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AgBr-rGO nanocomposite
Applied Surface Science 502 (2020) 144275
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Full Length Article
Boosted photocatalytic degradation of Rhodamine B pollutants with Zscheme CdS/AgBr-rGO nanocomposite
T
⁎
Jianfeng Zhanga,b, , Zhiqiang Zhanga,b, Weihuang Zhua,b, Xiaoguang Mengc a
Shaanxi Key Laboratory of Environmental Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, People’s Republic of China Key Laboratory of Northwest Water Resource, Environment and Ecology, MOE, Xi’an University of Architecture and Technology, Xi’an 710055, People’s Republic of China c Center for Environmental Systems, Stevens Institute of Technology, Hoboken, NJ 07030, USA b
A R T I C LE I N FO
A B S T R A C T
Keywords: CdS AgBr Reduced graphene oxide Z-scheme photocatalytic system Rhodamine B
High-efficiency photocatalytic degradation of a harmful organic pollutant, i.e., Rhodamine B (RhB), was achieved using an rGO-mediated CdS/AgBr (CdS/AgBr-rGO) photocatalyst composed of AgBr, CdS and reduced graphene oxide (rGO). The batch experiments demonstrated that the first-order kinetic constants of as-synthesized photocatalyst prepared CdS/AgBr-rGO with a mass ratio of 10:1:0.005 was 0.051 min−1. The value was 1.96 and 5.67 times higher than those of CdS and AgBr alone for RhB degradation, respectively. Using benzoquinone (BZQ), isopropyl alcohol (ISA) and ethylenediamine tetraacetic acid disodium salt (Na2-EDTA) as scavengers, the results of quenching experiment indicated that %O2−, %OH and h+ are the dominant reactive species for RhB decomposition. The electrochemical analysis demonstrated that CdS and AgBr formed a Zscheme photocatalytic system via rGO at the interface of the two semiconductors acting as a solid electron shuttle where the photogenerated electron-hole pairs were efficiently separated through electron transfer. Furthermore, the photostability of CdS/AgBr-rGO heterojunction photocatalysts was investigated over four successive runs.
1. Introduction Industrial wastewater is a dominant source of organic dyes causing esthetic pollution, eutrophication, and perturbation to aquatic life in freshwater systems [1]. Rhodamine B (RhB) is widely used in the textile industry as an organic dyes, and it can threaten public health due to its toxicity [2,3]. Methods for the degradation of RhB in wastewater include adsorption, advanced chemical oxidation, photodegradation, and microbiological discoloration [4]. Of these, semiconductor photocatalytic oxidation technology is the least intensive and easiest to apply with complete degradation of organic pollutants; therefore, it has been developed as a favorable technology in the field of environmental remediation and has been the focus of many research studies [5–8]. As opposed to common semiconductor materials (e.g., TiO2, ZnO, Fe2O3, ZnS, WO3 and α-Fe2O3), CdS, with a more narrow band gap of 2.3 eV, is an ideal visible-light responsive material for photocatalysis and solar cells; however, a critical obstacle to CdS application is hard to separate the photogenerated electron-hole pair [9]. Numerous efforts have been made to increase CdS photoactivity by constructing CdS with other semiconductors, incorporating CdS
particles into mesoporous materials and polymer matrices to form hybrid photocatalysts, and optimizing synthetic approaches [10–13]. Of these, constructing a Z-scheme heterostructure in which the photogenerated holes in CdS can be filled with photoinduced electrons from another semiconductor has been shown to effectively restrain the combination of photogenerated electron-hole pairs [14,15]. Candidate semiconductors for assembling a Z-scheme photocatalyst with CdS include BiVO4 [16], WO3 [17,18], ZnO [19], AgPO4 [20], TiO2 [21] and CdWO4 [22] among others. The photosensitive characteristics of silver halides (AgX, X = Cl, Br, I) could improve the photon adsorption activity under visible light [23,24]. AgBr exhibits higher photocatalytic performance under visible light versus AgCl [24,25]. Facile synthesis methods to construct a Z-scheme photosystem using AgBr materials remains challenging. Recently, tremendous reports have demonstrated that reduced graphene oxide (rGO) can be used as an excellent solid electron mediator between two photocatalysts in a Zscheme photosystem [26–28]. The introduction of an electronic shuttle to regulate the Z-scheme photocatalyst could prompt the rapid movement of photogenerated electrons, which in turn can improve the photocatalytic behaviors [29]. However, there is a lack of data
⁎ Corresponding author at: Shaanxi Key Laboratory of Environmental Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, People’s Republic of China. E-mail address: [email protected] (J. Zhang).
https://doi.org/10.1016/j.apsusc.2019.144275 Received 26 June 2019; Received in revised form 2 October 2019; Accepted 3 October 2019 Available online 14 October 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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regarding rGO as an electron mediator for CdS and AgBr photocatalyst, and the relevant mechanisms remain unresolved. Here, a novel CdS/AgBr-rGO composite was synthesized using a hydrothermal protocol. Given that the valence band (VB) potentials of AgBr are more active than CdS and the conduction band (CB) of CdS potentials are more negative than AgBr. The transmission and separation of photoexcited electron–hole pairs through their heterojunctions can be accelerated with the intervention of rGO. Additionally, the results of RhB degradation experiments indicated that the novel CdS/ AgBr-rGO heterostructure displays the higher photocatalytic activity and stability than those of the bare CdS and AgBr particles under visible light irradiation.
(Shimadzu, UV-2450) was recorded on a Shimadzu with BaSO4 as a reference. The photocurrent was analyzed with Electrochemical Workstation (PARSATA 2273, USA) and a conventional three-electrode system including a silver-silver chloride (Ag/AgCl) reference electrode, a Pt wire counter electrode, and a fluorine-doped tin oxide (FTO) glass for the working electrode. Photoluminescence spectra (PL) of photocatalysts were tested with Hitachi F-7000 fluorescence spectrometer. Specific surface area values (SBET) and pore structures were tested with MICROMERITICS ASAP 2460. Electron spin resonance (ESR) was record on an MXmicro-6/1/P/L spectrometer.
2. Experimental
The photocatalytic properties of the as-synthesized CAR photocatalysts were investigated by the degradation of RhB pollutant. The visible light source was a 500 W Xe lamp with a 420 nm cut-off filter. In this photocatalysis experiment, 60 mg of CAR was dispersed in 100 mL RhB (50 µmol·L−1) aqueous solution, and stirred for 20 min in the dark at room temperature. Then, a 3.0 mL suspension was withdrawn at 10minute intervals and centrifuged under irradiation conditions. The concentration of RhB was determined via the change in absorption intensity at 554 nm wavelength on a UV–vis spectrophotometer.
2.4. Photocatalytic activity
2.1. Material and reagents Cadmium chloride hemidihydrate (CdCl2·2.5H2O), sodium diethyldithiocarbamate trihydrate ((C2H5)2NCSSNa3H2O), ethylenediamine (C2H8N2), silver nitrate (AgNO3), potassium bromide (KBr), benzoquinone (C6H4O2), isopropyl alcohol ((CH3)2CHOH) and ethylenediamine tetraacetic acid disodium salt (C10H14N2Na2O8·2H2O) were from the Kemiou Chemical Reagent Company (Tianjin, China) and used as received.
2.5. Photoelectrochemical (PEC) measurements
2.2. Photocatalyst preparation
A 0.3-M Na2SO4 aqueous solution was prepared as the working electrolyte in the three-electrode system. The working electrodes consisted of as-synthesized photocatalysts deposited on fluorine-doped tin oxide (FTO) glass with an irradiation area of 2.5 cm2. Typically, the FTO working electrodes were assembled as follows: First, 10 mg of photocatalyst was dispersed in a 1 mL sonicated homogeneous mixture (ethanol/Nafion = 95/5, v/v), and the mixture was sonicated to obtain a homogeneous suspension. Then, the 0.25 mL obtained suspension was dripped dropwise on FTO glass and dried at 60 °C for 24 h to ensure good adhesion of the catalyst.
The method reported by Jamble et al. was used to prepare CdS [30]. A total of 3.865 g of C9H18NNaS2 and 1.985 g of CdCl2·2.5H2O were dissolved in 125 mL of ethylenediamine. First, the solution was ultrasonicated for 30 min following by stirring for 1 h. The solution was transferred to a 150-mL Teflon-lined stainless steel autoclave heated to 180 °C for 24 h in air oven. After cooling naturally, the final products was collected by centrifugation and repeated rinsing with deionized (DI) water and alcohol and finally dried at 60 °C for 24 h. Graphene oxide (GO) powders were synthesized from commercial graphite flakes using a modified Hummers method [31]. To prepare the CdS/AgBr-rGO (CAR) photocatalyst, as-prepared CdS powder was first added in 20 mL of a 20 mg/L GO aqueous solution. Next, 40 mL of a KBr (0.01 M) aqueous solution was dropped slowly into the system, and the mixture was stirred for 5 min. An AgNO3 (0.01 M) aqueous solution (40 mL) was then added slowly while stirring. After 30 min of continuous stirring, the mixture was transferred to a 150-mL Teflon-lined autoclave and heated at 180 °C for 6 h. Finally, centrifugation was used to collect the CAR powder, which was then dried in the oven at 60 °C for 24 h. The CdS mass of 0.3756 g, 0.7512 g, and 1.1268 g with 0.075 g of AgBr resulted in CdS to AgBr mass ratios of 5, 10 and 15, respectively. The acquired composites were denoted as CAR-x, where x (x = 5, 10 and 15); this represents the mass ratio of CdS to AgBr with which to prepare sample. A single AgBr photocatalyst was prepared similarly with the above method without the GO/CdS aqueous solution and heating at 180 °C. The preparation method of AgBr-rGO was the same as that of CAR without the addition of bare CdS. The preparation method of CdS/AgBr was the same as that of CAR without GO and heating. It used a similar denotation method as that of CAR, i.e., CA-x refers to a material prepared with a CdS/AgBr mass ratio of x.
3. Results and discussion 3.1. Structure and morphology The XRD patterns of the as-synthesized photocatalysts are shown in Fig. 1a. For pure CdS, peaks at 2θ = 24.7°, 28.1°, 43.6°, 47.7°and 52.8° were attributed to the characteristic of diffraction CdS phases (JCPDS No. 75-0581) [32]. For bare AgBr, the peaks located at 26.4°, 31.0°, 44.0°, 55.1°and 64.6° corresponded to the (1 1 1), (2 0 0), (2 2 0), (2 2 2) and (4 0 0) plane diffraction of the AgBr cubic phase (JCPDS No. 060438), respectively [33]. Besides, no peak assigned to Ag0 could be observed in the XRD patterns, which indicated that Ag-AgBr photocatalytic system was not constructed in this study [34]. The trace amount of loaded rGO and the low atomic number suggested that no peak attributed to rGO was observed in the XRD spectra of AgBr-rGO. Fig. 1b shows that the use of rGO in the composite material changed the diffraction peak position of the AgBr (from 44.0° to 44.12°) based on the JCPDS data for (2 2 0) diffraction peak of AgBr. This indicates that rGO was included successfully into the AgBr lattice [24]. In the XRD patterns of CA-10 and CAR-10, the diffraction peaks characteristic of AgBr were not observed because of the low content of AgBr. Meanwhile, compared to the (1 0 3) diffraction peak of CdS, the peaks had a certain deflection by the close up view could be found for CA-10 and CAR-10 (Fig. 1c) suggesting the existence of an interfacial interaction between CdS and AgBr or AgBr-rGO; the as-synthesized photocatalysts (i.e. CAR) were likely not simple hybrids [35]. The dimensional and morphological features of as-synthesized CAR10 were demonstrated by TEM and HRTEM. CdS in the CAR-10 nanocomposite appears as aggregates of several elongated nanorod particles with a radius of about 60 nm (Fig. 2). The AgBr particles have diameters
2.3. Characterization X-ray diffraction (XRD) patterns were determined by scanning from a 2θ of 10° to 80° on an X-ray diffractometer (Ultiman IV, Rigaku Corporation, Japan). The morphology and size of the samples were observed by transmission electron microscopy (TEM) (JEM-2100). The elemental composition and chemical valence was also determined using X-ray photoelectron spectroscopy (XPS) (Thermo Fisher Scientific, USA). Ultraviolet–visible diffuse reflectance spectroscopy (DRS) 2
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Fig. 1. XRD patterns of (a) synthesized photocatalysts CdS, AgBr, AgBr-rGO, CA-10 and CAR-10; (b) the (2 2 0) diffraction peak of AgBr in AgBr and AgBr-rGO; (c) the (1 0 3) diffraction peak of CdS in CA-10 and CAR-10.
AgBr [38], and Cd 3d5/2 and Cd 3d3/2 of Cd2+ in CdS [39], respectively. In Fig. 3e, the S 2p XPS spectrum exhibited S2- peaks of CdS at 160.25 (S 2p3/2) and 161.7 eV (2p1/2) [40]. In Fig. 3f, a peak at 284.5 eV was shown in C 1s, which can be ascribed to the Csp2–Csp2 bonds in a pure carbonaceous environment possibly due to graphite or rGO. The weak peaks at 289.2 and 286.5 eV likely correspond to the COOH groups and sp2 CeC bond, respectively. The peak intensity reflective of oxidized carbon species of rGO has a significant decrease versus GO. This result confirmed the reduction of GO to rGO under solvothermal conditions during hybrid composite synthesis [32,41]. The XPS results showed that AgBr, CdS and rGO were successfully compounded consistent with the results of XRD and TEM. 3.3. UV-DRS analysis Light absorption can confirm the corresponding band gap of materials, and thus is an important parameter for semiconductors. Fig. 4a shows the UV–Vis DRS of CdS, AgBr and CAR-10 composites. Compared with CdS, pure AgBr has a narrow light absorption band in the ultraviolet region. The CAR-10 composite exhibits similar DRS spectra with bare CdS and AgBr in the optical light region demonstrating mixed light absorption properties of both CdS and AgBr photocatalysts. The absorption edge of composites exhibits a small red-shift to the higher wave length relative to the AgBr photocatalyst. The band gap of CdS, AgBr and CAR-10 can be calculated from the absorption edge using the Kubelka-Munk equation:
Fig. 2. TEM image of CAR-10 nanoparticles. (Inset shows HRTEM images of CdS and AgBr).
varying from 10 to 100 nm; these were clearly observed growing on the CdS nanorods. rGO, which showed wrinkles. These were clearly observed at the interface between CdS and AgBr. The HRTEM images (inset in Fig. 2) show that the lattice fringes were identified to the (0 0 2) crystallographic phase of CdS with an interplanar spacing d = 0.335 nm; the (1 1 1) crystalline plane of AgBr had d = 0.333 nm. These are consistent with the literature [22,36].
αhν = A (hν − Eg)n/2 ,
3.2. XPS
(1)
where α, h, ν, A and Eg are the diffuse reflection absorption coefficient, Planck constant, frequency of light, absorption constant and band gap, respectively. n depends on the type of optical transition exhibited by the semiconductor (n = 1.0 or 4.0 are assigned to direct transition and indirect transition, respectively). The n for both CdS and AgBr is 1.0 [36,37]. Based on the plot of (αhν)1/2 versus (hν) shown in Fig. 4b, the Eg of CdS and AgBr was estimated to be about 2.3 eV and 2.4 eV,
XPS analysis were performed to demonstrate the chemical construct characteristics of CAR-10 sample. The peaks in XPS spectrum of CAR-10 sample can be seen in Fig. 3a, which is very consistent with the composition of the photocatalysts. In Fig. 3b–d, the peaks appeared at 367.6, 373.8 eV, 68.3, 69.0 eV, 404.5 and 411.3 eV correspond to Ag 3d5/2, Ag 3d3/2 of Ag+ in AgBr [37], Br 3d5/2 and Br 3d3/2 of Br− in 3
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Fig. 3. (a) XPS spectrum of CAR-10 composite containing Ag 3d (b), Br 3d (c), Cd 3d (d), S 2p (e) and C 1s (f).
3.4. Photoelectrochemical analysis
respectively. This is consistent with values reported in the literature [36,38]. More importantly, the band gap of the CAR-10 composite was narrower than that of CdS and AgBr; therefore, the excellent lighttrapping ability is an exception. Furthermore, the narrowed band gap of CAR-10 suggested a strong interfacial interaction between CdS and AgBr [42], which is consistent with the XRD data (3.1).
Photoelectrochemical analysis can provide obvious information on the interfacial charge transfer in photocatalysts [21]. Fig. 5 displays the transient photocurrent responses of pure CdS, pure AgBr, AgBr-rGO and CAR-10 under intermittent visible light irradiation in seven on–off cycles. As expected, CAR-10 exhibits the strongest photocurrent response
Fig. 4. (a) UV–vis DRS spectra of as-synthesized photocatalyst. (b) Relationships of (ahυ)1/2 versus photo energy (hυ) of CdS, AgBr and CAR-10. 4
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Fig. 7. PL spectra of CdS, AgBr and CAR-10.
lowest fluorescence intensity followed by CdS; the highest was AgBr. This confirmed the results of the photocatalytic degradation (3.7) and photocurrent (3.4).
Fig. 5. Photocurrent response of various photocatalysts sample in 0.3 M Na2SO4 solution.
(0.023 mA/cm2). This is about 3.28 times higher than that of pure CdS (0.007 mA/cm2) and 11.5 times that of pure AgBr (0.002 mA/cm2). These results indicate that the CAR-10 composite has the highest separation efficiency for the photoinduced charge transfer of the various photocatalysts. This is consistent with the results of RhB degradation discussed (3.7).
3.7. Photocatalytic performances Photocatalytic activity of as-synthesized photocatalysts was evaluated by the RhB degradation under artificial visible light conditions. Before irradiation, all systems were kept in the dark for 20 min to reach adsorption equilibrium. Fig. 8a shows the discoloration efficiency for RhB on the photocatalyst with various photocatalysts. The results show that 79.2%, 46.6% and 71.4% of RhB was discolored by pure CdS, AgBr, and AgBr-rGO, respectively, under 60 min of irradiation. There was an obvious improvement in the discoloration efficiency for CAR composites. The RhB discoloration ratio for CAR-5, CAR-10 and CAR-15 reached 90.6%, 95.8% and 89.5%, respectively. CAR-10 in particular displayed perfect photocatalytic performance, which was about 1.21 and 2.05 times higher than those of pure CdS and AgBr, respectively. The kinetic curves for RhB discoloration shown in Fig. 8b reveal a linear relationship between ln(C0/Ct) and irradiation time: The pseudofirst order model had the best fit to the experimental data [34,35]. The rate constants (slope k of the regression line) of pure CdS, pure AgBr, AgBr-rGO, CAR-5, CAR-10 and CAR-15 were estimated to be 0.026, 0.009, 0.019, 0.038, 0.051 and 0.037 min−1, respectively. As expected, CAR-10 showed the highest degradation rate constant, which was about 1.96 and 5.67 times higher than those of CdS and AgBr alone, respectively. The discoloration efficiency of the CAR-10 sample (Fig. 8) is slightly better than CAR-5 and CAR-10. This can be attributed to the required number balance between photogenerated electrons from AgBr with photogenerated holes from CdS [15]; this can be achieved by tuning the
3.5. N2 adsorption-desorption isotherms and pore structures Fig. 6 shows that N2 adsorption-desorption performance and pore size distribution of AgBr, CdS and CAR-10. Fig. 6a shows that the adsorption capacity of the AgBr, CdS and CAR-10 to N2 were small under the low relative pressure (P/P0 < 0.5). The BET specific surface areas of CAR-10 (16.38 m2·g−1) were about 2.8 and 1.8 times higher than those of AgBr (5.80 m2·g−1) and CdS (9.11 m2·g−1), respectively. Fig. 6b shows that the pore of all photocatalysts were 1–30 nm. The higher surface areas and the more active sites of the photocatalyst will facilitate the transfer of photogenerated charges; thus, better photocatalytic performance could be expected [43,44]. 3.6. PL analysis The PL spectrum was used to compare the recombination process of photogenerated carriers between various as-synthesized photocatalyst. The PL spectra with 395 nm excitation of AgBr, CdS and CAR-10 composites are shown in Fig. 7. Generally, a lower PL intensity means a photosynthetic lower recombination efficiency carrier and a better photocatalytic activity [45]. The results depicted that CAR-10 had the
Fig. 6. (a) Nitrogen adsorption-desorption isotherms and (b) the pore size distribution curves of CdS, AgBr and CAR-10. 5
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Fig. 8. (a) photodegradation of RhB with CdS, AgBr and series of CAR composites; (b) kinetic data for the degradation of RhB in the presence of different photocatalysts.
light photocatalytic performance of CAR-10 with respect to the degradation of RhB. These data suggests that the introduction of BZQ, Na2-EDTA and ISA had a negative influence on the photocatalytic performance of CAR-10 for the RhB degradation. The RhB removal efficiencies decreased from 95.8% to 18.8% (BZQ), 34.9% (Na2-EDTA) and 65.3% (ISA) within 60 min, respectively. The CA-5 and CA-15 also showed the same quenching characteristics as CAR-10. These data suggest that %O2−, h+ and %OH are the reactive substances during the photodegradation process. Moreover, %O2− is the controlled reactive species in the photocatalytic reaction, and the intermediate of the active radicals formed in the Z-scheme composite could largely promote pollutant degradation efficiencies. This experiment involved simultaneous electron excitation from the VB to the CB of the two individual semiconductors (i.e., CdS and AgBr). In Fig. 11, under the Z-scheme electron transfer routine, the photogenerated holes from CdS VB and the photogenerated electrons from AgBr CB are rapidly combined. This effectively separated the photogenerated holes from AgBr VB, and electrons can be photogenerated from CdS CB in this system. Afterwards, photogenerated electrons from CdS CB and photogenerated holes from AgBr VB produce active radical species capable of attacking and decomposing pollutant molecules. The holes from AgBr VB could react with H2O molecules to form %OH or oxidize organic contaminants directly. The photogenerated electrons from CdS CB can react with the adsorbed O2 on the surface of CdS to produce a reactive superoxide radical [15,55–57]. To further explore the mechanism of photocatalytic degradation, the CAR-10 was examined by DMPO spin-trapped ESR spectroscopy to monitor the active radicals that form during the photodegradation process. Fig. 10c shows that no signal was found for DMPO-%O2− in the dark; a stable DMPO-%O2− peak appeared in the system at 5 min, 10 min, and 15 min after illumination. Fig. 10d showed that there was no peak in DMPO-%OH in the absence of visible light irradiation. Under visible light conditions, the signal of DMPO-%OH displayed at 5 min, 10 min, and 15 min were not obvious. The EPR results were consistent with the results of the quenching experiment discussed above. That is, %O2− dominates the degradation process versus the %OH [35]. To further explore the role of rGO in the system, comparative photodegradation experiments were conducted using series of photocatalyst prepared with and without rGO (Fig. 10b). As expected, the photocatalytic performance of CAR was better than that of CA under same mass ratio of CdS to AgBr (x) conditions. Therefore, with the intervention of rGO, the electron conduction ability of the entire system was strengthened. This improved the efficiency of RhB degradation. The degradation mechanism of the photocatalyst can be summarized as follows (with rGO accelerating the transmission of active substances):
mass ratio of CdS to AgBr. We next compared the RhB degradation rate constant of CAR with the other Z-scheme photocatalysts ZnO/TiO2 [46], AgCl/Bi3O4Cl [47] and Fe3O4/BiOBr [48]. The results show that the as-synthesized CAR-10 has better photocatalytic degradation performance. The stability and reusability of the photocatalyst is of great significance to its practical application. The degradation efficiency of CAR10 shows a slight decrease after four repeated degradations (Fig. 9). This is attributed to the photochemical corrosion in CdS during the degradation process [49]. Compared to the other photocatalyst composites, i.e., CdS/CdWO4 [22], CdS/BiVO4 [50] and CdS/ZnWO4 [51], the photocatalytic activity of CAR-10 had no significant weakening even after four runs of repeated experiments. Moreover, the results of XRD patterns and the XPS spectra exhibited a crystal structure of CAR10 after four cycling runs that are consistent with the fresh photocatalyst (Fig. S1). Therefore, the stability in cycling experiments demonstrated that the construction of CAR-10 composites could retard the photocorrosion of CdS without reductive regents.
3.8. Discussion of photocatalytic mechanism The effects of various scavengers on the decolorization of RhB in the photocatalytic process were conducted to investigate the photocatalytic mechanism of CAR in more details. To the best of our knowledge, three main reactants including h+, %OH and %O2− play a crucial role in the photocatalytic process. Ethylenediamine tetraacetic acid disodium salt (Na2-EDTA), isopropyl alcohol (ISA), and benzoquinone (BZQ) were used to scavenge h+, %OH and %O2−, respectively [52–54]. Fig. 10a displays the influence of various scavengers on the visible-
Fig. 9. Cycle runs of RhB photocatalytic degradation with CAR-10 composites. 6
CdS + hv → h+ + e−,
(2)
AgBr + hv → h+ + e−,
(3)
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Fig. 10. (a) Photodegradation of RhB with series of CA and CAR photocatalysts after introducing various scavengers. (b) Photocatalytic degradation of RhB in the presence of a series of CA and CAR composites prepared with different mass ratios of CdS to AgBr (x); (c) and (d) ESR signals of the DMPO-%O2− and DMPO-%OH adducts recorded with a CAR-10 composite during light irradiation (> 420 nm).
rGO
e− (CB of AgBr) → combinate with h+
(VB of CdS),
h+ (VB of AgBr) + OH− → ·OH,
(5)
e− (CB of CdS) + O2 →· O2−
(6)
·O− 2
+
·OH
+ RhB → Oxidation products
introduction of rGO. This inhibits the recombination rate of photogenerated electrons-hole pairs and accelerates the separation of photogenerated electron-hole pairs. Meanwhile, the preparation of photocatalyst CAR is better than the above-mentioned ZnO/TiO2, AgCl/ Bi3O4Cl and Fe3O4/BiOBr composites. The degradation efficiency is higher. We envision that the photocatalytic application of CAR-10 composites can also target other refractory pollutants; the formation of Z-scheme systems between semiconductors is a facile method for boosting photocatalytic activity.
(4)
(7)
4. Conclusions In conclusion, a solid-state CAR-10 composite was successfully synthesized by growing AgBr on the surface of rGO-CdS, and RhB was photodegraded under visible light irradiation. The CAR-10 photocatalyst exhibited the highest RhB degradation efficiency versus bare CdS, bare AgBr and a series of composite materials with different amounts of CdS. The enhanced photocatalytic activity of CAR-10 composite is due to the heterostructure between AgBr, CdS and the
Declaration of Competing Interest The authors declared that there is no conflict of interest.
Fig. 11. Proposed mechanism for electron transfer in CAR-10 composite photocatalysts. 7
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Acknowledgments [26]
This work was supported by the National Key Project of Research and Development Plan of China (No. 2017YFC0403403-3).
[27]
Appendix A. Supplementary material [28]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2019.144275.
[29]
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Full Length Article
Boosted photocatalytic degradation of Rhodamine B pollutants with Zscheme CdS/AgBr-rGO nanocomposite
T
⁎
Jianfeng Zhanga,b, , Zhiqiang Zhanga,b, Weihuang Zhua,b, Xiaoguang Mengc a
Shaanxi Key Laboratory of Environmental Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, People’s Republic of China Key Laboratory of Northwest Water Resource, Environment and Ecology, MOE, Xi’an University of Architecture and Technology, Xi’an 710055, People’s Republic of China c Center for Environmental Systems, Stevens Institute of Technology, Hoboken, NJ 07030, USA b
A R T I C LE I N FO
A B S T R A C T
Keywords: CdS AgBr Reduced graphene oxide Z-scheme photocatalytic system Rhodamine B
High-efficiency photocatalytic degradation of a harmful organic pollutant, i.e., Rhodamine B (RhB), was achieved using an rGO-mediated CdS/AgBr (CdS/AgBr-rGO) photocatalyst composed of AgBr, CdS and reduced graphene oxide (rGO). The batch experiments demonstrated that the first-order kinetic constants of as-synthesized photocatalyst prepared CdS/AgBr-rGO with a mass ratio of 10:1:0.005 was 0.051 min−1. The value was 1.96 and 5.67 times higher than those of CdS and AgBr alone for RhB degradation, respectively. Using benzoquinone (BZQ), isopropyl alcohol (ISA) and ethylenediamine tetraacetic acid disodium salt (Na2-EDTA) as scavengers, the results of quenching experiment indicated that %O2−, %OH and h+ are the dominant reactive species for RhB decomposition. The electrochemical analysis demonstrated that CdS and AgBr formed a Zscheme photocatalytic system via rGO at the interface of the two semiconductors acting as a solid electron shuttle where the photogenerated electron-hole pairs were efficiently separated through electron transfer. Furthermore, the photostability of CdS/AgBr-rGO heterojunction photocatalysts was investigated over four successive runs.
1. Introduction Industrial wastewater is a dominant source of organic dyes causing esthetic pollution, eutrophication, and perturbation to aquatic life in freshwater systems [1]. Rhodamine B (RhB) is widely used in the textile industry as an organic dyes, and it can threaten public health due to its toxicity [2,3]. Methods for the degradation of RhB in wastewater include adsorption, advanced chemical oxidation, photodegradation, and microbiological discoloration [4]. Of these, semiconductor photocatalytic oxidation technology is the least intensive and easiest to apply with complete degradation of organic pollutants; therefore, it has been developed as a favorable technology in the field of environmental remediation and has been the focus of many research studies [5–8]. As opposed to common semiconductor materials (e.g., TiO2, ZnO, Fe2O3, ZnS, WO3 and α-Fe2O3), CdS, with a more narrow band gap of 2.3 eV, is an ideal visible-light responsive material for photocatalysis and solar cells; however, a critical obstacle to CdS application is hard to separate the photogenerated electron-hole pair [9]. Numerous efforts have been made to increase CdS photoactivity by constructing CdS with other semiconductors, incorporating CdS
particles into mesoporous materials and polymer matrices to form hybrid photocatalysts, and optimizing synthetic approaches [10–13]. Of these, constructing a Z-scheme heterostructure in which the photogenerated holes in CdS can be filled with photoinduced electrons from another semiconductor has been shown to effectively restrain the combination of photogenerated electron-hole pairs [14,15]. Candidate semiconductors for assembling a Z-scheme photocatalyst with CdS include BiVO4 [16], WO3 [17,18], ZnO [19], AgPO4 [20], TiO2 [21] and CdWO4 [22] among others. The photosensitive characteristics of silver halides (AgX, X = Cl, Br, I) could improve the photon adsorption activity under visible light [23,24]. AgBr exhibits higher photocatalytic performance under visible light versus AgCl [24,25]. Facile synthesis methods to construct a Z-scheme photosystem using AgBr materials remains challenging. Recently, tremendous reports have demonstrated that reduced graphene oxide (rGO) can be used as an excellent solid electron mediator between two photocatalysts in a Zscheme photosystem [26–28]. The introduction of an electronic shuttle to regulate the Z-scheme photocatalyst could prompt the rapid movement of photogenerated electrons, which in turn can improve the photocatalytic behaviors [29]. However, there is a lack of data
⁎ Corresponding author at: Shaanxi Key Laboratory of Environmental Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, People’s Republic of China. E-mail address: [email protected] (J. Zhang).
https://doi.org/10.1016/j.apsusc.2019.144275 Received 26 June 2019; Received in revised form 2 October 2019; Accepted 3 October 2019 Available online 14 October 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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regarding rGO as an electron mediator for CdS and AgBr photocatalyst, and the relevant mechanisms remain unresolved. Here, a novel CdS/AgBr-rGO composite was synthesized using a hydrothermal protocol. Given that the valence band (VB) potentials of AgBr are more active than CdS and the conduction band (CB) of CdS potentials are more negative than AgBr. The transmission and separation of photoexcited electron–hole pairs through their heterojunctions can be accelerated with the intervention of rGO. Additionally, the results of RhB degradation experiments indicated that the novel CdS/ AgBr-rGO heterostructure displays the higher photocatalytic activity and stability than those of the bare CdS and AgBr particles under visible light irradiation.
(Shimadzu, UV-2450) was recorded on a Shimadzu with BaSO4 as a reference. The photocurrent was analyzed with Electrochemical Workstation (PARSATA 2273, USA) and a conventional three-electrode system including a silver-silver chloride (Ag/AgCl) reference electrode, a Pt wire counter electrode, and a fluorine-doped tin oxide (FTO) glass for the working electrode. Photoluminescence spectra (PL) of photocatalysts were tested with Hitachi F-7000 fluorescence spectrometer. Specific surface area values (SBET) and pore structures were tested with MICROMERITICS ASAP 2460. Electron spin resonance (ESR) was record on an MXmicro-6/1/P/L spectrometer.
2. Experimental
The photocatalytic properties of the as-synthesized CAR photocatalysts were investigated by the degradation of RhB pollutant. The visible light source was a 500 W Xe lamp with a 420 nm cut-off filter. In this photocatalysis experiment, 60 mg of CAR was dispersed in 100 mL RhB (50 µmol·L−1) aqueous solution, and stirred for 20 min in the dark at room temperature. Then, a 3.0 mL suspension was withdrawn at 10minute intervals and centrifuged under irradiation conditions. The concentration of RhB was determined via the change in absorption intensity at 554 nm wavelength on a UV–vis spectrophotometer.
2.4. Photocatalytic activity
2.1. Material and reagents Cadmium chloride hemidihydrate (CdCl2·2.5H2O), sodium diethyldithiocarbamate trihydrate ((C2H5)2NCSSNa3H2O), ethylenediamine (C2H8N2), silver nitrate (AgNO3), potassium bromide (KBr), benzoquinone (C6H4O2), isopropyl alcohol ((CH3)2CHOH) and ethylenediamine tetraacetic acid disodium salt (C10H14N2Na2O8·2H2O) were from the Kemiou Chemical Reagent Company (Tianjin, China) and used as received.
2.5. Photoelectrochemical (PEC) measurements
2.2. Photocatalyst preparation
A 0.3-M Na2SO4 aqueous solution was prepared as the working electrolyte in the three-electrode system. The working electrodes consisted of as-synthesized photocatalysts deposited on fluorine-doped tin oxide (FTO) glass with an irradiation area of 2.5 cm2. Typically, the FTO working electrodes were assembled as follows: First, 10 mg of photocatalyst was dispersed in a 1 mL sonicated homogeneous mixture (ethanol/Nafion = 95/5, v/v), and the mixture was sonicated to obtain a homogeneous suspension. Then, the 0.25 mL obtained suspension was dripped dropwise on FTO glass and dried at 60 °C for 24 h to ensure good adhesion of the catalyst.
The method reported by Jamble et al. was used to prepare CdS [30]. A total of 3.865 g of C9H18NNaS2 and 1.985 g of CdCl2·2.5H2O were dissolved in 125 mL of ethylenediamine. First, the solution was ultrasonicated for 30 min following by stirring for 1 h. The solution was transferred to a 150-mL Teflon-lined stainless steel autoclave heated to 180 °C for 24 h in air oven. After cooling naturally, the final products was collected by centrifugation and repeated rinsing with deionized (DI) water and alcohol and finally dried at 60 °C for 24 h. Graphene oxide (GO) powders were synthesized from commercial graphite flakes using a modified Hummers method [31]. To prepare the CdS/AgBr-rGO (CAR) photocatalyst, as-prepared CdS powder was first added in 20 mL of a 20 mg/L GO aqueous solution. Next, 40 mL of a KBr (0.01 M) aqueous solution was dropped slowly into the system, and the mixture was stirred for 5 min. An AgNO3 (0.01 M) aqueous solution (40 mL) was then added slowly while stirring. After 30 min of continuous stirring, the mixture was transferred to a 150-mL Teflon-lined autoclave and heated at 180 °C for 6 h. Finally, centrifugation was used to collect the CAR powder, which was then dried in the oven at 60 °C for 24 h. The CdS mass of 0.3756 g, 0.7512 g, and 1.1268 g with 0.075 g of AgBr resulted in CdS to AgBr mass ratios of 5, 10 and 15, respectively. The acquired composites were denoted as CAR-x, where x (x = 5, 10 and 15); this represents the mass ratio of CdS to AgBr with which to prepare sample. A single AgBr photocatalyst was prepared similarly with the above method without the GO/CdS aqueous solution and heating at 180 °C. The preparation method of AgBr-rGO was the same as that of CAR without the addition of bare CdS. The preparation method of CdS/AgBr was the same as that of CAR without GO and heating. It used a similar denotation method as that of CAR, i.e., CA-x refers to a material prepared with a CdS/AgBr mass ratio of x.
3. Results and discussion 3.1. Structure and morphology The XRD patterns of the as-synthesized photocatalysts are shown in Fig. 1a. For pure CdS, peaks at 2θ = 24.7°, 28.1°, 43.6°, 47.7°and 52.8° were attributed to the characteristic of diffraction CdS phases (JCPDS No. 75-0581) [32]. For bare AgBr, the peaks located at 26.4°, 31.0°, 44.0°, 55.1°and 64.6° corresponded to the (1 1 1), (2 0 0), (2 2 0), (2 2 2) and (4 0 0) plane diffraction of the AgBr cubic phase (JCPDS No. 060438), respectively [33]. Besides, no peak assigned to Ag0 could be observed in the XRD patterns, which indicated that Ag-AgBr photocatalytic system was not constructed in this study [34]. The trace amount of loaded rGO and the low atomic number suggested that no peak attributed to rGO was observed in the XRD spectra of AgBr-rGO. Fig. 1b shows that the use of rGO in the composite material changed the diffraction peak position of the AgBr (from 44.0° to 44.12°) based on the JCPDS data for (2 2 0) diffraction peak of AgBr. This indicates that rGO was included successfully into the AgBr lattice [24]. In the XRD patterns of CA-10 and CAR-10, the diffraction peaks characteristic of AgBr were not observed because of the low content of AgBr. Meanwhile, compared to the (1 0 3) diffraction peak of CdS, the peaks had a certain deflection by the close up view could be found for CA-10 and CAR-10 (Fig. 1c) suggesting the existence of an interfacial interaction between CdS and AgBr or AgBr-rGO; the as-synthesized photocatalysts (i.e. CAR) were likely not simple hybrids [35]. The dimensional and morphological features of as-synthesized CAR10 were demonstrated by TEM and HRTEM. CdS in the CAR-10 nanocomposite appears as aggregates of several elongated nanorod particles with a radius of about 60 nm (Fig. 2). The AgBr particles have diameters
2.3. Characterization X-ray diffraction (XRD) patterns were determined by scanning from a 2θ of 10° to 80° on an X-ray diffractometer (Ultiman IV, Rigaku Corporation, Japan). The morphology and size of the samples were observed by transmission electron microscopy (TEM) (JEM-2100). The elemental composition and chemical valence was also determined using X-ray photoelectron spectroscopy (XPS) (Thermo Fisher Scientific, USA). Ultraviolet–visible diffuse reflectance spectroscopy (DRS) 2
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Fig. 1. XRD patterns of (a) synthesized photocatalysts CdS, AgBr, AgBr-rGO, CA-10 and CAR-10; (b) the (2 2 0) diffraction peak of AgBr in AgBr and AgBr-rGO; (c) the (1 0 3) diffraction peak of CdS in CA-10 and CAR-10.
AgBr [38], and Cd 3d5/2 and Cd 3d3/2 of Cd2+ in CdS [39], respectively. In Fig. 3e, the S 2p XPS spectrum exhibited S2- peaks of CdS at 160.25 (S 2p3/2) and 161.7 eV (2p1/2) [40]. In Fig. 3f, a peak at 284.5 eV was shown in C 1s, which can be ascribed to the Csp2–Csp2 bonds in a pure carbonaceous environment possibly due to graphite or rGO. The weak peaks at 289.2 and 286.5 eV likely correspond to the COOH groups and sp2 CeC bond, respectively. The peak intensity reflective of oxidized carbon species of rGO has a significant decrease versus GO. This result confirmed the reduction of GO to rGO under solvothermal conditions during hybrid composite synthesis [32,41]. The XPS results showed that AgBr, CdS and rGO were successfully compounded consistent with the results of XRD and TEM. 3.3. UV-DRS analysis Light absorption can confirm the corresponding band gap of materials, and thus is an important parameter for semiconductors. Fig. 4a shows the UV–Vis DRS of CdS, AgBr and CAR-10 composites. Compared with CdS, pure AgBr has a narrow light absorption band in the ultraviolet region. The CAR-10 composite exhibits similar DRS spectra with bare CdS and AgBr in the optical light region demonstrating mixed light absorption properties of both CdS and AgBr photocatalysts. The absorption edge of composites exhibits a small red-shift to the higher wave length relative to the AgBr photocatalyst. The band gap of CdS, AgBr and CAR-10 can be calculated from the absorption edge using the Kubelka-Munk equation:
Fig. 2. TEM image of CAR-10 nanoparticles. (Inset shows HRTEM images of CdS and AgBr).
varying from 10 to 100 nm; these were clearly observed growing on the CdS nanorods. rGO, which showed wrinkles. These were clearly observed at the interface between CdS and AgBr. The HRTEM images (inset in Fig. 2) show that the lattice fringes were identified to the (0 0 2) crystallographic phase of CdS with an interplanar spacing d = 0.335 nm; the (1 1 1) crystalline plane of AgBr had d = 0.333 nm. These are consistent with the literature [22,36].
αhν = A (hν − Eg)n/2 ,
3.2. XPS
(1)
where α, h, ν, A and Eg are the diffuse reflection absorption coefficient, Planck constant, frequency of light, absorption constant and band gap, respectively. n depends on the type of optical transition exhibited by the semiconductor (n = 1.0 or 4.0 are assigned to direct transition and indirect transition, respectively). The n for both CdS and AgBr is 1.0 [36,37]. Based on the plot of (αhν)1/2 versus (hν) shown in Fig. 4b, the Eg of CdS and AgBr was estimated to be about 2.3 eV and 2.4 eV,
XPS analysis were performed to demonstrate the chemical construct characteristics of CAR-10 sample. The peaks in XPS spectrum of CAR-10 sample can be seen in Fig. 3a, which is very consistent with the composition of the photocatalysts. In Fig. 3b–d, the peaks appeared at 367.6, 373.8 eV, 68.3, 69.0 eV, 404.5 and 411.3 eV correspond to Ag 3d5/2, Ag 3d3/2 of Ag+ in AgBr [37], Br 3d5/2 and Br 3d3/2 of Br− in 3
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Fig. 3. (a) XPS spectrum of CAR-10 composite containing Ag 3d (b), Br 3d (c), Cd 3d (d), S 2p (e) and C 1s (f).
3.4. Photoelectrochemical analysis
respectively. This is consistent with values reported in the literature [36,38]. More importantly, the band gap of the CAR-10 composite was narrower than that of CdS and AgBr; therefore, the excellent lighttrapping ability is an exception. Furthermore, the narrowed band gap of CAR-10 suggested a strong interfacial interaction between CdS and AgBr [42], which is consistent with the XRD data (3.1).
Photoelectrochemical analysis can provide obvious information on the interfacial charge transfer in photocatalysts [21]. Fig. 5 displays the transient photocurrent responses of pure CdS, pure AgBr, AgBr-rGO and CAR-10 under intermittent visible light irradiation in seven on–off cycles. As expected, CAR-10 exhibits the strongest photocurrent response
Fig. 4. (a) UV–vis DRS spectra of as-synthesized photocatalyst. (b) Relationships of (ahυ)1/2 versus photo energy (hυ) of CdS, AgBr and CAR-10. 4
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Fig. 7. PL spectra of CdS, AgBr and CAR-10.
lowest fluorescence intensity followed by CdS; the highest was AgBr. This confirmed the results of the photocatalytic degradation (3.7) and photocurrent (3.4).
Fig. 5. Photocurrent response of various photocatalysts sample in 0.3 M Na2SO4 solution.
(0.023 mA/cm2). This is about 3.28 times higher than that of pure CdS (0.007 mA/cm2) and 11.5 times that of pure AgBr (0.002 mA/cm2). These results indicate that the CAR-10 composite has the highest separation efficiency for the photoinduced charge transfer of the various photocatalysts. This is consistent with the results of RhB degradation discussed (3.7).
3.7. Photocatalytic performances Photocatalytic activity of as-synthesized photocatalysts was evaluated by the RhB degradation under artificial visible light conditions. Before irradiation, all systems were kept in the dark for 20 min to reach adsorption equilibrium. Fig. 8a shows the discoloration efficiency for RhB on the photocatalyst with various photocatalysts. The results show that 79.2%, 46.6% and 71.4% of RhB was discolored by pure CdS, AgBr, and AgBr-rGO, respectively, under 60 min of irradiation. There was an obvious improvement in the discoloration efficiency for CAR composites. The RhB discoloration ratio for CAR-5, CAR-10 and CAR-15 reached 90.6%, 95.8% and 89.5%, respectively. CAR-10 in particular displayed perfect photocatalytic performance, which was about 1.21 and 2.05 times higher than those of pure CdS and AgBr, respectively. The kinetic curves for RhB discoloration shown in Fig. 8b reveal a linear relationship between ln(C0/Ct) and irradiation time: The pseudofirst order model had the best fit to the experimental data [34,35]. The rate constants (slope k of the regression line) of pure CdS, pure AgBr, AgBr-rGO, CAR-5, CAR-10 and CAR-15 were estimated to be 0.026, 0.009, 0.019, 0.038, 0.051 and 0.037 min−1, respectively. As expected, CAR-10 showed the highest degradation rate constant, which was about 1.96 and 5.67 times higher than those of CdS and AgBr alone, respectively. The discoloration efficiency of the CAR-10 sample (Fig. 8) is slightly better than CAR-5 and CAR-10. This can be attributed to the required number balance between photogenerated electrons from AgBr with photogenerated holes from CdS [15]; this can be achieved by tuning the
3.5. N2 adsorption-desorption isotherms and pore structures Fig. 6 shows that N2 adsorption-desorption performance and pore size distribution of AgBr, CdS and CAR-10. Fig. 6a shows that the adsorption capacity of the AgBr, CdS and CAR-10 to N2 were small under the low relative pressure (P/P0 < 0.5). The BET specific surface areas of CAR-10 (16.38 m2·g−1) were about 2.8 and 1.8 times higher than those of AgBr (5.80 m2·g−1) and CdS (9.11 m2·g−1), respectively. Fig. 6b shows that the pore of all photocatalysts were 1–30 nm. The higher surface areas and the more active sites of the photocatalyst will facilitate the transfer of photogenerated charges; thus, better photocatalytic performance could be expected [43,44]. 3.6. PL analysis The PL spectrum was used to compare the recombination process of photogenerated carriers between various as-synthesized photocatalyst. The PL spectra with 395 nm excitation of AgBr, CdS and CAR-10 composites are shown in Fig. 7. Generally, a lower PL intensity means a photosynthetic lower recombination efficiency carrier and a better photocatalytic activity [45]. The results depicted that CAR-10 had the
Fig. 6. (a) Nitrogen adsorption-desorption isotherms and (b) the pore size distribution curves of CdS, AgBr and CAR-10. 5
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Fig. 8. (a) photodegradation of RhB with CdS, AgBr and series of CAR composites; (b) kinetic data for the degradation of RhB in the presence of different photocatalysts.
light photocatalytic performance of CAR-10 with respect to the degradation of RhB. These data suggests that the introduction of BZQ, Na2-EDTA and ISA had a negative influence on the photocatalytic performance of CAR-10 for the RhB degradation. The RhB removal efficiencies decreased from 95.8% to 18.8% (BZQ), 34.9% (Na2-EDTA) and 65.3% (ISA) within 60 min, respectively. The CA-5 and CA-15 also showed the same quenching characteristics as CAR-10. These data suggest that %O2−, h+ and %OH are the reactive substances during the photodegradation process. Moreover, %O2− is the controlled reactive species in the photocatalytic reaction, and the intermediate of the active radicals formed in the Z-scheme composite could largely promote pollutant degradation efficiencies. This experiment involved simultaneous electron excitation from the VB to the CB of the two individual semiconductors (i.e., CdS and AgBr). In Fig. 11, under the Z-scheme electron transfer routine, the photogenerated holes from CdS VB and the photogenerated electrons from AgBr CB are rapidly combined. This effectively separated the photogenerated holes from AgBr VB, and electrons can be photogenerated from CdS CB in this system. Afterwards, photogenerated electrons from CdS CB and photogenerated holes from AgBr VB produce active radical species capable of attacking and decomposing pollutant molecules. The holes from AgBr VB could react with H2O molecules to form %OH or oxidize organic contaminants directly. The photogenerated electrons from CdS CB can react with the adsorbed O2 on the surface of CdS to produce a reactive superoxide radical [15,55–57]. To further explore the mechanism of photocatalytic degradation, the CAR-10 was examined by DMPO spin-trapped ESR spectroscopy to monitor the active radicals that form during the photodegradation process. Fig. 10c shows that no signal was found for DMPO-%O2− in the dark; a stable DMPO-%O2− peak appeared in the system at 5 min, 10 min, and 15 min after illumination. Fig. 10d showed that there was no peak in DMPO-%OH in the absence of visible light irradiation. Under visible light conditions, the signal of DMPO-%OH displayed at 5 min, 10 min, and 15 min were not obvious. The EPR results were consistent with the results of the quenching experiment discussed above. That is, %O2− dominates the degradation process versus the %OH [35]. To further explore the role of rGO in the system, comparative photodegradation experiments were conducted using series of photocatalyst prepared with and without rGO (Fig. 10b). As expected, the photocatalytic performance of CAR was better than that of CA under same mass ratio of CdS to AgBr (x) conditions. Therefore, with the intervention of rGO, the electron conduction ability of the entire system was strengthened. This improved the efficiency of RhB degradation. The degradation mechanism of the photocatalyst can be summarized as follows (with rGO accelerating the transmission of active substances):
mass ratio of CdS to AgBr. We next compared the RhB degradation rate constant of CAR with the other Z-scheme photocatalysts ZnO/TiO2 [46], AgCl/Bi3O4Cl [47] and Fe3O4/BiOBr [48]. The results show that the as-synthesized CAR-10 has better photocatalytic degradation performance. The stability and reusability of the photocatalyst is of great significance to its practical application. The degradation efficiency of CAR10 shows a slight decrease after four repeated degradations (Fig. 9). This is attributed to the photochemical corrosion in CdS during the degradation process [49]. Compared to the other photocatalyst composites, i.e., CdS/CdWO4 [22], CdS/BiVO4 [50] and CdS/ZnWO4 [51], the photocatalytic activity of CAR-10 had no significant weakening even after four runs of repeated experiments. Moreover, the results of XRD patterns and the XPS spectra exhibited a crystal structure of CAR10 after four cycling runs that are consistent with the fresh photocatalyst (Fig. S1). Therefore, the stability in cycling experiments demonstrated that the construction of CAR-10 composites could retard the photocorrosion of CdS without reductive regents.
3.8. Discussion of photocatalytic mechanism The effects of various scavengers on the decolorization of RhB in the photocatalytic process were conducted to investigate the photocatalytic mechanism of CAR in more details. To the best of our knowledge, three main reactants including h+, %OH and %O2− play a crucial role in the photocatalytic process. Ethylenediamine tetraacetic acid disodium salt (Na2-EDTA), isopropyl alcohol (ISA), and benzoquinone (BZQ) were used to scavenge h+, %OH and %O2−, respectively [52–54]. Fig. 10a displays the influence of various scavengers on the visible-
Fig. 9. Cycle runs of RhB photocatalytic degradation with CAR-10 composites. 6
CdS + hv → h+ + e−,
(2)
AgBr + hv → h+ + e−,
(3)
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Fig. 10. (a) Photodegradation of RhB with series of CA and CAR photocatalysts after introducing various scavengers. (b) Photocatalytic degradation of RhB in the presence of a series of CA and CAR composites prepared with different mass ratios of CdS to AgBr (x); (c) and (d) ESR signals of the DMPO-%O2− and DMPO-%OH adducts recorded with a CAR-10 composite during light irradiation (> 420 nm).
rGO
e− (CB of AgBr) → combinate with h+
(VB of CdS),
h+ (VB of AgBr) + OH− → ·OH,
(5)
e− (CB of CdS) + O2 →· O2−
(6)
·O− 2
+
·OH
+ RhB → Oxidation products
introduction of rGO. This inhibits the recombination rate of photogenerated electrons-hole pairs and accelerates the separation of photogenerated electron-hole pairs. Meanwhile, the preparation of photocatalyst CAR is better than the above-mentioned ZnO/TiO2, AgCl/ Bi3O4Cl and Fe3O4/BiOBr composites. The degradation efficiency is higher. We envision that the photocatalytic application of CAR-10 composites can also target other refractory pollutants; the formation of Z-scheme systems between semiconductors is a facile method for boosting photocatalytic activity.
(4)
(7)
4. Conclusions In conclusion, a solid-state CAR-10 composite was successfully synthesized by growing AgBr on the surface of rGO-CdS, and RhB was photodegraded under visible light irradiation. The CAR-10 photocatalyst exhibited the highest RhB degradation efficiency versus bare CdS, bare AgBr and a series of composite materials with different amounts of CdS. The enhanced photocatalytic activity of CAR-10 composite is due to the heterostructure between AgBr, CdS and the
Declaration of Competing Interest The authors declared that there is no conflict of interest.
Fig. 11. Proposed mechanism for electron transfer in CAR-10 composite photocatalysts. 7
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Acknowledgments [26]
This work was supported by the National Key Project of Research and Development Plan of China (No. 2017YFC0403403-3).
[27]
Appendix A. Supplementary material [28]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2019.144275.
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