Preparation of CeO2 nanorods-reduced graphene oxide hybrid nanostructure with highly enhanced decolorization performance

Applied Surface Science 499 (2020) 143939

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Preparation of CeO2 nanorods-reduced graphene oxide hybrid nanostructure with highly enhanced decolorization performance ⁎

Xiaoqing Dua,b, Zhao Zhangb, , Hong Chena, Pei Liangc,

T

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a

School of Materials Science and Energy Engineering, Foshan University, Foshan, Guangdong 528000, China Department of Chemistry, Zhejiang University, Hangzhou, Zhejiang 310027, China c College of Optical and Electronic Technology, China Jiliang University, 310018 Hangzhou, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: CeO2 NRs-RGO Decolorization Adsorption Methyl blue Synergistic effect

Decolorization by both adsorption and catalysis method, especially for the synergistic effect of these two methods is very important. Here, we report that CeO2 nanorods (NRs) with different amounts (wt%) of reduced graphene oxide (RGO) have been prepared by a facile hydrothermal method. The decolorization tests show that decolorization of methyl blue (MB) is attributed to both catalysis and absorption, though its adsorption over the catalyst CeO2 NRs-RGO mainly influences the decolorization, the visible light irradiation plays a vital role in increasing the decolorization efficiency. Incorporating RGO into CeO2 NRs matrix not only changes surface morphology and area, particle size, band gap, crystal structure and oxygen vacancy content of the latter, but also causes mutual charge transfer between RGO and CeO2 NRs, which therefore increases the decolorization efficiency of CeO2 NRs-RGO to be 95.4% and to be as high as 99.5% after addition of electron scavenger H2O2. The study demonstrates the synergistic effect of adsorption and photo-catalysis are very important in MB decolorization process over CeO2 NRs-RGO (9.2 wt%) and provides a possible way in the design of decolorization agent.

1. Introduction Environmental pollution is one of the major problems hindering the development of the modern society. Pollution caused by industries and agricultures is especially serious due to the widely use of toxic, nonbiodegradable and potential carcinogenic organic synthetic dyes [1]. Many techniques have been applied in removing these pollutants (such as dyes) from industrial effluents, such as membrane filtration [2], coagulation [3], ion exchange [4], electrochemical oxidation [5], adsorption [4,6] and photocatalytic degradation [7–9]. Among these techniques, adsorption is especially welcomed because it can handle large amount of effluents and result in less toxic byproducts, such as wastewater or ozone [4]. Meanwhile, photodegradation has also attracted much attention for its complete mineralization of many toxic organic pollutants [10]. With rapid development of nanoscience and nanotechnology, it is realizable to produce nanomaterials with higher adsorption capacity and better photocatalytic performance which can efficiently remove various contaminants by a combination of adsorption and photo-degradation [11]. Recently, reduced graphene oxide (RGO)-based nanosemiconductor oxide composites have been widely investigated for their excellent specificities and potential applications in energy storage

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and photocatalysis [12–14]. Many researchers have studied the enhanced performance from the combination of RGO with metal oxides, such as TiO2 [15,16], ZnO [17] and WO3 [18]. Cerium oxide (CeO2) is one of the most attractive metal oxide semiconductors for its certain superior catalytic performance and potential technological applications [19–22]. However, the large band gap (3.2 eV) of CeO2 makes it absorb only UV light of the solar spectrum, and the fast recombination between electrons and holes also decreases its photocatalytic activity, both of which limits its application in dye decolorization [8,23]. Based on the above analysis, narrowing the bandgap [15,16] to make it responsive to wider spectral area, especially visible light region, or adding scavenger in the medium [24,25] would be the effective ways to enhance the photocatalytic degradation performance of CeO2 on organic dyes. Recently, the photocatalytic property in degradation of organic dyes by CeO2 has been improved by doping with noble metals, semiconducting metal oxides or other materials, such as Ag [25], Au [26,27], TiO2 [28], CuO [29] or reduced oxidized graphene (RGO) [7,9] and so on. Muthuraj [8] found that the band gap of CeO2 decreased from 3.14 eV to 2.83 eV by doping with Ag, which increased the photodegradation efficiency of methylene blue (MB) on CeO2 to 97%. Samdarshi [7] presented a new method to decorate CeO2 on RGO in situ

Corresponding authors. E-mail address: [email protected] (P. Liang).

https://doi.org/10.1016/j.apsusc.2019.143939 Received 26 April 2019; Received in revised form 30 August 2019; Accepted 9 September 2019 Available online 10 September 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.

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were also prepared by the same hydrothermal method except that GO or Ce(NO3)3·6H2O was not added. The morphology and crystallographic properties of CeO2 NRs and CeO2 NRs-RGO nanocomposites were investigated by field-emission scanning electron micrographs (FE-SEM) (Hitachi SU 8010), X-ray diffraction (XRD) (Rigaku Ultima IV) and high-resolution transmission electron micrographs (HRTEM) (JEOL 2100 F). The X-ray photoelectron spectroscopy (XPS) analysis were performed on a VG ESCALAB MARK II spectrometer with the Mg Ka radiation (1253.6 eV), operating at constant pass energy mode at 50 eV. The surface charging effect was corrected by fixing the C1s peak at the binding energy of 284.6 eV. The absorption of solutions containing different concentration of MB was measured by UV–vis spectrophotometry (Shanghai Xinmao Instrument Co., Ltd. UV-7502 PC). Diffuse reflectance spectra (DRS) of CeO2 NRs and CeO2 NRs-RGO were recorded in the wavelength range from 200 to 800 nm using a Shimadzu UV–vis spectrophotometer (UV 2550, Japan) with an integrating sphere. The Raman spectra were recorded with a LabRam HRUV spectrometer, equipped with a confocal microscope with a focal spot size about 40 μm, using an incident beam of 514 nm emitted by an argon laser. Specific surface area of catalyst was measured by nitrogen adsorption/desorption isotherms at 77 K using ASAP 2020 M instrument (USA), and analyzed by Brunauer-Emmet-Teller (BET) method. Fourier transform infrared (FTIR) spectroscopy (iS 50, 142 Nicolet) was used to analyze the changes in functional groups of CeO2 NRs-RGO nanocomposites with or without adding trapping agent. Zeta potential of CeO2 NRs-RGO nanoparticles dispersed in deionized water, as a function of pH, was measured via electrophoretic light scattering (ELS) combined with phase analysis light scattering (PALS) (Malvern Zetasizer Nano ZS) and the pH value of suspension was adjusted with 0.15 mol/L HCl and 0.1 mol/L NaOH solution. The decolorization efficiency (ηDE) of MB was calculated as the following equation:

and used the prepared CeO2-RGO nanocomposite as photocatalyst to degrade methyl orange (MO). They proposed that MO decoloration is mainly attributed to the enhanced photocatalytic activity of the nanocomposite which was caused by incorporating RGO, though RGO incorporation also enhances the adsorption of dye to some extent. Another research [9] showed that the prepared CeO2-RGO nanocomposite also exhibited enhanced photodegradation efficiency in MB decoloration under sunlight. Also they ascribed the excellent photocatalytic activity to a smaller band gap in CeO2-RGO nanocomposite and a decreased recombination rate of photo-generated electron and hole pairs, which was caused by incorporating RGO. While, the high electron mobility and large surface area provided by the unique 2D planar structure of RGO also makes it an excellent adsorbent [30,31].Theoretically, after incorporating RGO to CeO2, the surface area of the nanocomposites CeO2-RGO would increase greatly, which may also lead to a highly enhance in decoloration efficiency. Moreover, adsorption process is the first step during the whole decoloration process and the photocatalytic efficiency also depends partly on the amount of the adsorbed catalysts. Therefore, in order to clarify the reason for the increased decoloration efficiency of MB on CeO2-RGO nanocomposites, CeO2 nanorods (NRs) decorated with different amount of RGO (CeO2 NRs-RGO) nanocomposites have been prepared by a facile hydrothermal method. The atomistic structure, morphology, light absorption properties and decoloration ability of CeO2 NRs and CeO2 NRs-RGO both in dark and under visible light irradiation have been comparatively studied. At last the decoloration performances for MB on CeO2 NRs-RGO with or without adding scavengers have also been investigated so as to find out the possible complete decolorization pathway of MB over catalyst CeO2RGO. 2. Experimental sections

ηDE =

Graphite powder (≥99.8%), cerium (III) nitrate hexahydrate (Ce (NO3)3·6H2O), methylene blue trihydrate (MB·3H2O), sulfuric acid (H2SO4, 98.8%), potassium permanganate (KMnO4, ≥99.5%), sodium hydroxide (NaOH, AR), ammonium hydroxide (NH3·H2O, 25–28%), hydrogen peroxide (H2O2, 35%), hydrochloric acid (HCl, 36.5%) were bought from Sinopharm Chemical Reagent Co., Ltd. All the used reagents were of analytical grade without any further purification. Graphene oxide (GO) was prepared by modified Hummer's method [32,33]. CeO2 NRs-RGO nanocomposites with different feeding ratio of RGO to CeO2 NRs were synthesized via a simple hydrothermal method. Briefly, 35 mg of GO was first dispersed into 35 mL of DI water followed by ultra-sonication for 1 h to get the GO dispersed completely, then 1 mM (the resultant content of RGO in CeO2 NRs-RGO is 16.9 wt%), 2 mM (RGO: 9.2 wt%),4 mM (RGO: 4.84 wt%), 7 mM (RGO: 2.82 wt%), 13.5 mM (RGO: 1.49 wt%) and 40 mM (RGO: 0.49 wt%) of Ce (NO3)3·6H2O were slowly added into the above GO solution with continuous stirring. Subsequently, the mixed solution was kept stirring for 1 h to form a uniform homogeneous solution. Thereafter, 3 g NaOH (AR.) and 4 mL NH3·H2O (AR. 25–28%) were dissolved in 30 mL DI water by magnetic stirring till the mixture was dissolved completely, then cooled down and added drop-wise into the above mixed solution till the pH became 12. After the mixed solution was kept stirring for another 4 h, it was placed into a 100 mL Teflon-lined autoclave for 24 h at 120 °C, and then cooled down to room temperature. Finally, the resultant colloidal solution was washed several times using DI water and ethanol, then filtered and dried at 60 °C under vacuum condition for 12 h to get the objective black powder of CeO2 NRs-RGO. The complete synthesis process is shown in SI schematic 1, and design philosophy of this article and the mechanism of the hybrid composite is shown in the Fig. 1. The Fig. 1 indicates that the adsorption and the degradation may both affect the decolorization. For the CeO2/RGO case, the incorporation of RGO would give a positive effect on both absorption and degradation since the band gap would be narrowed. CeO2 NRs and RGO

C0 − CT × 100% C0

(1)

where, C0 and CT were the content of MB (μg /g) obtained from the characteristic absorption peak intensity at λ = 664 nm in the UV–vis spectrophotometry before and after decolorization, respectively. The details of the decoloration process of MB under different conditions (in dark circumstance, under visible light environment, with different pH values and different catalyst amounts) and density functional theory calculation used to describe the atomistic structure of the composites are both shown in the SI Supporting Methods. 3. Results and discussion Generally, the material's performance is mainly attributed to its chemical composition and structure, therefore, SEM, TEM, Raman, XPS and DFT are adopted to characterize CeO2-RGO (9.2 wt%) in details (Fig. 2 and Fig. S1). Fig. 2(A-B) and Fig. S1(A-B) show the surface morphologies, chemical composition and elements mapping of CeO2 and CeO2-RGO (9.2 wt%) obtained by SEM and EDS. It can be clearly seen that the shapes of both CeO2 and CeO2-RGO (9.2 wt%) particles are needle-like (or nanorod shaped), and both the length and the diameter of the well dispersed spindly CeO2 nanorods (NRs) (Fig. 2A) are much larger than those of CeO2 NRs-RGO (9.2 wt%) catalyst (Fig. 2B). Generally, the smaller particle size will shorten the migration distance between the photo-generated carriers (electrons and holes) and the reaction sites, leading a decrease in the recombination probability of electrons with holes, and therefore result in an enhanced photocatalytic activity [27]. Meanwhile, the smaller particle size also leads to a larger specific surface area (Table S1) which will result in greater adsorption capacity. Consequently, the addition of RGO should change the band gap (Table S2) and specific surface area, and finally enhance the decolorization performance of CeO2 NRs-RGO catalysts. On the other hand, the EDS results show that without adding RGO, the obtained 2

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Fig. 1. The possible decolorization mechanism of MB over the catalyst CeO2 NRs-RGO (the grey balls are carbon atoms; the red balls are oxygen atoms and the light grey balls are hydrogen atoms).

nanorods consists of two elements Ce and O (Fig. S1(A)) and the atomic ratio of Ce and O is almost 1:2 (inset in Fig. S1(A)), which undoubtedly suggests that the main component of the prepared compound is CeO2. While, after adding RGO(Fig. S1(B)), the nanocomposites consist of three elements C, O and Ce and the atomic ratio of Ce and O is larger than 1:2 (inset in Fig. S1(B)), which clearly proves that RGO has been incorporated into CeO2 NRs. Meanwhile, the uniform distribution of C, O, Ce on CeO2 NRs-RGO catalysts (Fig. 2B) means RGO has been well decorated by CeO2 NRs, which also confirm the strong coupling effect between RGO and CeO2 NRs [7], and the atomistic structure of the CeO2/RGO hybrid composite is shown in the Fig. 2H. Generally speaking, construction of semiconductor heterojunction with hierarchical architectures is highly effective for improving photocatalytic performance. Different heterojunction types with distinct mechanisms lead to different photocatalytic activity enhancement level, and thus the control on heterojunction type is meaningful [34]. Some of the previous report [35] also reported that the interfacial interaction can tune the band structure: the CeO2(111)/GR interface is a type-I heterojunction,

while a type-II staggered band alignment exists between the CeO2(111) surface and RGO. The smaller band gap, type-II heterojunction, and negatively charged O atoms on the RGO as active sites are responsible for the enhanced photoactivity of CeO2 /RGO composite. Moreover, all these can enhance the catalyst effect. Further, in order to illustrate detailed structure of the composite, the Figs. 2(C-D) and Fig. S1(C–F) show TEM images and SAED of CeO2 NRs-RGO (9.2 wt%) nanocomposites RGO and CeO2 NRs. The sheet-like RGO (Fig. S1C) with many wrinkles on its surface (Fig. 2C) possesses the inter-planar spacing (d-spacing) of 0.359 nm (Fig. S1D) and good crystallinity (the hexagonal bright dots shown as an insert in the SAED pattern, Fig. S1D), which is in good accordance with the XRD analysis (Fig. 2E) that only the diffraction peaks corresponding to 〈002〉 lattice plane of RGO are observed. When compared with pure CeO2 (Fig. S1E), only a small amount of needle-like CeO2 nanorods with smaller diameter exist in CeO2 NRs-RGO (9.2 wt%) nanocomposite (Fig. 2C). The SAED characteristics obtained from the HRTEM images (Fig. S1F for pure CeO2, Fig. 2D for CeO2 NRs-RGO) are much similar, both of the

Fig. 2. Morphology and structure of CeO2 and CeO2 NRs-RGO (9.2 wt%) nanocomposites. (A) SEM of CeO2, Inset A: elements mapping of CeO2 (Ce: green, O: red), (B) SEM image CeO2 NRs-RGO (9.2 wt%) Inset B: elements mapping of CeO2 NRs-RGO (9.2 wt%) (Ce: yellow, O: green, C: red) nanocomposites, (C) TEM images of CeO2 NRs-RGO (9.2 wt%), (D) high-resolution TEM image of CeO2 NRs-RGO (9.2 wt%). Inset D: SAED of CeO2 NRs-RGO (9.2 wt%). (E) XRD patterns of GO, RGO, CeO2 and CeO2-RGO of different RGO content (JCPDS: 34-0349, 49-1458), (F) Raman spectra of GO, RGO, CeO2 and CeO2 NRs-RGO (9.2 wt%) nanocomposites,(G) XPS spectra of GO, RGO, CeO2 and CeO2 NRs-RGO (9.2 wt%) nanocomposites, (H) the atomistic structure of the CeO2/RGO hybrid composite. 3

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two samples exhibit four kinds of crystal orientation, which are corresponding to crystal planes 〈111〉, 〈200〉, 〈220〉 and 〈311〉 (JCPDS: 340349), respectively in XRD (Fig. 2E). However, the brightness of the diffraction rings for pure CeO2 (Fig. S1F) seem to be brighter than those in CeO2 NRs-RGO nanocomposite (Fig. 2D), which means that pure CeO2 has better crystallization. This phenomenon is also reflected in XRD patterns (Fig. 2E) that the characteristic peaks of CeO2 become broader after addition of RGO. Because the vibrational energies of the sample molecules in Raman spectroscopy can be used to characterize the presence of defects and predict whether the crystal structure is disordered or not [9], Raman spectroscopy is carried out to analyze the change of electronic structure (Fig. 2F). It can be obviously seen that typical D (defects) and G (graphite) bands only appeared in GO, RGO and CeO2 NRs-RGO (9.2 wt%) nanocomposites with the range of 1250–1675 cm−1. It is reported that the D band is corresponding to sp3 defects in graphene [36], the G band is assigned to the planar stretching vibration of sp2 carbon bonds [37,38], and the larger of the ratio of D and G band intensities (ID/IG) always means the higher of the defects density in GO-based composites [16,39]. Therefore, Fig. 2F clearly demonstrates the result obtained by XRD that many defects have been introduced into the CeO2 crystal structure during its interaction with RGO. Both the obtained ID/IG of RGO (1.09) and CeO2 NRs-RGO (9.2 wt%) (1.11) are higher than that of GO, which also indicates that the defects density of CeO2 NRs-RGO (9.2 wt%) is higher and parts of GO have been reduced to RGO effectively. There exist two more peaks of different intensities in Raman spectra of CeO2 contained samples (Fig. 2F). For CeO2, the prominent peak located at 462 cm−1 is ascribed to the F2g mode of the symmetrical stretching vibration of CeeO8 bond [7], while the weak peak at 596 cm−1 is caused by the intrinsic oxygen defects [9]. However, for CeO2 NRs-RGO (9.2 wt%), the peak located at 462 cm−1 in CeO2 shifts to 454 cm−1 (blue shift), while the other shifts from 596 cm−1 in CeO2 to 607 cm−1 (red shift). Moreover, these two peaks for CeO2 NRs-RGO (9.2 wt%) are somewhat broader than those for pure CeO2 NRs. According to the studies of Xu [29] and Dezfuli [40], such shift and broadening of these peaks indicate the strong interaction between RGO and CeO2, which is beneficial for their mutual charge transfer. Finally, the symmetric 2D and 3S bands in GO and RGO spectra are originated from the boundary of Brillion zone phonon [19] and lattice disorders [41]. Fig. 2G and Fig. S2 show XPS spectra of RGO, CeO2 and CeO2 NRsRGO nanocomposites. The presence of Ce4d, C1s and O1s (Fig. 2G) indicates the successful fabrication of CeO2 NRs-RGO composite in the current conditions. The fitting results of the high-resolution Ce 3d spectra of CeO2 (Fig. S2A) and CeO2 NRs-RGO (Fig. S2B) show that the Ce 3d spectra of both CeO2 and CeO2 NRs-RGO can be divided into eight peaks, which are labeled using the notation of Burroughs in the spectra. The peaks U, U″, U″′ and V, V″ V″′ refer to 3d3/2 and 3d5/2, respectively, which are corresponding to Ce(IV)3d final states. The doublets U/V, U″/V″ and U″′/V″′ are assigned to the state of Ce(IV) 3d94f2O2p4, the hybridization state of Ce(IV) 3d94f1O2p5 and the final state of Ce(IV) 3d94f0O2p6, respectively [42]. While, the doublet U′ and V′ refer to 3d3/2 and 3d5/2 and are assigned to Ce(III)3d final states [42], which indicates the existence of Ce(III) ions and the presence of oxygen vacancies in CeO2 [26]. In order to clarify the difference of oxygen vacancy content between CeO2 and CeO2 NRs-RGO, Ce (III) contents in CeO2 and CeO2 NRs-RGO are determined by using the ratio of the integrated peak areas corresponding to Ce (III) peaks to the total area under Ce 3d peak [43]. The fitting results are listed in Table 1. It can be seen that Ce(III) content increases from 14.86% (CeO2) to 19.62% (CeO2 NRs-RGO), which indicates the density of oxygen vacancies in CeO2 NRs-RGO is higher than that in CeO2. This result agrees well with that obtained by Yang [44]. The increase of oxygen vacancies density may be an indication of charge transfer between the interface of GO and CeO2 NRs [45], which further supports the results obtained

Table 1 The fitting results of the Ce3d spectra of CeO2 NRs and CeO2 NRs-RGO (9.2 wt %) nanocomposites. Ce 3d

U″′ U″ U′ U V″′ V″ V′ V Total Ce (III) (III)%

CeO2 NRs-RGO

CeO2 NRs

Peak position(eV)

Peak area

Peak position(eV)

Peak area

915.18 907.87 904.74 899.69 896.70 888.68 886.14 881.13

6544.92 5213.95 4744.98 7395.73 9155.51 7994.90 6344.82 9535.73 56,530.54 11,089.80 19.62

916.27 907.26 904.63 900.73 897.84 890.06 887.69 882.32

6086.50 6800.05 2114.88 6362.09 9162.26 6948.92 6026.15 11,301.64 54,802.48 8141.02 14.86

from Raman spectrum that charge transfer exists between GO sheet and CeO2 NRs. The C1s XPS spectrum of CeO2 NRs-RGO (Fig. S2C) is much similar to that of RGO (Fig. S2D). There exist four fitting peaks at 279.6, 280.3, 282 and 283.6 eV for RGO, which are ascribed to C]C or CeC, CeC, COH, and O-C=O groups, respectively [7], whereas the binding energy of all the above peaks for CeO2 NRs-RGO shifts to lower value somewhat (Fig. S2C). This small shift in binding energy corresponding to (CeC) in the C 1 s spectrum of CeO2 NRs-RGO and RGO from 280 eV to 280.3 eV has also been adopted to suggest the existence of charge transfer between RGO and CeO2 NRs [46]. The UV–vis spectra of CeO2 with different amount (wt%) of RGO composites was shown in Fig. S3A. The band gap values of the nanocomposites were determined using Tauc's equation [8] by extrapolating the linear portion of the (ahν)2 vs. hν plots to intercept the photon energy axis (Fig. S3B and Fig. 3A), and the obtained results can be seen in Table S2. It is obvious that, except the CeO2-RGO (0.49 wt%, 1.49 wt % and 4.84 wt%) samples, the band gaps of other three CeO2-RGO catalysts are smaller than that of pure CeO2. Generally, the conduction band level of CeO2 are composed of empty orbitals of Ce4+ with d0 and d10 configurations which should not be lowered, but the valence band level always depends on crystal structure and bond character between metal and oxygen [28]. Therefore, it is reasonable to deduce that the decreased band gap of CeO2-RGO composites (Table S2) may be induced by the chemical bonding between CeO2 with RGO [7,9] or the changed crystal structure and morphology of CeO2 [47]. In order to get a deep insight into the hybrid composite, the DFT calculations are also used to get the band structure and the binding strength of the CeO2/RGO. The band structure and PDOS of the CeO2/ RGO are also given in the Fig. 3B, which indicates the incorporation of the graphene can increase the conductive of the composite and it maybe improve the decoloration properties. In the Fig. 3C, charge difference density of the CeO2/RGO are provided, which indicates the charge transfer between the CeO2 and RGO occurred, thus the binding strength is very strong for these two parts. Fig. 4 shows the ηDE of MB with time both in dark and under visible light condition over the catalyst CeO2-RGO. For both cases, ηDE increases initially markedly then slowly with the augment of RGO. In the case of the totally dark environment (Fig. 4A), ηDE increases sharply in the first 30 min and then slowly, except that of the sample containing 16.9 wt% RGO which seems to reach its highest value at 60 min. Fig. 4A also indicates that, after 30 min, there almost exists a quasi-equilibrium of adsorption-desorption for MB on catalysts, which is in accordance with the results obtained by other researchers [7,9,24] that the required duration for the adsorption-desorption equilibrium of MB on catalysts is ca. 30–60 min. However, the more apparent increase in ηDE for the samples of 4.84 wt% and 9.2 wt% RGO (Fig. 4A) seems to indicate that the time for these two samples to achieve the quasi-equilibrium is much 4

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Fig. 3. Tauc plots {(αhν)2 vs. (hν)} from UV–vis spectra of CeO2 with different amount (wt%) of RGO composites (A), in Fig. (A): a:pure CeO2, b:0.49%, c:1.49%, d:2.82%, e:4.84%, f:9.2%, g:16.9%, (B) the band structure and PDOS of the CeO2/RGO; (C) the charge density difference of the CeO2/RGO.

the effective surface of catalyst and the adsorption of UV light [50], increasing the amount of the catalyst, the number of active sites on the photocatalyst surface increases. Whereas, with the increase of the catalyst (above the optimal value), the light scattering and screening effects by catalyst particles are also enhanced which will lead to a reduce in irradiation field inside the reaction medium and finally a decrease in ηDE [48]. The influence of pH on ηDE was evaluated in 100 mg/L MB solution with different pH values over 0.1 g catalyst CeO2 NRs-RGO (9.2 wt%) and the pH of the solution was not controlled during the process. In Fig. 4(D-E), the obtained results are shown for MB decolorization as a function of the initial pH of the solution at various reaction times. In Fig. S4A, the zeta potentials of the catalyst in MB solution with different pH values have been tested and the results show that the pH changes have an influence on the surface charged state of the catalyst, which can thus affect the adsorption of MB onto the surface of the catalyst. When the pH is about 2.83, the zeta potential is zero. When the pH is 2.32 or 3.61 (both of the two pH values are pretty close to 2.83), the surface charge amount of the catalyst is smaller and the dispersion stability is worse than the other three pH values (pH = 6.85, 9.56 and 11.35), which will finally results in a relative smaller decoloration efficiency ηDE [51,52]. The ηDE of pH being 2.32 is even smaller than that

longer (> 60 min) than that of other samples. In the case of visible light irradiation (Fig. 4B), the variation trend of ηDE with both the content of RGO and the decoloration time is much similar to that of dark environment except that, when the visible light with wavelength of 415 nm is turned on (the point ‘light on’ in Fig. 4B), ηDE of the samples with higher RGO content (≥ 2.82 wt%) increases more rapidly, especially from 60 min to 70 min, to reach a relatively stable plateau of higher value (ca. 95.4%) than that in dark environment (Fig. 4A). While, it also can be seen that the final ηDE under visible light irradiation is somewhat higher than that in dark environment for all the seven samples, which should be ascribed to some reactions (photocatalytic degradation) [7,8] or further deep adsorption induced by light irradiation. The effects of the catalyst amount on photocatalytic decolorization of MB over the CeO2 NRs-RGO (9.2 wt%) under visible light condition are shown in Fig. 4C. The ηDE of MB increased with increasing the catalyst amount from 0.01 g to 0.1 g and decreased with further increase in the catalyst amount from 0.1 g to 0.75 g. This result proves that there was an optimum amount of catalyst loading under different experimental conditions, which is in accordance with the results from Ejhieh [48] and Yatmaz [49]. When the amount of catalyst is relatively low (below the optimal value), the ηDE of MB is mainly determined by

Fig. 4. Decoloration efficiency of MB with time (A)in dark; (B) under visible light over the catalyst CeO2 with different amount of RGO; (C) under visible light with different amounts of catalyst CeO2 NRs-RGO (9.2 wt%);(D-E) under visible light over the catalyst CeO2 NRs-RGO (9.2 wt%) in MB solution with different pH values; (F) after adding H2O2 or CH3CH2OH scavengers over CeO2 NRs-RGO (9.2 wt%), a: dark 1 h; b: light 1 h; c: light 1 h + CH3CH2OH; d: light 1 h + H2O2; e: dark 1 h + CH3CH2OH; f: dark 1 h + H2O2 (experimental conditions: 100 mL of 5 mg/ L MB solution with 0.1 g CeO2-RGO, immersion for 1 h in the dark). 5

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carboxylic group), 1230.45 cm−1 (C-O-C of epoxy) and 1068.69 cm−1 (C-OH of alkoxy group) in FTIR spectrum (Fig. 5). In Fig. 5, the other peaks at 1543.41 cm−1 and 1463.24 cm−1 are ascribed to C]C bond, peaks at 1316.95 cm−1 and 718.46 cm−1 correspond to CeC and CeH in CH2. The left peak at 448.39 cm−1 is assigned to CeeO in CeO2 and has been used to indicate the successful preparation of CeO2-RGO nanocomposite [7]. Interestingly, when inspecting the FTIR spectra in detail, it is found that, addition of H2O2 into the deionized water or the MB aqueous solutions (curves d and g in Fig. 5A) can cause the presence of one more characteristic peak at 3739.78 cm−1, which has already been ascribed to hydroxyl radical (OH·) [56]. Whereas, this peak at 3739.78 cm−1 is absent for all other FTIR spectra when the solution contains no H2O2. These findings may indicate that the strong oxidant H2O2 should preferentially oxidize the negatively charged RGO (Reaction 2) rather than the positively charged MB,

of pH being 3.61, this is because the surface of the catalyst is positively charged at pH 2.32, an electrostatic repulsion exists between the catalyst and the positively charged MB surface [47], which will lead to a relatively weak adsorption and a smaller decoloration efficiency [53]. With pH increasing from 3.87 to 11.68, the zeta potential changes from −32.1 mV to −35.8 mV, which indicates that the amount of negative charge on the catalyst surface increases with the increase of the pH in the range from 3.87 to 11.68. Also, the change of the zeta potential in the wide pH range (from 3.87 to 11.68) is relatively small, which means the catalyst is very stable in this pH range [52]. Because the surface of MB is positively charged [47], the increase of negative charge on the surface of the catalyst will undoubtedly enhance the adsorption of MB on the catalyst [48]. Moreover in the first 1 h in the dark during the decolorization process, the ηDE is mainly determined by the adsorption capacity of the catalyst, so in the Fig. 4(D-E), an obvious increase of ηDE can be seen with pH increasing from 2.32 to 11.35 in the first 1 h. While under the visible light (from the point ‘light on’ in Fig. 4), the ηDE difference in various pH values become smaller. This is because photodegradation induced by the photo holes (h+) oxidation is the mainly decolorization process [7,9] under the light condition though the exist of OH– could favor OH· formation and finally enhances the degradation efficiency [48]. Moreover, OH· formation could also consume photo holes (h+) which was used to oxidize and degrade MB. The final ηDE of MB in different pH values seems to have little difference, which may be ascribed to the stability of catalyst in the selected pH range and the synergistic effect between adsorption and photodegradation during the whole decolorization process. Fig. 4F and Fig. S4(B–C) show the influences of the electron scavenger H2O2 (1 mL, 30 wt%) and the hole scavenger CH3CH2OH (1 mL, AR.) on MB decolorization efficiency over CeO2 NRs-RGO (9.2 wt%) catalyst in the totally dark (curves a, e, f in Fig. 4F) or under the visible light irradiation environment (curves b, c, d in Fig. 4F). Before addition of H2O2 or CH3CH2OH (time < 60 min) into the decolorization solutions (Fig. 4F), no obvious difference in ηDE can be observed, the achieved higher ηDE is mainly attributed to the adsorption of MB on NRs-RGO (9.2 wt%) catalyst. However, in the totally dark environment, after addition of H2O2 or CH3CH2OH (Fig. S4B), the ηDE with scavenger CH3CH2OH (curve b in Fig. S4B) is somewhat higher than that of no scavenger, while the ηDE with scavenger H2O2 is the lowest (curve c in Fig. S4B). These differences may be caused not only by the different surface characteristics of RGO and MB, but also by the different natures of H2O2 and CH3CH2OH. It is reported that the surface of MB is positively charged [54], whereas the surface of CeO2-RGO is negatively charged due to the existence of carbonyl (C=O), epoxy group (C-O-C), alkoxy group (C-OH) and hydroxyl (OeH) [55]. Existence of such oxygen-containing groups on CeO2-RGO surface can not only be confirmed by XPS analysis (Fig. S2A), but also be verified by the presence of the characteristic peaks at 3462.69 cm−1 (physically absorbed OeH), 1725.56 cm−1 (C]O of

CeO2 − RGO (−) + H2 O2 → CeO2 − RGO + ∙OH

(2)

The reaction between H2O2 and RGO should change the size of CeO2-RGO nanoparticles. Accordingly, XRD technique and Scherrer equation (reaction S1) are adopted to characterize the CeO2-RGO samples pretreated under the same conditions as those for FTIR tests, and the obtained results are shown in Fig. 5B and Table S3, respectively. It is obvious that the nature of the investigated or used solutions does not change the characteristic peak position of CeO2 (Fig. 5B), but the particle size of CeO2-RGO pretreated in the solutions containing H2O2 is larger than those in the absence of H2O2 (Table S3). Therefore, according to the results of FTIR spectra (Fig. 5A) and XRD patterns (Fig. 5B and Table S3) of CeO2-RGO (9.2 wt%), it can be concluded that, even in totally dark environments, addition of electron scavenger H2O2 into solutions can not only cause the formation of OH·, but also increase the particle size of CeO2 in CeO2 NRs-RGO. Theoretically, the increase in particle size should decrease the adsorption capacity of CeO2-RGO and hence reduce its decolorization efficiency (ηDE), whereas the generated hydroxyl radical (OH·) with stronger oxidizing ability (Reaction 2) should improve the ηDE. The recyclability of catalyst CeO2 NRs-RGO (9.2 wt%) nanocomposite for decolorization of MB in Fig. S4D clearly shows that the decolorization of MB is mostly attributed to its adsorption over the catalyst CeO2 NRsRGO, especially, the amount of the generated OH· being very small due to the trace addition of H2O2, consequently, the ηDE with scavenger H2O2 (curve c in Fig. S4B) is the lowest. On the other hand, CH3CH2OH has negligible influence on the particle size of CeO2-RGO (Table S3), but its dispersal capacity is somewhat higher than that of pure deionized water. Consequently, the addition of CH3CH2OH into MB aqueous solution may improve the effective contact area between catalyst CeO2-RGO and MB, which hence enhances the decolorization efficiency a little bit (curve b in Fig. S4B), and therefore the ηDE with scavenger CH3CH2OH (curve b in Fig. S4B) Fig. 5. FTIR spectra (A) and XRD patterns (B) of CeO2-RGO (9.2 wt%) before and after immersion in the solutions containing different chemicals for 1 h under the dark environments. a: CeO2-RGO (9.2 wt%) before immersion (for comparison), b: H2O, c: H2O2+ ethanol, d: H2O + H2O2, e: MB (aq.), f: MB (aq.) + ethanol, g: MB (aq.) + H2O2.

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properties caused by physical adsorption. Also the increased adsorption can enlarge the contact area between CeO2 NRs-RGO and MB, which enhances the number of active sites in the photocatalytic reaction and consequently results in an enhanced decolorization properties. For the other reason is the narrowed band gap of CeO2 NRs-RGO caused by the energy levels transitions between CeO2 NRs and RGO. The decreased band gap makes the CeO2 NRs-RGO respond in visible light and increases the utilization of light, which finally improve the photocatalytic activity.

is the highest in totally dark environment. In the case of the first 1 h in the dark and the subsequently simultaneous addition of scavengers and irradiation of visible light (Fig. S4C), both addition of H2O2 and CH3CH2OH can improve the total decolorization efficiency efficiently, and the elevating effect of H2O2 (curve c in Fig. S4C)is more significant than that of CH3CH2OH(curve b in Fig. S4C). The reason may be that, under the irradiation of visible light, holes (h+) and electrons (e−) are generated on CeO2-RGO (Reaction 3) [7,9],

CeO2 − RGO + hν → CeO2 − RGO (h+ + e−)

(3)

4. Conclusion

Although the large number defects existed in CeO2 NRs-RGO (9.2 wt %) can always operate as the recombination centers for the photogenerated electrons and holes, which should result in a decrease in the photocatalytic activity [27], the addition of sacrificial reagents [27] can always consume one kind of photon-generated carriers (holes and electrons) so as to prevent their recombination, which consequently increases the total decolorization efficiency. Moreover, the photo-assisted MB decolorization efficiency without any scavengers (H2O2 and CH3CH2OH) is much higher than that in dark environment, which also demonstrates that the defects induced recombination of holes and electrons are insignificant. When the electron acceptors or electron scavengers (such as H2O2) are added into the MB contained aqueous solution, the photo-generated electrons in conduction band (of CeO2 NRs-RGO (9.2 wt%)) will be consumed by H2O2 to produce∙OH with stronger oxidizing ability (Reaction 4) [57], which then collaborates with the photo-generated holes to oxidize MB (Reactions 5–6) [7,9],

CeO2 − RGO (e−) + H2 O2 → CeO2 − RGO + ∙OH

(4)

∙OH + MB → Mineralized products

(5)

CeO2 − RGO (h+) + MB → CeO2 − RGO + Mineralized products

(6)

A series of CeO2 NRs-RGO nanocomposites with different RGO contents have been prepared by a facile hydrothermal method. The doping of RGO into CeO2 NRs not only changes the surface morphology and area, the particle size, the band gap, the crystal structure (especially the formation of new phase Ce2O3 and the defects) and the oxygen vacancy content of the latter, but also causes the mutual charge transfer between RGO and CeO2 NRs, which therefore increases the decolorization efficiency of CeO2 NRs-RGO. In the totally dark environments, the addition of the electron scavenger H2O2 into the decolorization solutions can not only cause the formation of OH·, but also increases the particle size of CeO2-RGO catalyst, whereas CH3CH2OH has negligible influence on the particle size of CeO2-RGO. The decolorization of MB is mostly attributed to its adsorption over the catalyst CeO2 NRs-RGO, but the visible light irradiation also plays a vital role in increasing the decolorization efficiency, and the decolorization efficiency of CeO2 NRs-RGO (9.2 wt%) under visible light irradiation is 95.4% and this can be as high as 99.5% after addition of H2O2. Synergistic effect of both adsorption and photo-catalysis are very important in MB decolorization process and also in the design of decolorization agent. Acknowledgement

On the other hand, when the hole scavengers (such as CH3CH2OH) are added into the MB contained aqueous solution, parts of the photogenerated holes (h+) in Reaction 3 will be taken away to irreversibly oxidize CH3CH2OH (Reaction 7) [58] instead of MB. The newly generated∙C2H4OH will be further transformed to CH3CHO (Reaction 8) [59], which may enrich the electrons in a photocatalyst reaction,

C2 H5 OH + CeO2 − RGO (h+) → ·C2 H4 OH + H+ + CeO2 − RGO

(7)

CeO2 − RGO + ·C2 H4 OH → CH3 CHO + H+ + CeO2 − RGO (e−)

(8)

CeO2 − RGO (e−) + O2 → CeO2 − RGO + O2.−

(9)

O2.−

+ MB → Mineralized products

The authors wish to acknowledge the financial supports from the National Natural Science Foundation of China (Project 51771173, 21073162, 21273199). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2019.143939. References

(10)

Although the generated electrons may also react with the dissolved oxygen (O2) to produce O2.− to oxidize MB (Reactions 9–10), the final degradation efficiency is largely dependent on the concentration of dissolved O2 but not the holes and electrons generated by photocatalyst CeO2 NRs-RGO [7]. Therefore, the influence of CH3CH2OH on MB decolorization efficiency is relatively smaller than that of H2O2, which is just the reason why the elevating effect of H2O2 is more significant than that of CH3CH2OH. Based on the decolorization results and the above analyses, it can be concluded that most part of MB is removed by its adsorption on CeO2 NRs-RGO (9.2 wt%), and a small part of MB is degraded by the photo-generated carriers or their oxidizing products (holes, ∙OH and O2.−). Combined the design philosophy in the Fig. 1 and the whole analysis above, we deduced that the whole decolorization process may consist of the following four steps, which are: (1) physical absorption of catalyst, (2) light absorption, (3) reaction between photon-generated carrier and dyes, (4) desorption of photocatalytic reaction product. The enhanced photocatalytic activity may be attributed to two reasons: one is that addition of RGO is benefit for the adsorption between the catalyst CeO2 NRs-RGO and MB, which not only leads to an enhanced decolorization

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