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Facile synthesis of [email protected]2S3@UiO-66 ternary composite with enhanced visible-light photodegradation activity for methyl orange
Journal of Photochemistry & Photobiology A: Chemistry 384 (2019) 112025
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Facile synthesis of rGO@In2S3@UiO-66 ternary composite with enhanced visible-light photodegradation activity for methyl orange
T
Chuanxian Gan, Chen Xu, Hang Wang, Na Zhang , Jianyong Zhang , Yongzheng Fang ⁎
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Shanghai Institute of Technology, Shanghai, 201418, PR China
ARTICLE INFO
ABSTRACT
Keywords: rGO@In2S3@UiO-66 Ternary composite Visible-light Photocatalysis Methyl orange
In this study, rGO@In2S3@UiO-66 ternary composites have been designed and fabricated via a facile two-step hydrothermal process and used as visible-light-driven photocatalyst. The as-prepared samples have been characterized by PXRD, SEM, TEM, XPS, UV–vis DRS, PC, PL and BET. The photocatalytic performance is evaluated by the degradation of methyl orange (MO) under visible-light irradiation. The results show that the photocatalytic performance of rGO@In2S3@UiO-66 is superior to that of In2S3, UiO-66 and In2S3@UiO-66. The enhanced photocatalytic performance of rGO@In2S3@UiO-66 can be attributed to the relative large specific surface area, the improving light absorption region caused by rGO and the promoting the separation of photo-induced carriers by the synergistic effect of rGO, In2S3 and UiO-66. In addition, the rGO@In2S3@UiO-66 composites also demonstrate outstanding photocatalytic stability and there is only 5.8% loss of degradation rate after five cycles. Furthermore, the reasonable photocatalytic mechanism for the degradation of MO by rGO@In2S3@UiO-66 was also proposed.
1. Introduction
researches to overcome these limitations. For instance, Khanchandani et al. [12] have reported the ZnO@In2S3 core/shell nanorod arrays, the photocatalytic activity under visible light to degrade rhodamine B (RhB) was enhanced by these core/shell nanostructures due to the formation of heterojunctions, which prolong the separation of photogenerated electrons and holes. Nevertheless, it is still of great interest to further improved the photocatalytic performance for practicalapplication in water treatment. Metal-organic-frameworks (MOFs) is a group of porous crystalline materials which synthesized via the self-assembly of metal ions/clusters and organic ligands [13,14]. Recently, MOFs are explored to apply in many promising fields, such as gas storage, chemical sensors, heterogeneous catalysis, and biomedicine due to their high specific surface area, tunable pore size, great chemical variety, and relatively good thermo stability [15–18]. Very recently, some certain MOFs have attracted huge attention in photocatalysis due to their semiconductor behavior [19], which means that those MOFs can be designed as photocatalysts due to they can facilitate charge transfer and directly harvest light irradiation [20–31]. Among them, UiO-66, a typical MOF which isconstructed form the Zr6(OH)4O4(CO2)12 cluster, can be regarded as a promising candidate for the development of heterogeneous photocatalysts for water treatment due to its extraordinarily chemical stability [31] and water stability as UiO-66 is capable to maintain the
In the past years, semiconductor-based photocatalysts has attracted enormous attentions in degradation of organic pollutants for water treatment due to their no secondary-pollutants generation and direct solar-to-chemical conversion [1,2]. Particularly, visible-light-driven semiconductors are more appealing for their effectual utilization of the solar energy [3,4]. Recently, metal sulfides has been exhibited as a group of advantageous photocatalysts for the degradation of organic pollutants in in aqueous solution, such as CdS [5], Bi2S3 [6] and In2S3 [7] Among them, β-In2S3, an III–VI group sulfide with defect spinel structure, has received a special attention due to its wide response to solar energy own to the suitable band-gap (2.0–2.3 eV) and stable in the photocatalytic reaction process own to the anti-photo-corrosion property [8–10]. More importantly, the conduction band (CB) of In2S3 is composed of d, s and p orbital, while the valence band (VB) consists of S 3p orbital, which are much more negative than O 2p orbital, the characteristics of band edges could lead to the reduction reactions more available [11]. However, in spite of the advantage in of In2S3 in photocatalysis, there are some problems which limit the practicalapplicationof In2S3 as follows: fast recombination of photo-generated electron (e−) and hole (h+) pairs, easy agglomeration and difficult separation from aqueous system for reuse. Recently, many scientists have done
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Corresponding author. E-mail addresses: [email protected] (N. Zhang), [email protected] (J. Zhang).
https://doi.org/10.1016/j.jphotochem.2019.112025 Received 31 May 2019; Received in revised form 2 August 2019; Accepted 5 August 2019 Available online 10 August 2019 1010-6030/ © 2019 Elsevier B.V. All rights reserved.
Journal of Photochemistry & Photobiology A: Chemistry 384 (2019) 112025
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structure in the water for several months [32,33]. However, due to the relatively wide band gap, UiO-66 can hardly response to visible light, which make a limit to UiO-66 for practical application [33]. To overcome this problem, incorporating UiO-66 together with other certain semiconductor material for a visible-light-driven heterostructure photocatalyst is a great strategy. For that semiconductor@UiO-66 heterostructure, the semiconductor nanoparticles can disperse on UiO-66 surface due to the large surface area of UiO-66. UiO-66 acts as a host matrix and mediator in the preparation process, and the synergistic effect between semiconductor and UiO-66 can prolong the transmission path of photo-generated carriers and inhibit the recombination of photo-generated electron (e−)-hole (h+) pairs. For instance, CdS@UiO66 [34] and Bi2WO6@UiO-66 [35], those composites obtained by incorporating UiO-66 together with a certain narrow band gap semiconductors demonstrate an enhanced photocatalytic performance than each pristine semiconductor and UiO-66. Recently, we have also prepared a core–shell CdS@NH2-MIL-125(Ti) photocatalyst.The photodegradation efficiencies under visible light for organic pollutants (such as rhodamine B, phenol and oxytetracycline) were higher than that of pure CdS and pure NH2-MIL-125(Ti) due to the opened porous structure, effective transfer of photo-generated carriers and the synergistic effect between NH2-MIL-125(Ti) and CdS [30]. Graphene (GR) including graphene oxide (GO) and reduced graphene oxide (rGO) is a unique sp2 hybrid carbon network material [36] with superior charge carrier mobility, and high electrical conductivity [37]. Due to those excellent characteristics, GR was always used to combine with other semiconductor to form a heterostructure to promote the separation of photo-generated charge carriers and thus increase their lifetime, therefore, improving the photocatalytic performance. It has been reported that the heterostructure photocatalyst formed by coupling GR and semiconductor, such as rGO@TiO2 [38], rGO@Ag3PO4 [39] and rGO@MIL-53(Fe) [40]. In our previous work [41], the In2S3@UiO-66 composites were fabricated and exhibited enhanced photocatalytic ability than pure In2S3 and UiO-66 under visible light irradiation. To find photocatalyst with better photocatalytic performance, the rGO was introduced into the In2S3@UiO-66 system, and the rGO plays a supporting role in ternary composite catalysts. It contacts with the binary composite catalysts consisting of UiO-66 and In2S3. The electrons generated by the binary composite catalysts can migrate to rGO, which increases the active sites of the reaction, so that the photocatalytic performance of the ternary catalysts can be improved. Based on these, the rGO@In2S3@UiO-66 ternary composite photocatalyst has been tried to prepare. Finally the resulting rGO@ In2S3@UiO-66 ternary composite were believed to demonstrate superior photocatalytic ability than In2S3@UiO-66 composite. Herein, in this paper, rGO@In2S3@UiO-66 ternary composites with enhanced photocatalytic ability were prepared. Firstly, the octahedral UiO-66 was used as substrate and mediator for the growth of In2S3 nanoparticles during the hydrothermal process. After that, the In2S3@ UiO-66 composite was dispersed in GO solution to obtain rGO@In2S3@ UiO-66 ternary composite during the hydrothermal process. The photocatalytic performance and recyclability of freshly-synthesized rGO@ In2S3@UiO-66composites were investigated in detail by employing the photo-degradation of MO as model reaction under visible light irradiation.
2.2. Synthesis and preparation 2.2.1. Synthesis of UiO-66 UiO-66 octahedrons were synthesized via a solvothermal method following the previous report [41]. Briefly, ZrCl4 (1.0 mmol, 233.0 mg) and H2BDC (1.0 mmol, 166.0 mg) were dissolved in the mixed solution consist of 100 mL N,N′-dimethylformamide (DMF) and 23 mL acetic acid which was added to turn the morphology of UiO-66. The obtained mixture was stirred for 1 h under room temperature and then sealed in a Teflon-lined autoclave and heated in an oven at 120 °C for 24 h. After cooling to room temperature, the white solid products were collected by filtration and thoroughly washed by deionized (DI) water and ethanol for three times, and then dried at 60 °C in the vacuum drying oven for 6 h. 2.2.2. Preparation of In2S3@UiO-66 composites In2S3@UiO-66 composite were prepared by the growth of In2S3 nanoparticles upon the surface of UiO-66 octahedrons via a facile hydrothermal method. Typically, In(NO3)3·xH2O (149.0 mg, 0.496 mmol) was dissolved in 50 ml DI water, thereafter, 200 mg as-obtained UiO-66 was dispersed in In(NO3)3·xH2O solution, after stirring for 1 h under room temperature, Na2S·9H2O (357.0 mg, 1.488 mmol) was added. After that, the total mixture was stirred vigorously for another 1 h, and then sealed in a Teflon-lined autoclave and heated in an oven at 180 °C for 16 h. After cooling to room temperature, the solid products were collected by filtration and thoroughly washed by DI water and ethanol for three times, and then dried at 60 °C in the vacuum drying oven for 6 h, the as-obtained In2S3@UiO-66 composite process a weight ratio of In2S3:UiO-66 of 40%, which has been proved as an optimal ratio according to our previous work [42]. The pure In2S3 was also prepared under same condition except the absence of UiO-66. 2.2.3. Preparation of rGO@In2S3@UiO-66 composite Graphene oxide (GO) was synthesized by a modified Hummers method [39]. The rGO@In2S3@UiO-66 composite were prepared by a facile two-step hydrothermal method. Briefly, 20.0 mg GO was ultrasonically dispersed in 40.0 ml DI water for 4 h. After that, 200.0 mg asobtained In2S3@UiO-66 was added in the GO solution. The resulting suspension was stirred vigorously for another 1 h and then sealed in a Teflon-lined autoclave and heated in an oven at 150 °C for 8 h. After cooling to room temperature, the solid products were collected by filtration and thoroughly washed by DI water and ethanol for three times, and then dried at 60 °C in the vacuum drying oven for 6 h. The pure rGO was also prepared under same condition except the absence of In2S3@ UiO-66. 2.3. Characterization Powder X-ray diff ;raction (PXRD) was investigated via a diffractometer (Bruker D8 Advance) using Cu Kα radiation at 35 mA and 35 kV with a scanning speed of 5° min−1. The morphologies and size of samples were conducted through a scanning electron microscopy (SEM) (Hitachi S-4800 II) and transmission electron microscopy (TEM) (FEIJEM-2100). The surface valence state and the chemical composition of samples were explored by X-ray photoelectron spectroscopy (XPS) on a Physical Electronics spectrometer (PHI-5702) equipped with a monochromatic Al K X-ray source at 150 W. The UV–Vis spectra were recorded on a spectrophotometer (Agilent Cary 5000) with a 200–800 nm wavelength range. The photoluminescence (PL) spectra were obtained by a Varian Cary Eclipse spectrometer (Hitachi F-7000) with an excitation wavelength of 297 nm. N2 isothermal adsorption experiments were conducted on a micromeritics surface area analyzer (ASAP-2020) at 77 K, and the samples were degassed at 120 °C for 6 h in vacuum before the measurements.
2. Experimental 2.1. Materials Sodium sulphide (Na2S·9H2O), indium nitrate (In(NO3)3·xH2O), 1,4benzendicarboxylic acid (H2BDC), zirconium tetrachloride (ZrCl4), graphite powder, potassium permanganate (KMnO4), sodium nitrate (NaNO3), hydrogen peroxide solution (H2O2), sulfuric acid (H2SO4), acetic acid (CH3COOH) and N,N′-dimethylformamide (DMF) are of analytical grade and used as received without further purification. 2
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smooth surfaces、relative large average edge length (∼400 nm) and good dispersion, all this features are beneficial for UiO-66 acting as a matrix for In2S3 nanoparticles growth. For In2S3@UiO-66 composite (Fig. 2c), it can be observed that UiO-66 still maintain the original octahedral shape with good dispersion, indicating that the UiO-66 crystalline structure have not been destroyed after the growth of In2S3 nanoparticles, which is consistent with the previous PXRD analysis. However, the surface of In2S3@UiO-66 block are not smooth yet, but coated with many small nanoparticles which can be believed as In2S3 nanoparticles. It was very interesting to find that the size of In2S3 nanoparticles embedded on the UiO-66 was smaller than the pure In2S3, implying the confinement effect of the UiO-66 [45]. These observations suggest that the UiO-66 has been functionalized with the In2S3 nanoparticles firmly, yielding the In2S3@UiO-66 heterostructures. As shown in Fig. 2d, the UiO-66 is intimately enwrapped by rGO nanosheets under the pronounced electrostatic interactions due to the negatively charged GO and the positively charged UiO-66, and meanwhile, the In2S3 nanoparticles are coating on the UiO-66 surface, hence, the rGO@ In2S3@UiO-66 ternary composite was obtained. To further explore the morphologies and microstructures of rGO@ In2S3@UiO-66, TEM and HRTEM are used. As shown in Fig. 3a, the big block coated with small nanoparticles should be In2S3@UiO-66, and the rGO (in the yellow circle marked in Fig. 3a) can also be detected, which enwrapped with In2S3@UiO-66 block as well as act as a bridge to connect every adjacent In2S3@UiO-66 blocks. Hence, in such structure, photo-generated electrons can fast migrate from In2S3@UiO-66 heterojunction to rGO nanosheets, which can effectively promote the separation of photo-generated electrons (e−) and holes (h+), and inhibit their recombination. As shown in Fig. 3b, the lattice spacing around 0.323 nm can still be found, which agree well with the crystallographic (109) spacing of In2S3 [46], further confirming that the growth of In2S3 nanoparticles onto UiO-66 surface. In addition, it can be observed that there is a clear interface between In2S3 and UiO-66, which is act as conductive channel to the transmission and separation of photo-generated carriers. The elemental composition and surface valence state of as-prepared samples was investigated by X-ray photoelectron spectroscopy (XPS). Fig. 4 shows the XPS survey spectrum of the In2S3 and rGO@In2S3@ UiO-66, and high-resolution XPS spectra of In 3d, S 2p, Zr 3d and C 1s in the rGO@In2S3@UiO-66 composite. As shown in Fig. 4a, the rGO@ In2S3@UiO-66 composite mainly consists of In, S, Zr, C and O elements, as a comparison, the pure In2S3 only contains In and S. Fig. 4b shows that the high-resolution In 3d peaks are centered at approximate 444.8 eV and 452.4 eV, which can be attributed to the binding energies of In 3d5/2 and In 3d3/2, Fig. 4c shows that the high-resolution S 2p peak can be divided into two peaks at 161.5 eV and 162.5 eV, ascribing at the binding energy of S 2p3/2 and S 2p1/2, respectively. Therefore, it can be deduced that the In and S existed as In3+ and S2−, which providing a clear evidence of the existence of In2S3 in rGO@In2S3@UiO-66 composite [41]. Fig. 4d shows the high-resolution Zr 3d peaks, the apparent two peaks observed at 182.7 eV and 185.1 eV correspond to the binding energies of Zr 3d5/2 and Zr 3d3/2, which can be believed that the Zr element was came from UiO-66 [33]. As shown in Fig. 4e, the high-resolution C 1s can be deconvoluted into three surface components, the binding energy of 284.7 eV and 287.7 are assigned to CeC/C]C and OeC]C, respectively, corresponding to the sp2 carbon of rGO and benzoic rings of BDC linkers, and the binding energy of 286.1 eV is also assigned to CeO, corresponding to rGO [38,43]. The existence of rGO in rGO@In2S3@UiO-66 composite will further confirm by Raman spectra. To investigate the vibrational properties of the as-prepared samples, Raman measurements have been carried out. Fig. 5 shows the Raman spectra of GO, rGO and rGO@In2S3@UiO-66. For the sample related to GO and rGO, it can be observed that two prominent bands at 1341 cm−1 and 1593 cm−1, corresponding to the local defects or disorder atomic arrangement of sp3-carbon (D-band) and the plane
Fig. 1. The PXRD patterns of different samples.
2.4. Photocatalytic measurement The photo-degradation experiments of various as-obtained samples were executed under visible light irradiation. The experiment details were shown as follows: 30 mg as-obtained sample was dispersed in 60 mL MO (15 mg/L) solution in a 100 mL beaker placed on top of a magnetic stirrer with water bath. Before irradiation, the mixture was stirred under dark for 40 min to achieve the adsorption-desorption equilibrium. Thereafter, the mixture was irradiated by a 500 W Xe lamp equipped with a UV-cut-off filter (λ =420 nm), and the irradiation intensity in the center of the beaker is100 mW cm−2. At a certain time of 10 min intervals, 3 ml reaction solution was extracted and centrifuged to separate the photocatalyst and clear liquid. After that, the clear liquid was monitored by the UV–vis absorbance at 464 nm to measure the concentration of MO. 3. Results and discussion Fig. 1 shows the PXRD patterns of different samples. The diffraction pattern of pure UiO-66 fits well with previous report [40], indicating the successful synthesis of UiO-66. GO shows a sharp and intense diffraction peak at 10.3°, which can be attribute to the (001) diffraction planes of GO. After hydrothermal treatment of GO, the sharp and intense diffraction peak at 10.3° is disappeared, meanwhile, a broad and weak diffraction peak at 25.0° appears instead, indicating that the GO has been converted to rGO during the hydrothermal treatment process [41,44]. The diffraction peaks of the pure In2S3 appears at 27.40°, 33.45°, and 47.90°, which can be attributed to the (109), (0012) and (2212) diffraction planes of β-In2S3 phase (JCPDS card no. 73-1366), respectively [45]. The In2S3@UiO-66 composite show a mixed diffraction peak of both In2S3 and UiO-66, suggesting that the In2S3 nanoparticles incorporate successfully with UiO-66, and the UiO-66 crystalline structure have not been destroyed after the growth of In2S3 nanoparticles, the same phenomenon can be also observed in the diffraction pattern of rGO@In2S3@UiO-66 composite. However, no apparent characteristic diff ;raction peaks of rGO at 25.0°can be observed in rGO@In2S3@UiO-66, a reasonable explanation is that the rGO is of relatively low content and crystallinity in the composite, and this phenomenon is reported in previous work [38]. The existence of rGO in rGO@In2S3@UiO-66 will be verified by the next XPS and TEM tests. The scanning electron microscope (SEM) is used to provide a direct insight of the as-prepared samples. As shown in Fig. 2a, the panoramic view of the pure In2S3 nanoparticles demonstrate poor dispersion morphology and large agglomeration, which is unfavorable to photocatalysis activity due to the reducing of active sites. As shown in Fig. 2b, the as-obtained UiO-66 are octahedral microcrystals with sharp edges、 3
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Fig. 2. SEM images of (a) pure In2S3, (b) pure UiO-66, (c) In2S3@UiO-66, (d) rGO@In2S3@UiO-66.
vibration of the sp2-carbon in the two-dimensional lattice (G-band), respectively [34]. The intensity ratio between the D and G bands (ID/IG) has been widely used to evaluate the oxygen containing functional groups on the surface of carbon nanomaterials, the intensity ratio of D to G bands (ID/IG) of GO is 0.99, which is lower than that of rGO (ID/ IG = 1.06), confirming the efficient reduction of GO during hydrothermal process [40]. In the case of rGO@In2S3@UiO-66 composite, the intensity ratio of D to G bands is 1.19, indicating the existence of rGO, the result further confirm the successful combination between rGO nanosheets and In2S3@UiO-66. Meanwhile, the optical absorption of as-prepared rGO@In2S3@UiO66 composite was characterized by UV–vis diff ;use reflectance spectroscopy (DRS), and for comparison, In2S3, UiO-66 and In2S3@UiO-66 were also executed. As shown in Fig. 6a, the pure UiO-66 nanocrystals only exhibit an optical absorption edge about 300 nm, corresponding to the white color. As a comparison, the pure In2S3 demonstrate an optical absorption edge about 580 nm, corresponding to the orange color, indicating that In2S3 possesses a wide response range to visible light. For the sample related to In2S3@UiO-66, the spectra show a mixed absorption properties of both In2S3 and UiO-66, further confirming the combination between In2S3 and UiO-66, correspondingly, the color changes from orange to light-orange. After coupling an appropriate amount of rGO, it can be observed that the rGO@In2S3@UiO-66
composite keep the absorption feature compare with In2S3 and UiO-66 in the range of 200–500 nm, meanwhile, the absorption intensity in the visible region (about 500 ˜ 800 nm) increase obviously [48]. The changes of absorption edge are corresponding with the change of the sample’s color, and it can be observed that the color is turn to black in rGO@In2S3@UiO-66 composite. Obviously, the enhanced absorption in visible region is also advantageous in promoting the photocurrent response of rGO@In2S3@UiO-66 composite. Moreover, the band gap energy (Eg) of as-obtained In2S3 and UiO-66 samples were calculated by the following equation: (ahv)2 = A(hv-Eg)
(1)
where a, h, v, A and Eg indicate absorption coefficient, Planck constant, light frequency, proportionality and band gap energy, respectively [43]. The results are shown in Fig. 6b, the Eg of In2S3 and UiO-66 were calculated as 2.2 eV and 4.1 eV, respectively, which is similar to the previous reports [34,46]. Fig. 6c shows the photoelectric current with time (i–t) curves for all the samples under visible-light irradiation. The rGO@In2S3@UiO-66 composite display highest photocurrent intensity among In2S3, UiO-66, In2S3@UiO-66 and rGO@In2S3@UiO-66, demonstrating that the separation efficiency of the photo-generated electron-hole pairs of rGO@ In2S3@UiO-66 composite is highest. The result is well in agreement
Fig. 3. (a) TEM, (b) HRTEM images of rGO@In2S3@UiO-66. 4
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Fig. 4. The XPS spectra: (a) survey scan, (b) In 3d (c) S 2p, (d) Zr 3d, (e) C 1s.
exhibits a strong luminescence centered at about 380 nm, which could be attributed to the band gap recombination of electron-hole pairs, corresponding to previous report [28]. Comparatively, the PL intensity of In2S3@UiO-66 decreases significantly, which can be attributed to the efficient separation and recombination of photo-generated electronhole pairs due to the fabrication of heterostructure between In2S3 and UiO-66. Furthermore, after rGO modification, the rGO@In2S3@UiO-66 demonstrates the lowest PL intensity, suggesting the lowest recombination rate of photo-generated electron-hole pairs, which can be attributed to that rGO can serve as an eff ;ective acceptor of the photogenerated electrons, hence, the photo-generated electrons can be transferred to rGO sheets, thus eff ;ectively suppressing the charge recombination and improving the photocatalytic activity [49]. The N2 adsorption-desorption isotherms as well as the corresponding values of the BET surface area (SBET) and total pore volume (Vt) have shown in Fig. 6e and Table 1, respectively. For the pure In2S3 nanoparticles and UiO-66, the N2 isotherms are type-I and without hysteresis loops, and the SBET are 90.3 m2 g−1 and 1347.8 m2 g−1, and the Vt are 0.107 cm3 g−1 and 0.684 cm3 g−1, respectively, noting that In2S3 can be negligible compared with UiO-66. For the In2S3@UiO-66 composite, the N2 isotherms are still type-I, but with small hysteresis loops at high relative pressure (P/P0∼1), indicating the appearance of mesopores arised from the stacking of In2S3 nanoparticles, and corresponding, the SBET and Vt are 629.5 m2 g−1 and 0.401 cm3 g−1, which is reduced compared with UiO-66. For the rGO@In2S3@UiO-66 ternary
Fig. 5. The Raman spectra of GO、rGO and rGO@In2S3@UiO-66.
with the afore-mentioned PL studies and the photocatalytic experimental results. Photoluminescence (PL) test was employed to investigated the eff ;ect of rGO modification on separation and recombination of photogenerated electron–hole pairs in rGO@In2S3@UiO-66 composite. All the samples were excited by the photo-excitation wavelength of 297 nm at room temperature and the results were shown in Fig. 6d, UiO-66 5
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Fig. 6. (a) UV–vis absorption spectra of different samples; (b) plots of the (ahv)2 versus (hv) for In2S3 and UiO-66 samples; (c) the photocurrent responses for different samples; (d) room-temperature stable state photoluminescence spectra of different samples; (e) The N2 adsorption-desorption isotherms of diff ;erent samples obtained at 77 K; (f) the photo of the different samples.
equilibrium in 40 min. The pure In2S3 demonstrates an obvious weaker adsorption performance than pure UiO-66 and its composites and the adsorption efficient is only 5.5%, which may be due to the relative low specific surface area. The pure UiO-66 exhibits the highest adsorption efficient as 68.5%, which can be attributed to its large specific surface and porous structure [42]. The adsorption efficient of 51.2% of In2S3@ UiO-66 composite is lower than that of pure UiO-66, which can be ascribed to the reducing of specific surface area of In2S3@UiO-66 compared with UiO-66. After coupling with rGO, the rGO@In2S3@UiO66 composite possesses the weak adsorption performance than In2S3@ UiO-66, which is consistent with the BET analysis. After visible light irradiation for 1 h, no MO self-photolysis can be observed without photocatalyst in blank experiment, indicating that the impact of MO self-degradation can be ignored during the photocatalytic process. In addition, the pure UiO-66 demonstrates a very weak photocatalytic activity due to its wide band gap (4.1 eV). The rGO@In2S3@UiO-66 composite exhibits the highest total MO removal efficient of 98.1%, asacontrast, the total MO removal efficient of In2S3@UiO-66 and In2S3 is 96.2% and 39.0%, respectively. Furthermore, to investigate the photocatalytic ability in a more systematic manner, the pseudo-first-order kinetic model [47] is used.
Table 1 The SBET, pore size and pore volume data obtained for the various samples. Samples
SBET (m2 g−1)
Vt (cm3 g−1)
In2S3 UiO-66 In2S3@UiO-66 rGO@In2S3@UiO-66
90.3 1347.8 629.5 487.7
0.107 0.684 0.401 0.306
composite, the SBET and Vt further reduce to 487.7 m2 g−1 and 0.306 cm3 g−1, the reduced SBET and Vt can be attribute to the enwrapping of rGO. The adsorption and photocatalytic performances of as-obtained samples are evaluated through the removal of the MO, since MO is a typical organic dye. To eliminate the impact of self-degradation of MO in the photocatalytic process, blank experiment was excluded under the same condition without photocatalyst. Fig. 7a shows the total MO removal efficiency of different as-obtained samples. The removal efficiency is estimated by the 100(1-C/C0)%, where C0 and C is the initial and actual concentration of MO in the reaction system, respectively. It can be observed that all the samples can reach the adsorption 6
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Fig. 7. a) MO degradation performance of diff ;erent samples, (b) pseudo first-order kinetics of MO photodegradation of different samples, (c) the stability of the rGO@In2S3@UiO-66 over 5 cycles to MO degradation.
UiO-66,which can be attributedto the superior charge carrier mobility and high electrical conductivity as well as improving light absorption region due to the doped rGO. The stability of a photocatalyst is very important role for practical application. Thus, the circulating experiment over rGO@In2S3@UiO-66 sample is carried out to monitor the stability. As shown in Fig. 7c, the degradation ratio is decreasing from 98.1% to 92.3% after 5 cycles, which is only an approximate 5.8% loss. The results demonstrate the outstanding stability of rGO@In2S3@UiO-66 composite, and open a new door in the field of MOF-based photocatalysts for photocatalytic water treatment. In order to explore the active species for MO degradation of the catalyst, the sample rGO@In2S3@UiO-66 with the highest catalytic activity was taken as an example. In heterogeneous photocatalytic reaction, three active substances, superoxide radical (O2%−), hydroxyl radical (·OH) and hole (h+), are mainly involved [13,50,51]. Therefore, aiming at the above active substances, isopropanol (IPA), p-benzoquinone (BQ) and ethylenediaminetetraacetic acid (EDTA) were used as scavengers of %OH, O2%− and h+, respectively, to judge the active substances produced in the process of photocatalytic degradation [52]. Results As shown in Fig. 8, compared with the degradation curve without any eliminating agent, it was found that adding 500 mM IPA in the photocatalytic reaction system had no obvious effect on the degradation rate, indicating that %OH was not the main active substance in the degradation process; however, after adding 1 mM BQ and 1 mM EDTA respectively, the degradation process became slower obviously, indicating that O2%−, and h+ played a major role in the reaction system. Based on the above analysis, a possible photocatalysis mechanism for the photodegradation of MO over rGO@In2S3@UiO-66 under visible-light irradiation is proposed as shown in Scheme 1. The Eg of In2S3 (2.2 eV) is lower than that of UiO-66 (4.1 eV), and according to our previous work [34], the ECB of In2S3 and UiO-66 are −0.8 eV (vs. NHE) and −0.6 eV (vs. NHE), the EVB of In2S3 and UiO-66 are 1.4 eV (vs.NHE) and 3.5 eV (vs. NHE), respectively. Due to the ECB and EVB of In2S3 are both higher than these of UiO-66, hence, the In2S3@UiO-
Fig. 8. The effects of different scavengers on the degradation of MO in the presence of rGO@In2S3@UiO-66 under visible-light irradiation.
The corresponding formula is as follows: -ln(C/C0)=kt, where t is the reaction time, C0 is the concentration after equilibrium for 40 min and C is the concentration at t time in the reaction system, and k is the relative degradation rate constant and its value is equal to the slope of -ln (C/C0) versus time plots. As shown in Fig. 7b, the k value of various samples follow the following sequence: rGO@In2S3@UiO-66 (0.04142 min−1) > > In2S3 (0.05857 min−1) > In2S3@UiO-66 −1 −1 (0.00437 min ) > UiO-66 (0.00064 min ). According to this order, In2S3@UiO-66 demonstrates a superior photocatalytic ability than that of pure In2S3 and UiO-66, indicating that the synergistic effect between In2S3 and UiO-66 in In2S3@UiO-66 heterostructure can effectively promotes the separation and suppresses the secondary recombination of photo-induced carriers, leading to the enhanced photocatalytic performance. Significantly, rGO@In2S3@UiO-66 composites exhibit an enhanced photocatalytic activities than In2S3@ 7
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Xue, Improving photocatalytic hydrogen production of metal–organic framework UiO-66 octahedrons by dye-sensitization, Appl. Catal. B: Environ. 168–169 (2015) 572–576. [24] Y. Yuan, L. Wu, Y. Tong, L. Gu, Z. Xue, MOFs as electron-transfer-bridge for dye photosensitizer and low cost Ni2P co-catalyst to increased photocatalytic H2 generation, Sustain. Energy Fuels 2 (2018) 2502–2506. [25] X. Hou, L. Wu, L. Gu, G. Xue, H. Du, Y. Yuan, Maximizing the photocatalytic hydrogen evolution of Z-scheme UiO-66-NH2@Au@CdS by aminate-functionalized linkers, Mater. Sci. Mater. Electr. 30 (5) (2019) 5203–5211. [26] J. Qiu, X. Zhang, Y. Feng, X. Zhang, H. Wang, J. Yao, Modified metal-organic frameworks as photocatalysts, Appl. Catal. B: Environ. 231 (2018) 317–342. [27] Y. Xue, P. Wang, C. Wang, Y. Ao, Efficient degradation of atrazine by BiOBr/UiO-66 composite photocatalyst under visible light irradiation: environmental factors, mechanisms and degradation pathways, Chemosphere 203 (2018) 497–505.
Scheme 1. A possible photocatalysis mechanism for the photodegradation of MO under visible-light irradiation.
66 heterostructure is formed. With the visible-light irradiation, the photo-generated electrons (e−) can be excited in the VB of In2S3 and then transfer to the CB, leaving photo-generated holes (h+) on the VB. The h+ can directly oxidize MO [41]. Meanwhile, the e− on the In2S3 nanoparticles are inclined to transfer to the CB of UiO-66 via the In2S3 and UiO-66 intimate contact interfaces, because the ECB of In2S3 is more negative than that of UiO-66. Furthermore, the e−on the In2S3@UiO66 heterojunction can further captured by rGO sheets as rGO is an excellent electrons acceptor, then the e−can migrate quickly throughout the rGO sheets due to its superior charge carrier mobility and high electrical conductivity. Therefore, in such way, the rGO sheets with can facilitate charge separation and reduce the recombination of electron–hole pairs in the rGO@In2S3@UiO-66 photocatalyst, and the similar conclusions have appeared in previous literature [15,16]. As a result, more O2 in water acting as the electron scavenger could be reduced by e− to produce O2%−. Notably, the porous structures of UiO-66 are beneficial for the MO adsorption onto the surface of rGO@In2S3@ UiO-66, then these adsorbed MO molecules can be rapidly decomposed by O2%−, hence, promoting the degradation of MO. 4. Conclusion In summary, an rGO@In2S3@UiO-66 ternary composite has been designed and fabricated via a facile two-step hydrothermal process. In the rGO@In2S3@UiO-66 composite, the In2S3 nanoparticles are coating on the surface of UiO-66 with good dispersion and low agglomeration to form heterostructure, and the rGO enwrap upon In2S3@UiO-66 block as well as act as a bridge to connect every adjacent In2S3@UiO-66 blocks. The rGO@In2S3@UiO-66 composite possess a superior photocatalytic performance under visible-light irradiation compared with In2S3, UiO-66, rGO@In2S3 andIn2S3@UiO-66. The reason of the enhancement in photocatalytic performance can be deduced to the following: (1) The relative large SBET can provide more photocatalytic active sites, and the porous structures of UiO-66 are beneficial for the MO adsorption onto the surface of rGO@In2S3@UiO-66; (2) rGO can significantly improve the light absorption region of the composite; (3) The synergistic effect of rGO, In2S3 and UiO-66 can promotes the separation of photo-induced carriers and suppresses their secondary recombination. Declaration of Competing Interest There are no conflicts to declare. Acknowledgements The authors acknowledge the financial support of the National Natural Science Foundation of China (21707093, 51472162, 51672177 and 21503081), the Shanghai Alliance Project (LM201841, LM201843) 8
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Journal of Photochemistry & Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem
Facile synthesis of rGO@In2S3@UiO-66 ternary composite with enhanced visible-light photodegradation activity for methyl orange
T
Chuanxian Gan, Chen Xu, Hang Wang, Na Zhang , Jianyong Zhang , Yongzheng Fang ⁎
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Shanghai Institute of Technology, Shanghai, 201418, PR China
ARTICLE INFO
ABSTRACT
Keywords: rGO@In2S3@UiO-66 Ternary composite Visible-light Photocatalysis Methyl orange
In this study, rGO@In2S3@UiO-66 ternary composites have been designed and fabricated via a facile two-step hydrothermal process and used as visible-light-driven photocatalyst. The as-prepared samples have been characterized by PXRD, SEM, TEM, XPS, UV–vis DRS, PC, PL and BET. The photocatalytic performance is evaluated by the degradation of methyl orange (MO) under visible-light irradiation. The results show that the photocatalytic performance of rGO@In2S3@UiO-66 is superior to that of In2S3, UiO-66 and In2S3@UiO-66. The enhanced photocatalytic performance of rGO@In2S3@UiO-66 can be attributed to the relative large specific surface area, the improving light absorption region caused by rGO and the promoting the separation of photo-induced carriers by the synergistic effect of rGO, In2S3 and UiO-66. In addition, the rGO@In2S3@UiO-66 composites also demonstrate outstanding photocatalytic stability and there is only 5.8% loss of degradation rate after five cycles. Furthermore, the reasonable photocatalytic mechanism for the degradation of MO by rGO@In2S3@UiO-66 was also proposed.
1. Introduction
researches to overcome these limitations. For instance, Khanchandani et al. [12] have reported the ZnO@In2S3 core/shell nanorod arrays, the photocatalytic activity under visible light to degrade rhodamine B (RhB) was enhanced by these core/shell nanostructures due to the formation of heterojunctions, which prolong the separation of photogenerated electrons and holes. Nevertheless, it is still of great interest to further improved the photocatalytic performance for practicalapplication in water treatment. Metal-organic-frameworks (MOFs) is a group of porous crystalline materials which synthesized via the self-assembly of metal ions/clusters and organic ligands [13,14]. Recently, MOFs are explored to apply in many promising fields, such as gas storage, chemical sensors, heterogeneous catalysis, and biomedicine due to their high specific surface area, tunable pore size, great chemical variety, and relatively good thermo stability [15–18]. Very recently, some certain MOFs have attracted huge attention in photocatalysis due to their semiconductor behavior [19], which means that those MOFs can be designed as photocatalysts due to they can facilitate charge transfer and directly harvest light irradiation [20–31]. Among them, UiO-66, a typical MOF which isconstructed form the Zr6(OH)4O4(CO2)12 cluster, can be regarded as a promising candidate for the development of heterogeneous photocatalysts for water treatment due to its extraordinarily chemical stability [31] and water stability as UiO-66 is capable to maintain the
In the past years, semiconductor-based photocatalysts has attracted enormous attentions in degradation of organic pollutants for water treatment due to their no secondary-pollutants generation and direct solar-to-chemical conversion [1,2]. Particularly, visible-light-driven semiconductors are more appealing for their effectual utilization of the solar energy [3,4]. Recently, metal sulfides has been exhibited as a group of advantageous photocatalysts for the degradation of organic pollutants in in aqueous solution, such as CdS [5], Bi2S3 [6] and In2S3 [7] Among them, β-In2S3, an III–VI group sulfide with defect spinel structure, has received a special attention due to its wide response to solar energy own to the suitable band-gap (2.0–2.3 eV) and stable in the photocatalytic reaction process own to the anti-photo-corrosion property [8–10]. More importantly, the conduction band (CB) of In2S3 is composed of d, s and p orbital, while the valence band (VB) consists of S 3p orbital, which are much more negative than O 2p orbital, the characteristics of band edges could lead to the reduction reactions more available [11]. However, in spite of the advantage in of In2S3 in photocatalysis, there are some problems which limit the practicalapplicationof In2S3 as follows: fast recombination of photo-generated electron (e−) and hole (h+) pairs, easy agglomeration and difficult separation from aqueous system for reuse. Recently, many scientists have done
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Corresponding author. E-mail addresses: [email protected] (N. Zhang), [email protected] (J. Zhang).
https://doi.org/10.1016/j.jphotochem.2019.112025 Received 31 May 2019; Received in revised form 2 August 2019; Accepted 5 August 2019 Available online 10 August 2019 1010-6030/ © 2019 Elsevier B.V. All rights reserved.
Journal of Photochemistry & Photobiology A: Chemistry 384 (2019) 112025
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structure in the water for several months [32,33]. However, due to the relatively wide band gap, UiO-66 can hardly response to visible light, which make a limit to UiO-66 for practical application [33]. To overcome this problem, incorporating UiO-66 together with other certain semiconductor material for a visible-light-driven heterostructure photocatalyst is a great strategy. For that semiconductor@UiO-66 heterostructure, the semiconductor nanoparticles can disperse on UiO-66 surface due to the large surface area of UiO-66. UiO-66 acts as a host matrix and mediator in the preparation process, and the synergistic effect between semiconductor and UiO-66 can prolong the transmission path of photo-generated carriers and inhibit the recombination of photo-generated electron (e−)-hole (h+) pairs. For instance, CdS@UiO66 [34] and Bi2WO6@UiO-66 [35], those composites obtained by incorporating UiO-66 together with a certain narrow band gap semiconductors demonstrate an enhanced photocatalytic performance than each pristine semiconductor and UiO-66. Recently, we have also prepared a core–shell CdS@NH2-MIL-125(Ti) photocatalyst.The photodegradation efficiencies under visible light for organic pollutants (such as rhodamine B, phenol and oxytetracycline) were higher than that of pure CdS and pure NH2-MIL-125(Ti) due to the opened porous structure, effective transfer of photo-generated carriers and the synergistic effect between NH2-MIL-125(Ti) and CdS [30]. Graphene (GR) including graphene oxide (GO) and reduced graphene oxide (rGO) is a unique sp2 hybrid carbon network material [36] with superior charge carrier mobility, and high electrical conductivity [37]. Due to those excellent characteristics, GR was always used to combine with other semiconductor to form a heterostructure to promote the separation of photo-generated charge carriers and thus increase their lifetime, therefore, improving the photocatalytic performance. It has been reported that the heterostructure photocatalyst formed by coupling GR and semiconductor, such as rGO@TiO2 [38], rGO@Ag3PO4 [39] and rGO@MIL-53(Fe) [40]. In our previous work [41], the In2S3@UiO-66 composites were fabricated and exhibited enhanced photocatalytic ability than pure In2S3 and UiO-66 under visible light irradiation. To find photocatalyst with better photocatalytic performance, the rGO was introduced into the In2S3@UiO-66 system, and the rGO plays a supporting role in ternary composite catalysts. It contacts with the binary composite catalysts consisting of UiO-66 and In2S3. The electrons generated by the binary composite catalysts can migrate to rGO, which increases the active sites of the reaction, so that the photocatalytic performance of the ternary catalysts can be improved. Based on these, the rGO@In2S3@UiO-66 ternary composite photocatalyst has been tried to prepare. Finally the resulting rGO@ In2S3@UiO-66 ternary composite were believed to demonstrate superior photocatalytic ability than In2S3@UiO-66 composite. Herein, in this paper, rGO@In2S3@UiO-66 ternary composites with enhanced photocatalytic ability were prepared. Firstly, the octahedral UiO-66 was used as substrate and mediator for the growth of In2S3 nanoparticles during the hydrothermal process. After that, the In2S3@ UiO-66 composite was dispersed in GO solution to obtain rGO@In2S3@ UiO-66 ternary composite during the hydrothermal process. The photocatalytic performance and recyclability of freshly-synthesized rGO@ In2S3@UiO-66composites were investigated in detail by employing the photo-degradation of MO as model reaction under visible light irradiation.
2.2. Synthesis and preparation 2.2.1. Synthesis of UiO-66 UiO-66 octahedrons were synthesized via a solvothermal method following the previous report [41]. Briefly, ZrCl4 (1.0 mmol, 233.0 mg) and H2BDC (1.0 mmol, 166.0 mg) were dissolved in the mixed solution consist of 100 mL N,N′-dimethylformamide (DMF) and 23 mL acetic acid which was added to turn the morphology of UiO-66. The obtained mixture was stirred for 1 h under room temperature and then sealed in a Teflon-lined autoclave and heated in an oven at 120 °C for 24 h. After cooling to room temperature, the white solid products were collected by filtration and thoroughly washed by deionized (DI) water and ethanol for three times, and then dried at 60 °C in the vacuum drying oven for 6 h. 2.2.2. Preparation of In2S3@UiO-66 composites In2S3@UiO-66 composite were prepared by the growth of In2S3 nanoparticles upon the surface of UiO-66 octahedrons via a facile hydrothermal method. Typically, In(NO3)3·xH2O (149.0 mg, 0.496 mmol) was dissolved in 50 ml DI water, thereafter, 200 mg as-obtained UiO-66 was dispersed in In(NO3)3·xH2O solution, after stirring for 1 h under room temperature, Na2S·9H2O (357.0 mg, 1.488 mmol) was added. After that, the total mixture was stirred vigorously for another 1 h, and then sealed in a Teflon-lined autoclave and heated in an oven at 180 °C for 16 h. After cooling to room temperature, the solid products were collected by filtration and thoroughly washed by DI water and ethanol for three times, and then dried at 60 °C in the vacuum drying oven for 6 h, the as-obtained In2S3@UiO-66 composite process a weight ratio of In2S3:UiO-66 of 40%, which has been proved as an optimal ratio according to our previous work [42]. The pure In2S3 was also prepared under same condition except the absence of UiO-66. 2.2.3. Preparation of rGO@In2S3@UiO-66 composite Graphene oxide (GO) was synthesized by a modified Hummers method [39]. The rGO@In2S3@UiO-66 composite were prepared by a facile two-step hydrothermal method. Briefly, 20.0 mg GO was ultrasonically dispersed in 40.0 ml DI water for 4 h. After that, 200.0 mg asobtained In2S3@UiO-66 was added in the GO solution. The resulting suspension was stirred vigorously for another 1 h and then sealed in a Teflon-lined autoclave and heated in an oven at 150 °C for 8 h. After cooling to room temperature, the solid products were collected by filtration and thoroughly washed by DI water and ethanol for three times, and then dried at 60 °C in the vacuum drying oven for 6 h. The pure rGO was also prepared under same condition except the absence of In2S3@ UiO-66. 2.3. Characterization Powder X-ray diff ;raction (PXRD) was investigated via a diffractometer (Bruker D8 Advance) using Cu Kα radiation at 35 mA and 35 kV with a scanning speed of 5° min−1. The morphologies and size of samples were conducted through a scanning electron microscopy (SEM) (Hitachi S-4800 II) and transmission electron microscopy (TEM) (FEIJEM-2100). The surface valence state and the chemical composition of samples were explored by X-ray photoelectron spectroscopy (XPS) on a Physical Electronics spectrometer (PHI-5702) equipped with a monochromatic Al K X-ray source at 150 W. The UV–Vis spectra were recorded on a spectrophotometer (Agilent Cary 5000) with a 200–800 nm wavelength range. The photoluminescence (PL) spectra were obtained by a Varian Cary Eclipse spectrometer (Hitachi F-7000) with an excitation wavelength of 297 nm. N2 isothermal adsorption experiments were conducted on a micromeritics surface area analyzer (ASAP-2020) at 77 K, and the samples were degassed at 120 °C for 6 h in vacuum before the measurements.
2. Experimental 2.1. Materials Sodium sulphide (Na2S·9H2O), indium nitrate (In(NO3)3·xH2O), 1,4benzendicarboxylic acid (H2BDC), zirconium tetrachloride (ZrCl4), graphite powder, potassium permanganate (KMnO4), sodium nitrate (NaNO3), hydrogen peroxide solution (H2O2), sulfuric acid (H2SO4), acetic acid (CH3COOH) and N,N′-dimethylformamide (DMF) are of analytical grade and used as received without further purification. 2
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smooth surfaces、relative large average edge length (∼400 nm) and good dispersion, all this features are beneficial for UiO-66 acting as a matrix for In2S3 nanoparticles growth. For In2S3@UiO-66 composite (Fig. 2c), it can be observed that UiO-66 still maintain the original octahedral shape with good dispersion, indicating that the UiO-66 crystalline structure have not been destroyed after the growth of In2S3 nanoparticles, which is consistent with the previous PXRD analysis. However, the surface of In2S3@UiO-66 block are not smooth yet, but coated with many small nanoparticles which can be believed as In2S3 nanoparticles. It was very interesting to find that the size of In2S3 nanoparticles embedded on the UiO-66 was smaller than the pure In2S3, implying the confinement effect of the UiO-66 [45]. These observations suggest that the UiO-66 has been functionalized with the In2S3 nanoparticles firmly, yielding the In2S3@UiO-66 heterostructures. As shown in Fig. 2d, the UiO-66 is intimately enwrapped by rGO nanosheets under the pronounced electrostatic interactions due to the negatively charged GO and the positively charged UiO-66, and meanwhile, the In2S3 nanoparticles are coating on the UiO-66 surface, hence, the rGO@ In2S3@UiO-66 ternary composite was obtained. To further explore the morphologies and microstructures of rGO@ In2S3@UiO-66, TEM and HRTEM are used. As shown in Fig. 3a, the big block coated with small nanoparticles should be In2S3@UiO-66, and the rGO (in the yellow circle marked in Fig. 3a) can also be detected, which enwrapped with In2S3@UiO-66 block as well as act as a bridge to connect every adjacent In2S3@UiO-66 blocks. Hence, in such structure, photo-generated electrons can fast migrate from In2S3@UiO-66 heterojunction to rGO nanosheets, which can effectively promote the separation of photo-generated electrons (e−) and holes (h+), and inhibit their recombination. As shown in Fig. 3b, the lattice spacing around 0.323 nm can still be found, which agree well with the crystallographic (109) spacing of In2S3 [46], further confirming that the growth of In2S3 nanoparticles onto UiO-66 surface. In addition, it can be observed that there is a clear interface between In2S3 and UiO-66, which is act as conductive channel to the transmission and separation of photo-generated carriers. The elemental composition and surface valence state of as-prepared samples was investigated by X-ray photoelectron spectroscopy (XPS). Fig. 4 shows the XPS survey spectrum of the In2S3 and rGO@In2S3@ UiO-66, and high-resolution XPS spectra of In 3d, S 2p, Zr 3d and C 1s in the rGO@In2S3@UiO-66 composite. As shown in Fig. 4a, the rGO@ In2S3@UiO-66 composite mainly consists of In, S, Zr, C and O elements, as a comparison, the pure In2S3 only contains In and S. Fig. 4b shows that the high-resolution In 3d peaks are centered at approximate 444.8 eV and 452.4 eV, which can be attributed to the binding energies of In 3d5/2 and In 3d3/2, Fig. 4c shows that the high-resolution S 2p peak can be divided into two peaks at 161.5 eV and 162.5 eV, ascribing at the binding energy of S 2p3/2 and S 2p1/2, respectively. Therefore, it can be deduced that the In and S existed as In3+ and S2−, which providing a clear evidence of the existence of In2S3 in rGO@In2S3@UiO-66 composite [41]. Fig. 4d shows the high-resolution Zr 3d peaks, the apparent two peaks observed at 182.7 eV and 185.1 eV correspond to the binding energies of Zr 3d5/2 and Zr 3d3/2, which can be believed that the Zr element was came from UiO-66 [33]. As shown in Fig. 4e, the high-resolution C 1s can be deconvoluted into three surface components, the binding energy of 284.7 eV and 287.7 are assigned to CeC/C]C and OeC]C, respectively, corresponding to the sp2 carbon of rGO and benzoic rings of BDC linkers, and the binding energy of 286.1 eV is also assigned to CeO, corresponding to rGO [38,43]. The existence of rGO in rGO@In2S3@UiO-66 composite will further confirm by Raman spectra. To investigate the vibrational properties of the as-prepared samples, Raman measurements have been carried out. Fig. 5 shows the Raman spectra of GO, rGO and rGO@In2S3@UiO-66. For the sample related to GO and rGO, it can be observed that two prominent bands at 1341 cm−1 and 1593 cm−1, corresponding to the local defects or disorder atomic arrangement of sp3-carbon (D-band) and the plane
Fig. 1. The PXRD patterns of different samples.
2.4. Photocatalytic measurement The photo-degradation experiments of various as-obtained samples were executed under visible light irradiation. The experiment details were shown as follows: 30 mg as-obtained sample was dispersed in 60 mL MO (15 mg/L) solution in a 100 mL beaker placed on top of a magnetic stirrer with water bath. Before irradiation, the mixture was stirred under dark for 40 min to achieve the adsorption-desorption equilibrium. Thereafter, the mixture was irradiated by a 500 W Xe lamp equipped with a UV-cut-off filter (λ =420 nm), and the irradiation intensity in the center of the beaker is100 mW cm−2. At a certain time of 10 min intervals, 3 ml reaction solution was extracted and centrifuged to separate the photocatalyst and clear liquid. After that, the clear liquid was monitored by the UV–vis absorbance at 464 nm to measure the concentration of MO. 3. Results and discussion Fig. 1 shows the PXRD patterns of different samples. The diffraction pattern of pure UiO-66 fits well with previous report [40], indicating the successful synthesis of UiO-66. GO shows a sharp and intense diffraction peak at 10.3°, which can be attribute to the (001) diffraction planes of GO. After hydrothermal treatment of GO, the sharp and intense diffraction peak at 10.3° is disappeared, meanwhile, a broad and weak diffraction peak at 25.0° appears instead, indicating that the GO has been converted to rGO during the hydrothermal treatment process [41,44]. The diffraction peaks of the pure In2S3 appears at 27.40°, 33.45°, and 47.90°, which can be attributed to the (109), (0012) and (2212) diffraction planes of β-In2S3 phase (JCPDS card no. 73-1366), respectively [45]. The In2S3@UiO-66 composite show a mixed diffraction peak of both In2S3 and UiO-66, suggesting that the In2S3 nanoparticles incorporate successfully with UiO-66, and the UiO-66 crystalline structure have not been destroyed after the growth of In2S3 nanoparticles, the same phenomenon can be also observed in the diffraction pattern of rGO@In2S3@UiO-66 composite. However, no apparent characteristic diff ;raction peaks of rGO at 25.0°can be observed in rGO@In2S3@UiO-66, a reasonable explanation is that the rGO is of relatively low content and crystallinity in the composite, and this phenomenon is reported in previous work [38]. The existence of rGO in rGO@In2S3@UiO-66 will be verified by the next XPS and TEM tests. The scanning electron microscope (SEM) is used to provide a direct insight of the as-prepared samples. As shown in Fig. 2a, the panoramic view of the pure In2S3 nanoparticles demonstrate poor dispersion morphology and large agglomeration, which is unfavorable to photocatalysis activity due to the reducing of active sites. As shown in Fig. 2b, the as-obtained UiO-66 are octahedral microcrystals with sharp edges、 3
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Fig. 2. SEM images of (a) pure In2S3, (b) pure UiO-66, (c) In2S3@UiO-66, (d) rGO@In2S3@UiO-66.
vibration of the sp2-carbon in the two-dimensional lattice (G-band), respectively [34]. The intensity ratio between the D and G bands (ID/IG) has been widely used to evaluate the oxygen containing functional groups on the surface of carbon nanomaterials, the intensity ratio of D to G bands (ID/IG) of GO is 0.99, which is lower than that of rGO (ID/ IG = 1.06), confirming the efficient reduction of GO during hydrothermal process [40]. In the case of rGO@In2S3@UiO-66 composite, the intensity ratio of D to G bands is 1.19, indicating the existence of rGO, the result further confirm the successful combination between rGO nanosheets and In2S3@UiO-66. Meanwhile, the optical absorption of as-prepared rGO@In2S3@UiO66 composite was characterized by UV–vis diff ;use reflectance spectroscopy (DRS), and for comparison, In2S3, UiO-66 and In2S3@UiO-66 were also executed. As shown in Fig. 6a, the pure UiO-66 nanocrystals only exhibit an optical absorption edge about 300 nm, corresponding to the white color. As a comparison, the pure In2S3 demonstrate an optical absorption edge about 580 nm, corresponding to the orange color, indicating that In2S3 possesses a wide response range to visible light. For the sample related to In2S3@UiO-66, the spectra show a mixed absorption properties of both In2S3 and UiO-66, further confirming the combination between In2S3 and UiO-66, correspondingly, the color changes from orange to light-orange. After coupling an appropriate amount of rGO, it can be observed that the rGO@In2S3@UiO-66
composite keep the absorption feature compare with In2S3 and UiO-66 in the range of 200–500 nm, meanwhile, the absorption intensity in the visible region (about 500 ˜ 800 nm) increase obviously [48]. The changes of absorption edge are corresponding with the change of the sample’s color, and it can be observed that the color is turn to black in rGO@In2S3@UiO-66 composite. Obviously, the enhanced absorption in visible region is also advantageous in promoting the photocurrent response of rGO@In2S3@UiO-66 composite. Moreover, the band gap energy (Eg) of as-obtained In2S3 and UiO-66 samples were calculated by the following equation: (ahv)2 = A(hv-Eg)
(1)
where a, h, v, A and Eg indicate absorption coefficient, Planck constant, light frequency, proportionality and band gap energy, respectively [43]. The results are shown in Fig. 6b, the Eg of In2S3 and UiO-66 were calculated as 2.2 eV and 4.1 eV, respectively, which is similar to the previous reports [34,46]. Fig. 6c shows the photoelectric current with time (i–t) curves for all the samples under visible-light irradiation. The rGO@In2S3@UiO-66 composite display highest photocurrent intensity among In2S3, UiO-66, In2S3@UiO-66 and rGO@In2S3@UiO-66, demonstrating that the separation efficiency of the photo-generated electron-hole pairs of rGO@ In2S3@UiO-66 composite is highest. The result is well in agreement
Fig. 3. (a) TEM, (b) HRTEM images of rGO@In2S3@UiO-66. 4
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Fig. 4. The XPS spectra: (a) survey scan, (b) In 3d (c) S 2p, (d) Zr 3d, (e) C 1s.
exhibits a strong luminescence centered at about 380 nm, which could be attributed to the band gap recombination of electron-hole pairs, corresponding to previous report [28]. Comparatively, the PL intensity of In2S3@UiO-66 decreases significantly, which can be attributed to the efficient separation and recombination of photo-generated electronhole pairs due to the fabrication of heterostructure between In2S3 and UiO-66. Furthermore, after rGO modification, the rGO@In2S3@UiO-66 demonstrates the lowest PL intensity, suggesting the lowest recombination rate of photo-generated electron-hole pairs, which can be attributed to that rGO can serve as an eff ;ective acceptor of the photogenerated electrons, hence, the photo-generated electrons can be transferred to rGO sheets, thus eff ;ectively suppressing the charge recombination and improving the photocatalytic activity [49]. The N2 adsorption-desorption isotherms as well as the corresponding values of the BET surface area (SBET) and total pore volume (Vt) have shown in Fig. 6e and Table 1, respectively. For the pure In2S3 nanoparticles and UiO-66, the N2 isotherms are type-I and without hysteresis loops, and the SBET are 90.3 m2 g−1 and 1347.8 m2 g−1, and the Vt are 0.107 cm3 g−1 and 0.684 cm3 g−1, respectively, noting that In2S3 can be negligible compared with UiO-66. For the In2S3@UiO-66 composite, the N2 isotherms are still type-I, but with small hysteresis loops at high relative pressure (P/P0∼1), indicating the appearance of mesopores arised from the stacking of In2S3 nanoparticles, and corresponding, the SBET and Vt are 629.5 m2 g−1 and 0.401 cm3 g−1, which is reduced compared with UiO-66. For the rGO@In2S3@UiO-66 ternary
Fig. 5. The Raman spectra of GO、rGO and rGO@In2S3@UiO-66.
with the afore-mentioned PL studies and the photocatalytic experimental results. Photoluminescence (PL) test was employed to investigated the eff ;ect of rGO modification on separation and recombination of photogenerated electron–hole pairs in rGO@In2S3@UiO-66 composite. All the samples were excited by the photo-excitation wavelength of 297 nm at room temperature and the results were shown in Fig. 6d, UiO-66 5
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Fig. 6. (a) UV–vis absorption spectra of different samples; (b) plots of the (ahv)2 versus (hv) for In2S3 and UiO-66 samples; (c) the photocurrent responses for different samples; (d) room-temperature stable state photoluminescence spectra of different samples; (e) The N2 adsorption-desorption isotherms of diff ;erent samples obtained at 77 K; (f) the photo of the different samples.
equilibrium in 40 min. The pure In2S3 demonstrates an obvious weaker adsorption performance than pure UiO-66 and its composites and the adsorption efficient is only 5.5%, which may be due to the relative low specific surface area. The pure UiO-66 exhibits the highest adsorption efficient as 68.5%, which can be attributed to its large specific surface and porous structure [42]. The adsorption efficient of 51.2% of In2S3@ UiO-66 composite is lower than that of pure UiO-66, which can be ascribed to the reducing of specific surface area of In2S3@UiO-66 compared with UiO-66. After coupling with rGO, the rGO@In2S3@UiO66 composite possesses the weak adsorption performance than In2S3@ UiO-66, which is consistent with the BET analysis. After visible light irradiation for 1 h, no MO self-photolysis can be observed without photocatalyst in blank experiment, indicating that the impact of MO self-degradation can be ignored during the photocatalytic process. In addition, the pure UiO-66 demonstrates a very weak photocatalytic activity due to its wide band gap (4.1 eV). The rGO@In2S3@UiO-66 composite exhibits the highest total MO removal efficient of 98.1%, asacontrast, the total MO removal efficient of In2S3@UiO-66 and In2S3 is 96.2% and 39.0%, respectively. Furthermore, to investigate the photocatalytic ability in a more systematic manner, the pseudo-first-order kinetic model [47] is used.
Table 1 The SBET, pore size and pore volume data obtained for the various samples. Samples
SBET (m2 g−1)
Vt (cm3 g−1)
In2S3 UiO-66 In2S3@UiO-66 rGO@In2S3@UiO-66
90.3 1347.8 629.5 487.7
0.107 0.684 0.401 0.306
composite, the SBET and Vt further reduce to 487.7 m2 g−1 and 0.306 cm3 g−1, the reduced SBET and Vt can be attribute to the enwrapping of rGO. The adsorption and photocatalytic performances of as-obtained samples are evaluated through the removal of the MO, since MO is a typical organic dye. To eliminate the impact of self-degradation of MO in the photocatalytic process, blank experiment was excluded under the same condition without photocatalyst. Fig. 7a shows the total MO removal efficiency of different as-obtained samples. The removal efficiency is estimated by the 100(1-C/C0)%, where C0 and C is the initial and actual concentration of MO in the reaction system, respectively. It can be observed that all the samples can reach the adsorption 6
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Fig. 7. a) MO degradation performance of diff ;erent samples, (b) pseudo first-order kinetics of MO photodegradation of different samples, (c) the stability of the rGO@In2S3@UiO-66 over 5 cycles to MO degradation.
UiO-66,which can be attributedto the superior charge carrier mobility and high electrical conductivity as well as improving light absorption region due to the doped rGO. The stability of a photocatalyst is very important role for practical application. Thus, the circulating experiment over rGO@In2S3@UiO-66 sample is carried out to monitor the stability. As shown in Fig. 7c, the degradation ratio is decreasing from 98.1% to 92.3% after 5 cycles, which is only an approximate 5.8% loss. The results demonstrate the outstanding stability of rGO@In2S3@UiO-66 composite, and open a new door in the field of MOF-based photocatalysts for photocatalytic water treatment. In order to explore the active species for MO degradation of the catalyst, the sample rGO@In2S3@UiO-66 with the highest catalytic activity was taken as an example. In heterogeneous photocatalytic reaction, three active substances, superoxide radical (O2%−), hydroxyl radical (·OH) and hole (h+), are mainly involved [13,50,51]. Therefore, aiming at the above active substances, isopropanol (IPA), p-benzoquinone (BQ) and ethylenediaminetetraacetic acid (EDTA) were used as scavengers of %OH, O2%− and h+, respectively, to judge the active substances produced in the process of photocatalytic degradation [52]. Results As shown in Fig. 8, compared with the degradation curve without any eliminating agent, it was found that adding 500 mM IPA in the photocatalytic reaction system had no obvious effect on the degradation rate, indicating that %OH was not the main active substance in the degradation process; however, after adding 1 mM BQ and 1 mM EDTA respectively, the degradation process became slower obviously, indicating that O2%−, and h+ played a major role in the reaction system. Based on the above analysis, a possible photocatalysis mechanism for the photodegradation of MO over rGO@In2S3@UiO-66 under visible-light irradiation is proposed as shown in Scheme 1. The Eg of In2S3 (2.2 eV) is lower than that of UiO-66 (4.1 eV), and according to our previous work [34], the ECB of In2S3 and UiO-66 are −0.8 eV (vs. NHE) and −0.6 eV (vs. NHE), the EVB of In2S3 and UiO-66 are 1.4 eV (vs.NHE) and 3.5 eV (vs. NHE), respectively. Due to the ECB and EVB of In2S3 are both higher than these of UiO-66, hence, the In2S3@UiO-
Fig. 8. The effects of different scavengers on the degradation of MO in the presence of rGO@In2S3@UiO-66 under visible-light irradiation.
The corresponding formula is as follows: -ln(C/C0)=kt, where t is the reaction time, C0 is the concentration after equilibrium for 40 min and C is the concentration at t time in the reaction system, and k is the relative degradation rate constant and its value is equal to the slope of -ln (C/C0) versus time plots. As shown in Fig. 7b, the k value of various samples follow the following sequence: rGO@In2S3@UiO-66 (0.04142 min−1) > > In2S3 (0.05857 min−1) > In2S3@UiO-66 −1 −1 (0.00437 min ) > UiO-66 (0.00064 min ). According to this order, In2S3@UiO-66 demonstrates a superior photocatalytic ability than that of pure In2S3 and UiO-66, indicating that the synergistic effect between In2S3 and UiO-66 in In2S3@UiO-66 heterostructure can effectively promotes the separation and suppresses the secondary recombination of photo-induced carriers, leading to the enhanced photocatalytic performance. Significantly, rGO@In2S3@UiO-66 composites exhibit an enhanced photocatalytic activities than In2S3@ 7
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Scheme 1. A possible photocatalysis mechanism for the photodegradation of MO under visible-light irradiation.
66 heterostructure is formed. With the visible-light irradiation, the photo-generated electrons (e−) can be excited in the VB of In2S3 and then transfer to the CB, leaving photo-generated holes (h+) on the VB. The h+ can directly oxidize MO [41]. Meanwhile, the e− on the In2S3 nanoparticles are inclined to transfer to the CB of UiO-66 via the In2S3 and UiO-66 intimate contact interfaces, because the ECB of In2S3 is more negative than that of UiO-66. Furthermore, the e−on the In2S3@UiO66 heterojunction can further captured by rGO sheets as rGO is an excellent electrons acceptor, then the e−can migrate quickly throughout the rGO sheets due to its superior charge carrier mobility and high electrical conductivity. Therefore, in such way, the rGO sheets with can facilitate charge separation and reduce the recombination of electron–hole pairs in the rGO@In2S3@UiO-66 photocatalyst, and the similar conclusions have appeared in previous literature [15,16]. As a result, more O2 in water acting as the electron scavenger could be reduced by e− to produce O2%−. Notably, the porous structures of UiO-66 are beneficial for the MO adsorption onto the surface of rGO@In2S3@ UiO-66, then these adsorbed MO molecules can be rapidly decomposed by O2%−, hence, promoting the degradation of MO. 4. Conclusion In summary, an rGO@In2S3@UiO-66 ternary composite has been designed and fabricated via a facile two-step hydrothermal process. In the rGO@In2S3@UiO-66 composite, the In2S3 nanoparticles are coating on the surface of UiO-66 with good dispersion and low agglomeration to form heterostructure, and the rGO enwrap upon In2S3@UiO-66 block as well as act as a bridge to connect every adjacent In2S3@UiO-66 blocks. The rGO@In2S3@UiO-66 composite possess a superior photocatalytic performance under visible-light irradiation compared with In2S3, UiO-66, rGO@In2S3 andIn2S3@UiO-66. The reason of the enhancement in photocatalytic performance can be deduced to the following: (1) The relative large SBET can provide more photocatalytic active sites, and the porous structures of UiO-66 are beneficial for the MO adsorption onto the surface of rGO@In2S3@UiO-66; (2) rGO can significantly improve the light absorption region of the composite; (3) The synergistic effect of rGO, In2S3 and UiO-66 can promotes the separation of photo-induced carriers and suppresses their secondary recombination. Declaration of Competing Interest There are no conflicts to declare. Acknowledgements The authors acknowledge the financial support of the National Natural Science Foundation of China (21707093, 51472162, 51672177 and 21503081), the Shanghai Alliance Project (LM201841, LM201843) 8
Journal of Photochemistry & Photobiology A: Chemistry 384 (2019) 112025
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