive protein. Sens. Actuators B Chem. 284, 354–361. Gao, S., Fan, R., Li, B., Qiang, L., Yang, Y., 2016. Porous carbon-coated ZnO nanoparticles derived from low carbon content formic acid-based Zn(II) metal-organic frameworks towards long cycle lithium-ion anode material. Electrochim. Acta 215, 171–178. Hafizovic, J., Bjørgen, M., Olsbye, U., Dietzel, P.D., Bordiga, S., Prestipino, C., Lamberti, C., Lillerud, K.P., 2007. The inconsistency in adsorption properties and powder XRD data of MOF-5 is rationalized by framework interpenetration and the presence of organic and inorganic species in the nanocavities. J. Am. Chem. Soc. 129, 3612–3620. Hou, X., Stanley, S.L., Zhao, M., Zhang, J., Zhou, H., Cai, Y., Huang, F., Wei, Q., 2019. MOF-based C-doped coupled TiO2/ZnO nanofibrous membrane with crossed network connection for enhanced photocatalytic activity. J. Alloys Compd. 777, 982–990. Hu, C., Lu, L., Zhu, Y., Li, R., Xing, Y., 2018. Morphological controlled preparation and photocatalytic activity of zinc oxide. Mater. Chem. Phys. 217, 182–191. Huang, L., Wang, H., Chen, J., Wang, Z., Sun, J., Zhao, D., Yan, Y., 2003. Synthesis, morphology control, and properties of porous metal–organic coordination polymers. Microporous Mesoporous Mater. 58, 105–114. Hussain, M.Z., Schneemann, A., Fischer, R.A., Zhu, Y., Xia, Y., 2018. MOF derived porous ZnO/C nanocomposites for efficient dye photodegradation. ACS Appl. Energy Mater. 1, 4695–4707. Hussain, M.Z., Pawar, G.S., Huang, Z., Tahir, A.A., Fischer, R.A., Zhu, Y., Xia, Y., 2019. Porous ZnO/carbon nanocomposites derived from metal organic frameworks for highly efficient photocatalytic applications: a correlational study. Carbon 146, 348–363. Jayarambabu, N., KumarI, B.S., Rao, K.V., Prabhu, Y., 2015. Beneficial role of zinc oxide nanoparticles on green crop production. Int. J. Multidiscip. Adv. Res. Trends 2, 273–282. Jiang, H.L., Liu, B., Lan, Y.Q., Kuratani, K., Akita, T., Shioyama, H., Zong, F., Xu, Q., 2011. From metal–organic framework to nanoporous carbon: toward a very high surface area and hydrogen uptake. J. Am. Chem. Soc. 133, 11854–11857. Jiao, W., Zhang, L., 2018. Fabrication of new C/ZnO/ZnO composite material and their enhanced gas sensing properties. Mater. Sci. Semicond. Process. 86, 63–68. Joharian, M., Morsali, A., Tehrani, A.A., Carlucci, L., Proserpio, D.M., 2018. Water-stable fluorinated metal–organic frameworks (F-MOFs) with hydrophobic properties as efficient and highly active heterogeneous catalysts in aqueous solution. Green Chem.
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