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Preparation and photocatalytic properties of visible light driven Ag-AgBr-RGO composite
Separation and Purification Technology 190 (2018) 278–287
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Preparation and photocatalytic properties of visible light driven Ag-AgBrRGO composite
MARK
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Yanqing Yang , Weike Zhang, Ruixiang Liu, Jiamin Cui, Chuan Deng School of Environmental Science and Engineering, Taiyuan University of Technology, 79 West Yingze Street, Taiyuan 030024, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Ag-AgBr-RGO Photocatalytic Degradation Photoactive radicals
A visible light responsive photocatalyst (Ag-AgBr-RGO) consisting of Ag-AgBr composite dispersed over reduced graphene oxide (RGO) was synthesized via a facile method. The as-prepared samples were characterized by Xray diffraction (XRD), Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), Nitrogen adsorption-desorption measurements, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectrum measurements (FTIR), and UV–visible diffused reflectance spectra (UV–vis DRS). The photocatalytic activity of the composite was evaluated by degradation of rhodamine B (RhB) and p-nitrophenol (PNP) under visible light irradiation. The results indicated that the Ag-AgBr-RGO catalyst showed more enhanced photocatalytic activity and stability than the pure Ag-AgBr. The excellent photocatalytic activity of Ag-AgBr-RGO catalyst could be attributed to the heterojunctions among three materials (Ag0, AgBr and RGO). Finally, a possible photo-degradation mechanism was postulated based on the tests of photoactive radicals.
1. Introduction
activity under visible light irradiation because the transformation of Ag+ to Ag0 was usually accompanied during the photocatalytic process. In addition, the reported Ag-AgX photocatalysts often have larger size and wider size distribution. Therefore, it is rather necessary to further improve the photocatalytic activity and stability of Ag-AgX composite. Graphene, with a single layer sp2-bonded carbon atoms arranged in a honeycomb lattice, has been suggested to be a promising supporting material to disperse and stabilize inorganic nanoparticles for potential applications in the catalysis fields. Coupling with graphene and graphene derivatives, such as graphene oxide (GO) and reduced graphene oxide (RGO), has been proved to be an effective strategy to improve the quantum yield of a semiconductor photocatalyst [13,14]. What’s more, GO could provide abundant opportunities for the construction of GObased photocatalysts because of its large specific surface area, high optical transmittance, and unique electronic properties caused by the locally conjugated aromatic system [15–17]. Furthermore, GO are decorated with diverse oxygen-containing functional groups, including carbonxyl, hydroxyl, epoxide, which increases its solubility and provide fertile opportunities for the construction of GO-based hybrid composites [18,19]. Therefore, in virtue of the excellent properties of GO, promising results can be expected from coupling GO with Ag-AgX as it is expected to improve the efficiency of photocatalytic activity and fabricate the Ag-AgX species with controlled size and shape. Herein, considering the broad interests of GO and the significant concerns of visible light driven plasmonic photocatalysts, we developed
Water pollution caused by organic dyes and aromatic compounds has a negative impact on human health and ecosystem. However, it is difficult to degrade them completely by conventional physical, chemical or biological techniques because of their structural stability and resistance to biodegradation [1,2]. Therefore, developing a highly-efficient and cost-effective clean technology for decomposition of the harmful organics has always been the pursuit of environmental remediation [3]. In recent years, the semiconductor-aided photocatalysis using solar energy has been recognized as one of the most effective and green technologies to solve the existing environmental problems [4]. Of the well-known photocatalysts, Although TiO2 has proven to be one of the most excellent and most cost-effective materials for the degradation of organic pollutants due to its relatively high reactivity, innocuousness, chemical and biological stability [5–7], its practical industrial application is significantly limited by widely-accepted two bottleneck factors, including a poor absorption of visible light with a wide band gap and low quantum efficiency arising from the rapid recombination of photo-induced electrons and holes [8,9]. Recently, it was demonstrated that Ag-AgX (X = Cl, Br, I) could work as a prospective photocatalyst under visible light irradiation, which originates from the surface plasmon resonance of metallic Ag0 and its synergistic effect together with the photosensitive characteristic of AgX [10–12]. Unfortunately, the semiconductor had no stable
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Corresponding author. E-mail address: [email protected] (Y. Yang).
http://dx.doi.org/10.1016/j.seppur.2017.09.003 Received 12 July 2017; Received in revised form 30 August 2017; Accepted 1 September 2017 Available online 02 September 2017 1383-5866/ © 2017 Elsevier B.V. All rights reserved.
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2.4. Evaluation of photocatalytic activity
a facile solution route to synthesize a kind of Ag-AgBr-RGO composites. Here, the hybridized RGO sheets serve as acceptors of the generated electrons of semiconductors and then effectively suppress the charge recombination. Moreover, the Ag-AgBr-RGO catalyst exhibited excellent activity and stability for the photodegradation of organic pollutants under visible light irradiation. Finally, a possible enhanced photocatalytic mechanism was investigated and presented on the basis of our experimental tests and modern spectra characterizations. This work should provide an easy avenue and open new opportunities for an in-deep investigation on graphene-based photocatalysts for their practical applications.
The photocatalytic performance of the as-prepared photocatalyst was evaluated by decomposing RhB and PNP under visible light irradiation. A 300 W Xe lamp was used as the light source with a 400 nm cutoff filter to ensure complete removal of radiation below 400 nm. The photodegradation experiments were carried out at room temperature as follows: aqueous suspensions of RhB (100 mL, 10 mg/L) or PNP (100 mL, 5 mg/L) with the as-prepared catalyst were placed in the beaker. Prior to irradiation, the dispersions were magnetically stirred in the dark for 30 min to disperse the photocatalyst sufficiently. At given irradiation time intervals, about 6 mL dispersions were collected and centrifuged to remove the particles. The absorption UV–vis spectrum of the centrifugated solution was then recorded using an UV–visible spectrophotometer (Unico UV-2102PC). The photocatalytic degradation efficiency was calculated using the formula as follows:
2. Experimental section 2.1. Materials
D% = [(C0−Ct )/C0] ∗100% = [(A 0−At)/A 0] ∗100%
Silver nitrate (AgNO3) and Hexadecyl trimethyl ammonium bromide (CTAB) were purchased from Sinopharm Chemical Reagent Co., Ltd, China. Graphene oxide aqueous solution (12.35 mg/mL) was supplied by Shanxi Coal Chemical Research Institute, China. Polyvinyl pyrrolidone (PVP) and absolute ethanol were obtained from Tianjin, P. R. China. All chemicals were of analytical grade without further purification. Distilled water was used in the whole experiments.
(1)
where C0 and A0 denotes the initial concentration and absorption balance value, respectively. Ct and At is the concentration and absorption balance value of t time. 3. Results and discussion 3.1. Formation mechanism
2.2. Preparation
Fig. 1 displays the graphical illustration for the formation mechanism of the synthesized Ag-AgBr-RGO composite. The GO can be well dispersed in water to form a homogeneous and stable light-brown solution owing to the existence of oxygenous groups, such as hydroxyl group, carboxyl group, oxygen-containing groups [20]. In the GO solution, the negative functional groups can easily combined with Ag+ to form Ag+ -GO by an electrostatic attraction interaction after the addition of AgNO3 solution. The well dispersion of Ag+ ions leads to the formation of AgBr crystal nucleus. What’s more, in the process of AgBr grain growth, the wrap of GO sheets can strain the diffusion of Ag+ and Br- ions, thus slowing down the growth rate of AgBr. Following the solvothermal and photoreduction process, GO was reduced to RGO and certain amounts of Ag+ ions were reduced to Ag0 nanoparticles under laboratory light conditions, leading to the final formation of Ag-AgBrRGO composite.
Ag-AgBr-RGO composite was synthesized via a facile and fast solvothermal-photoreduction method. In a typical procedure, GO was added to polyvinyl pyrrolidone (PVP) ethanol solution with vigorous stirring for 10 min, the AgNO3 ethanol solution was then added drop by drop. The mixture was then ultrasonicated with probe ultrasonic for 30 min and marked as solution A. Next CTAB solution was added to solution A at a rate of 0.4 mL/min. Subsequently, the mixture was transferred into a Teflon-lined stainless steel autoclave, and kept at 100 °C for 30 min. After cooling the mixture down to room temperature, the samples were collected, washed with distilled water and absolute ethanol for three times. The samples were then dried at 70 °C to obtain AgBr-RGO composite. The reduction of some Ag+ ions to Ag0 was carried out via irradiation by a 300 W UV–vis light source for 30 min. The precipitate was then collected and dried in air to obtain the Ag-AgBr-RGO composite. Reference AgBr and Ag-AgBr were prepared using the same procedures without the irradiation steps and introduction of GO, respectively.
3.2. XRD analysis XRD was applied to detect the phase composition and phase structure of the as-prepared samples. The XRD peaks observed at 26.7°, 30.9°, 44.3°, 52.5°, 55.1°, 64.5° and 73.2° can be perfectly indexed to the (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2), (4 0 0) and (4 2 0) planes coinciding with the standard face centered cubic AgBr phase (JCPDS No. 06-0438). The peak at 38.2° can be indexed to (1 1 1) reflection of Ag0 (JCPDS No. 65-2871). However, no peak attributed to RGO was observed in the XRD pattern because the trace amount of loaded RGO with a low atomic number could not be resolved by XRD. Besides, the addition of GO did not change the diffraction peak position of AgBr in the composite when compared with the JCPDS standard data of AgBr, which indicated that RGO was not incorporated into the lattice of AgBr.
2.3. Characterization The structural information of the as-synthesized powders was collected by X-ray diffraction (XRD) performed on a D8 ADVANCE A25 using Cu Kα radiation (λ = 1.5406 Å). The morphologies were further examined with scanning electron microscopy (SEM, JSM-7001F) and transmission electron microscopy (TEM, JEM-2100F) operated at an accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) analysis was carried out by using a Kratos Analytical AXIS Ultra DLD spectrometer using a monochromatic Al Kα source. The BrunauerEmmett–Teller (BET) surface area and porosity were measured on a Quantachrome Autosorb-1 (USA) automated sorption system. Fourier transform infrared spectra (FTIR) were recorded from KBr pellet in a Thermo Nicolet 380 spectrophotometer. The optical properties of the samples were studied by the UV–visible diffuse reflectance spectroscopy (DRS) using a UV–vis spectrometer (UV2550, Shimadzu, Japan) in the range of 250–700 nm, in which BaSO4 was used as the reflectance standard material. The concentration of Ag ions in remaining solutions was measured by inductively coupled plasma atomic emission spectrometry (ICP-AES, Thermo iCAP6300, USA).
3.3. SEM and TEM images To further obtain the microscopic morphology and structure information, the SEM and TEM analysis of the as-prepared samples were investigated. As shown in Fig. 3a, the pure Ag-AgBr particles exhibited the irregular and aggregated grains with size of several micrometers. In contrast, when the GO was added to the reaction system, the Ag-AgBr particles were well dispersed on the surface of RGO sheets, and the average size of Ag-AgBr decreased to ca. 0.6 μm. This could be due to 279
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Fig. 1. Schematic illustration of the formation of the AgAgBr-RGO composite.
the interaction between the oxygen-functional groups and hydroxyl groups of RGO and that firmly bonded Ag-AgBr particles, thus preventing their aggregation. TEM images of Ag-AgBr-RGO composite further revealed the evidence of Ag-AgBr particles deposited on the surface of RGO, and exhibited a homogeneous distribution on the RGO sheets. The observed lattice spacing of 0.204 nm in Fig. 3d corresponded to the (2 0 0) crystallographic planes of Ag0. In view of the AgBr phase in Fig. 2 and Ag0 particles in Fig. 3d, it is clear that the asprepared photocatalyst can be referred to the Ag-AgBr particles that are well loaded on the RGO surface.
element composition and chemical status of elements. As shown in Fig. 4, the survey spectrum in Fig.4a indicates that the main elements on the surface of Ag-AgBr-RGO composite are Ag, Br, C and O. Furthermore, the typical high-resolution XPS spectra of Ag 3d of the AgBrRGO and the Ag-AgBr-RGO composite are shown in Fig. 4b. As observed in Fig. 4b, the binding energies of Ag 3d shift to the higher binding energies after irradiated by visible light (λ > 400 nm). The changes in the binding energy indicate the presence of Ag0 generated after photoreduction [21]. In addition, quantitative analysis was performed by utilizing the XPS peak area of individual element and its corresponding sensitivity factor according to the following equation [22]:
3.4. XPS analysis
n(E1)/n(E2) = [A(E1)/S(E1)]/[A(E2)/S(E2)]
The samples were further analyzed by XPS to investigate the surface
(2)
where n is the atomic number, E is the element and S is the elemental Fig. 3. SEM images of (a) pure Ag-AgBr, (b) Ag-AgBr-RGO; TEM images of Ag-AgBr-RGO (c) and (d).
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Fig. 2. XRD patterns of GO, Ag-AgBr and Ag-AgBr-RGO samples.
Fig. 5. FTIR spectra of Ag-AgBr-RGO and GO samples.
sensitivity factor. Based on the XPS results, the atomic ratio between the silver and the bromine species was determined as 1.2:1. This value is larger than the theoretic stoichiometric atomic ratio with the value of 1:1 between the silver and the bromine element of AgBr. The results indicated that the photoinduced reduction of metallic Ag0 occurred in the AgBr-RGO surface, which further implied the coexistence of Ag0
and Ag+ in the composite. The XPS spectra of the Ag-AgBr-RGO composite in the C1s region show three peaks at 284.6 eV, 286.6 eV and 288.6 eV, which can be assigned to the sp2-hybridized carbon (CeC), epoxy/hydroxyls carbon (CeO) and carboxyl (OeC]O), respectively [23,24]. Compared to
Fig. 4. The XPS survey spectra of (a) the Ag-AgBr-RGO, (b) high resolution XPS spectra of Ag 3d; XPS spectra of C1s of the (c) GO and (d) Ag-AgBr-RGO.
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unreduced GO (Fig. 4c), the intensity of the peaks ascribed to the CeO and OeC]O apparently decreased, indicating the successful deoxygenation of GO to RGO.
3.5. FTIR analysis In order to attain a better insight into the chemical structure of AgAgBr-RGO, FTIR analysis was carried out (Fig. 5). GO shows a broad peak around 3400 cm−1 which corresponds to OeH stretching vibration, and the two peaks at 1716 cm−1 and 1222 cm−1 correspond to the carboxylates or ketones C]O stretching and CeOeC stretching vibrations [25–27]. The peak around 1051 cm−1 for the CeO stretching vibration indicates the presence of the epoxide group in the GO layer, and peak at 1620 cm−1 is attributed to the vibrations of the adsorbed water molecules and unoxidize graphite [28]. After the solvothermal and photoreduction process, the intensity of characteristics bands corresponding to oxygen functional groups exhibit a significant decrease, suggesting the effective reduction of GO to RGO, which is in good agreement with the XPS results.
Fig. 7. UV–vis diffuse reflectance spectra of samples (a) Ag-AgBr-RGO, (b) Ag-AgBr and (c) AgBr.
3.7. UV–vis DRS analysis 3.6. N2 adsorption-desorption isotherms
The UV–vis diffuse reflectance spectra in the wavelength range of 250–700 nm for AgBr, Ag-AgBr and Ag-AgBr-RGO are depicted in Fig. 7. It can be clearly seen that the pure AgBr cannot strongly absorb visible light. However, the curve (b) reveals that Ag-AgBr sample exhibits the stronger visible light absorption ability to a certain extent based on the SPR of Ag0 nanoparticles, which derives from the enhancement consequence of the local electromagnetic field due to the collective response of the metal free electrons at specific wavelength [29–32]. Notably, the Ag-AgBr-RGO sample shows the strongest light absorption feature at the wavelength of 400–700 nm. This observation indicates that the introduction of RGO should generate an impurity band and narrow the band gap of AgBr [33]. Their corresponding band gap energy of was estimated by the equation [34]:
The Brunauer-Emmett-Teller (BET) specific surface area and porosity of the Ag-AgBr-RGO composite was investigated by the nitrogen adsorption-desorption isotherms. As shown in Fig. 6, the isotherms are identified as type IV with a distinct hysteresis loop observed in the range of (0–1.0) P/P0 (BDDT classification), which indicates the presence of mesopores in the size range of 2–50 nm. The BET specific surface area of Ag-AgBr-RGO was calculated to be 162.58 m2/g, which was much larger than that of Ag-AgBr (62.38 m2/g). This result can be attributed to the rougher surface of the investigated composite and the existence of RGO that had much larger specific surface area. The larger specific surface area of photocatalyst should offer more interface-reaction sites and thus enhancing the photocatalytic efficiency. In addition, the pore size distribution of Ag-AgBr-RGO was determined by using the (BJH) method (inset in Fig. 6), which indicates the presence of mesopores in the Ag-AgBr-RGO composite. The high specific surface area and porous structure should provide more active sites and allow the reactive molecules to easily diffuse during the photocatalytic degradation process.
E = 1240/ λ
(3)
which λ is the maximum absorption wavelength of photon. The calculated band gap of AgBr, Ag-AgBr and Ag-AgBr-RGO are 3.42 eV, 2.51 eV and 1.93 eV, respectively. In other words, due to the SPR effect of Ag0 NPs and the introduction of RGO, Ag-AgBr-RGO exhibits the enhanced absorption in the visible region compared with the pure AgBr and Ag-AgBr, which is in favor of the improved photocatalytic Fig. 6. N2 adsorption-desorption isotherms and (inset) pore size distribution of Ag-AgBr-RGO catalyst.
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Fig. 8. Photodegradation efficiencies of RhB (a) under different conditions, (b) under different content of RGO in Ag-AgBr-RGO.
performance. 3.8. Photocatalytic performance RhB, a chemically stable molecule, was chosen as a representative model pollutant to evaluate the photocatalytic performance of samples. As clearly shown in Fig. 8a, a blank test in the absence of photocatalyst under visible light irradiation shows that the photolysis of RhB was negligible. For comparison, degradation of RhB on pure Ag-AgBr was also performed under the same conditions. Fig. 8a shows the visible light photocatalytic activity of Ag-AgBr-RGO was obviously higher than that of Ag-AgBr. The remarkably enhanced visible photocatalytic activity of Ag-AgBr-RGO is mainly ascribed to the synergetic effect between Ag-AgBr and RGO. On one hand, the specific surface area of the RGO is very high, thus rendering Ag-AgBr-RGO with specific surface area and strong adsorption capacity. On another hand, the excellent electron-mobility of graphene could suppress the charge recombination, and consequently enhancing the photocatalytic activity. Notably, the photocatalytic activity was not gradually enhanced with the increasing of RGO content in the composite (seen in Fig. 8(b)). This phenomenon may be ascribed to the following reasons: (i) RGO may absorb and consume a part of visible light energy, thus resulting in a light competition between Ag-AgBr and RGO, which decreases the photocatalytic performance to a certain extent [26,35]. (ii) The excessive RGO can act as a kind of recombination center and then promote the recombination of e--h+ pairs in RGO surface [36]. The mineralization degree of RhB over Ag-AgBr-RGO composite was examined by TOC measurement. As shown in Fig. 9, the discoloration and TOC removal efficiency of RhB in 30 min reached 100% and 82%, respectively, indicating that the Ag-AgBr-RGO composite can effectively mineralize the organic contaminants during the photocatalysis process. To further study the photocatalytic degradation process of RhB, the variation of the UV–vis spectra of RhB degraded in the presence of AgAgBr-RGO was illustrated in Fig. 10. The maximum absorbance shifts from 554 nm to 498 nm along with the rapid decrease of RhB during the photodegradation process. The hypsochromic-shift should be ascribed to the de-ethylation process, suggesting a chemical change of RhB, other than an adsorption during the degradation process [37,38]. The rapid decrease of RhB is caused by the cleavage of the whole conjugated chromophore structure of RhB, which indicates that the RhB can be efficiently degraded to small molecules, and be finally mineralized to CO2. The result demonstrates that both the N-deethylation process and
Fig. 9. Discoloration and TOC removal efficiency of RhB over Ag-AgBr-RGO composite under visible light irradiation.
Fig. 10. UV–vis absorption spectra of RhB solution at different contact times with AgAgBr-RGO, inset: the color change of RhB during the process.
the cleavage of conjugated chromophore structure could occur simultaneously during the photocatalytic reaction [39]. It is well-known that PNP is a priority aromatic compound and has 283
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Fig. 11. Degradation and TOC removal of PNP and (inset) time-dependence absorption spectra during the photocatalytic process.
Fig. 12. Photocatalytic stability for the degradation of RhB over the as-prepared Ag-AgBr (a) and Ag-AgBr-RGO (b) samples.
Fig. 13. FTIR spectra of (a) RhB, (b) original Ag-AgBr-RGO and (c) Ag-AgBr-RGO after 5th run reaction.
Fig. 14. Ag ions concentration in solution during the photocatalytic process.
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Fig. 15. The mechanism of Ag+ concentration changed with time.
the relatively high toxicity and persistence in aqueous media [40]. In order to further investigate the catalytic activity of the as-prepared AgAgBr-RGO composite, the degradation of PNP was also carried out. The inset of Fig. 11 shows the time-dependent absorption spectra of PNP in the presence of Ag-AgBr-RGO composite. It can be seen that the characteristic peak of PNP at 317 nm diminished quickly with the increase of irradiation time. Besides, the mineralization degree of PNP was further examined by analyzing the decrease in TOC (Fig. 11). It is found that the degradation and TOC removal efficiency of PNP could reach 99.3% and 87% after 180 min visible light irradiation, indicating that the Ag-AgBr-RGO catalyst is efficient for the degradation of PNP as well. As the stability of photocatalysts is a general requirement for the practical application, the reusability of the Ag-AgBr-RGO composite was investigated. For comparison, the cycling experiments of Ag-AgBr were also carried out to evaluate its long-term serving life. The recycled Ag-AgBr-RGO and Ag-AgBr catalyst was separated easily from the reaction solution by a simple precipitation method and was used for the next run. As shown in Fig. 12b, Ag-AgBr-RGO composite exhibits enough photocatalytic stability during the repeated experiments without any significant loss of photocatalytic activity. In contrast, when using Ag-AgBr as catalyst, the degradation efficiency of RhB decreased apparently after five runs. Only ca. 37% RhB molecules were decomposed in the fifth run, which may be caused by inevitable photoreduction of Ag+ ions to metallic Ag0 under visible light irradiation. However, in the presence of RGO, the electrons derived from AgBr and Ag0 nanoparticles can be transferred to RGO, and thus prevent the photoreduction property during the light irradiation process. It is thus confirmed that the RGO greatly enhanced the stability of the Ag-AgBr catalyst. Also can be seen from Fig. 13, no changes of chemical bonds in the FTIR spectrum of the Ag-AgBr-RGO composite were observed after five cycles compared with the fresh one, indicating that the Ag-AgBr-RGO composite was stable and active in successive cycles. In addition, the Ag ions concentration in the RhB aqueous solution versus irradiation time in the photocatalytic system is shown in Fig. 14. The Ag ions concentration increased with increasing the irradiation time and attained a peak value at 10 min when the degradation of RhB reached the maximum. Notably, the Ag ions concentration in the RhB aqueous solution clearly decreased with the decrease of photodegradation efficiency. This phenomenon can be illustrated by Fig. 15. Photogenerated electrons and holes are produced under visible light
Fig. 16. The photocatalytic degradation of RhB with different quenchers under visible light irradiation.
Fig. 17. Proposed photocatalytic mechanism over Ag-AgBr-RGO under visible light irradiation.
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irradiation, and the electrons can be transferred to the RGO surface. On the other hand, the holes transfer to AgBr with its surface negatively charged by Br− to form Br0. Ag ions concentration in the RhB aqueous solution increased during this time. Br0 can oxidize RhB and be reduced to Br− again. The leaching Ag+ ions further react with Br- to form AgBr, maintaining the balance of between Ag0 and AgBr, thus ensuring the stability of Ag-AgBr-RGO composite.
Innovation Programs of Higher Education Institutions in Shanxi (STIP, 2016147), Natural Science Foundation of Shanxi (2015021062) and the Science Foundation of Taiyuan University of Technology for Young Teachers (1205-04020102). The authors thank Sehrina Eshon for the English polishing.
3.9. Visible light photodegradation mechanism
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In general, the photo-induced active species such as trapped holes (h+), hydroxyl radicals (HO·) and superoxide radical anions (O2·–) are well-known as the important active species in the photocatalytic process. To investigate the mechanism of the photocatalytic process, the main active species generated during the decomposition process of RhB were inspected by trapping experiments. As shown in Fig. 16, it is found that the addition of T-butyl alcohol, a well known scavenger of HO· radicals [41], will result in the decrease of photocatalytic degradation efficiency to a certain degree, implying that the free HO·radicals has effect on RhB degradation. Additionally, the photodegradation efficiency was evidently decreased in the presence of h+ radical scavenger (Ammonium oxalate, AO) [42], which suggest that the holes were the main active species in the photocatalytic reaction. Meanwhile, the degradation efficiency of RhB was depressed evidently with the addition of O2·– scavenger (1,4-Benzoquinone, BQ) [43,44], which suggests that the O2·– was an another important active species in the reaction. Based on the obtained results, a possible photocatalytic mechanism for the Ag-AgBr-RGO composite is shows in Fig. 17. AgBr and Ag0 can be excited by visible light due to the narrow band gap and surface plasmon resonance, respectively. Under visible light irradiation, the photocatalytic reaction is initiated by the absorption of visible light photons to fabricate the photogenerated electrons and holes. Generally, the majority of e−-h+ pairs rapidly recombines with each other and only a fraction of them participate in the photocatalytic degradation process [45]. In the presence of RGO, the photoinduced electrons can transfer to the RGO surface to produce O2·–, which is a strong oxidant species to degrade RhB and PNP molecules. As a consequence, the RGO nanosheets work as a highly efficient co-catalyst for the rapid transfer of photogenerated electrons to promote the charge separation and to enhance the photocatalytic activity [46]. In addition, the HO% radicals generated from the reaction of H2O and active holes can degrade organic pollutants effectively under visible light irradiation. Furthermore, reactive holes at the valence band are able to oxidize organic pollutants directly because of the strong oxidation potential. All in all, higher surface area, better optical absorption property, faster transferring rate of charges and lower recombination rate of electron-hole pairs contribute to the photocatalytic activity of Ag-AgBr-RGO composite. 4. Conclusions The well-monodispersed Ag-AgBr particles bonded on the RGO sheets have been successfully synthesized via a facile solvothermalphotoreduction method. The as-prepared Ag-AgBr-RGO composite, which integrates the synergetic effect of the nanosized plasmonic photocatalyst of Ag-AgBr and the support of RGO, displays much higher photocatalytic activity and more excellent stability for degradation of RhB and PNP than pure Ag-AgBr. Photocatalytic mechanism investigations demonstrate that the degradation of RhB over Ag-AgBrRGO composite under visible light irradiation is mainly via O2·– oxidation mechanism and direct h+ oxidation pathway. Such a composite photocatalyst is promising for water purification applications and environmental remediation. Acknowledgements This work was supported by the Scientific and Technological 286
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Preparation and photocatalytic properties of visible light driven Ag-AgBrRGO composite
MARK
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Yanqing Yang , Weike Zhang, Ruixiang Liu, Jiamin Cui, Chuan Deng School of Environmental Science and Engineering, Taiyuan University of Technology, 79 West Yingze Street, Taiyuan 030024, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Ag-AgBr-RGO Photocatalytic Degradation Photoactive radicals
A visible light responsive photocatalyst (Ag-AgBr-RGO) consisting of Ag-AgBr composite dispersed over reduced graphene oxide (RGO) was synthesized via a facile method. The as-prepared samples were characterized by Xray diffraction (XRD), Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), Nitrogen adsorption-desorption measurements, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectrum measurements (FTIR), and UV–visible diffused reflectance spectra (UV–vis DRS). The photocatalytic activity of the composite was evaluated by degradation of rhodamine B (RhB) and p-nitrophenol (PNP) under visible light irradiation. The results indicated that the Ag-AgBr-RGO catalyst showed more enhanced photocatalytic activity and stability than the pure Ag-AgBr. The excellent photocatalytic activity of Ag-AgBr-RGO catalyst could be attributed to the heterojunctions among three materials (Ag0, AgBr and RGO). Finally, a possible photo-degradation mechanism was postulated based on the tests of photoactive radicals.
1. Introduction
activity under visible light irradiation because the transformation of Ag+ to Ag0 was usually accompanied during the photocatalytic process. In addition, the reported Ag-AgX photocatalysts often have larger size and wider size distribution. Therefore, it is rather necessary to further improve the photocatalytic activity and stability of Ag-AgX composite. Graphene, with a single layer sp2-bonded carbon atoms arranged in a honeycomb lattice, has been suggested to be a promising supporting material to disperse and stabilize inorganic nanoparticles for potential applications in the catalysis fields. Coupling with graphene and graphene derivatives, such as graphene oxide (GO) and reduced graphene oxide (RGO), has been proved to be an effective strategy to improve the quantum yield of a semiconductor photocatalyst [13,14]. What’s more, GO could provide abundant opportunities for the construction of GObased photocatalysts because of its large specific surface area, high optical transmittance, and unique electronic properties caused by the locally conjugated aromatic system [15–17]. Furthermore, GO are decorated with diverse oxygen-containing functional groups, including carbonxyl, hydroxyl, epoxide, which increases its solubility and provide fertile opportunities for the construction of GO-based hybrid composites [18,19]. Therefore, in virtue of the excellent properties of GO, promising results can be expected from coupling GO with Ag-AgX as it is expected to improve the efficiency of photocatalytic activity and fabricate the Ag-AgX species with controlled size and shape. Herein, considering the broad interests of GO and the significant concerns of visible light driven plasmonic photocatalysts, we developed
Water pollution caused by organic dyes and aromatic compounds has a negative impact on human health and ecosystem. However, it is difficult to degrade them completely by conventional physical, chemical or biological techniques because of their structural stability and resistance to biodegradation [1,2]. Therefore, developing a highly-efficient and cost-effective clean technology for decomposition of the harmful organics has always been the pursuit of environmental remediation [3]. In recent years, the semiconductor-aided photocatalysis using solar energy has been recognized as one of the most effective and green technologies to solve the existing environmental problems [4]. Of the well-known photocatalysts, Although TiO2 has proven to be one of the most excellent and most cost-effective materials for the degradation of organic pollutants due to its relatively high reactivity, innocuousness, chemical and biological stability [5–7], its practical industrial application is significantly limited by widely-accepted two bottleneck factors, including a poor absorption of visible light with a wide band gap and low quantum efficiency arising from the rapid recombination of photo-induced electrons and holes [8,9]. Recently, it was demonstrated that Ag-AgX (X = Cl, Br, I) could work as a prospective photocatalyst under visible light irradiation, which originates from the surface plasmon resonance of metallic Ag0 and its synergistic effect together with the photosensitive characteristic of AgX [10–12]. Unfortunately, the semiconductor had no stable
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Corresponding author. E-mail address: [email protected] (Y. Yang).
http://dx.doi.org/10.1016/j.seppur.2017.09.003 Received 12 July 2017; Received in revised form 30 August 2017; Accepted 1 September 2017 Available online 02 September 2017 1383-5866/ © 2017 Elsevier B.V. All rights reserved.
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2.4. Evaluation of photocatalytic activity
a facile solution route to synthesize a kind of Ag-AgBr-RGO composites. Here, the hybridized RGO sheets serve as acceptors of the generated electrons of semiconductors and then effectively suppress the charge recombination. Moreover, the Ag-AgBr-RGO catalyst exhibited excellent activity and stability for the photodegradation of organic pollutants under visible light irradiation. Finally, a possible enhanced photocatalytic mechanism was investigated and presented on the basis of our experimental tests and modern spectra characterizations. This work should provide an easy avenue and open new opportunities for an in-deep investigation on graphene-based photocatalysts for their practical applications.
The photocatalytic performance of the as-prepared photocatalyst was evaluated by decomposing RhB and PNP under visible light irradiation. A 300 W Xe lamp was used as the light source with a 400 nm cutoff filter to ensure complete removal of radiation below 400 nm. The photodegradation experiments were carried out at room temperature as follows: aqueous suspensions of RhB (100 mL, 10 mg/L) or PNP (100 mL, 5 mg/L) with the as-prepared catalyst were placed in the beaker. Prior to irradiation, the dispersions were magnetically stirred in the dark for 30 min to disperse the photocatalyst sufficiently. At given irradiation time intervals, about 6 mL dispersions were collected and centrifuged to remove the particles. The absorption UV–vis spectrum of the centrifugated solution was then recorded using an UV–visible spectrophotometer (Unico UV-2102PC). The photocatalytic degradation efficiency was calculated using the formula as follows:
2. Experimental section 2.1. Materials
D% = [(C0−Ct )/C0] ∗100% = [(A 0−At)/A 0] ∗100%
Silver nitrate (AgNO3) and Hexadecyl trimethyl ammonium bromide (CTAB) were purchased from Sinopharm Chemical Reagent Co., Ltd, China. Graphene oxide aqueous solution (12.35 mg/mL) was supplied by Shanxi Coal Chemical Research Institute, China. Polyvinyl pyrrolidone (PVP) and absolute ethanol were obtained from Tianjin, P. R. China. All chemicals were of analytical grade without further purification. Distilled water was used in the whole experiments.
(1)
where C0 and A0 denotes the initial concentration and absorption balance value, respectively. Ct and At is the concentration and absorption balance value of t time. 3. Results and discussion 3.1. Formation mechanism
2.2. Preparation
Fig. 1 displays the graphical illustration for the formation mechanism of the synthesized Ag-AgBr-RGO composite. The GO can be well dispersed in water to form a homogeneous and stable light-brown solution owing to the existence of oxygenous groups, such as hydroxyl group, carboxyl group, oxygen-containing groups [20]. In the GO solution, the negative functional groups can easily combined with Ag+ to form Ag+ -GO by an electrostatic attraction interaction after the addition of AgNO3 solution. The well dispersion of Ag+ ions leads to the formation of AgBr crystal nucleus. What’s more, in the process of AgBr grain growth, the wrap of GO sheets can strain the diffusion of Ag+ and Br- ions, thus slowing down the growth rate of AgBr. Following the solvothermal and photoreduction process, GO was reduced to RGO and certain amounts of Ag+ ions were reduced to Ag0 nanoparticles under laboratory light conditions, leading to the final formation of Ag-AgBrRGO composite.
Ag-AgBr-RGO composite was synthesized via a facile and fast solvothermal-photoreduction method. In a typical procedure, GO was added to polyvinyl pyrrolidone (PVP) ethanol solution with vigorous stirring for 10 min, the AgNO3 ethanol solution was then added drop by drop. The mixture was then ultrasonicated with probe ultrasonic for 30 min and marked as solution A. Next CTAB solution was added to solution A at a rate of 0.4 mL/min. Subsequently, the mixture was transferred into a Teflon-lined stainless steel autoclave, and kept at 100 °C for 30 min. After cooling the mixture down to room temperature, the samples were collected, washed with distilled water and absolute ethanol for three times. The samples were then dried at 70 °C to obtain AgBr-RGO composite. The reduction of some Ag+ ions to Ag0 was carried out via irradiation by a 300 W UV–vis light source for 30 min. The precipitate was then collected and dried in air to obtain the Ag-AgBr-RGO composite. Reference AgBr and Ag-AgBr were prepared using the same procedures without the irradiation steps and introduction of GO, respectively.
3.2. XRD analysis XRD was applied to detect the phase composition and phase structure of the as-prepared samples. The XRD peaks observed at 26.7°, 30.9°, 44.3°, 52.5°, 55.1°, 64.5° and 73.2° can be perfectly indexed to the (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2), (4 0 0) and (4 2 0) planes coinciding with the standard face centered cubic AgBr phase (JCPDS No. 06-0438). The peak at 38.2° can be indexed to (1 1 1) reflection of Ag0 (JCPDS No. 65-2871). However, no peak attributed to RGO was observed in the XRD pattern because the trace amount of loaded RGO with a low atomic number could not be resolved by XRD. Besides, the addition of GO did not change the diffraction peak position of AgBr in the composite when compared with the JCPDS standard data of AgBr, which indicated that RGO was not incorporated into the lattice of AgBr.
2.3. Characterization The structural information of the as-synthesized powders was collected by X-ray diffraction (XRD) performed on a D8 ADVANCE A25 using Cu Kα radiation (λ = 1.5406 Å). The morphologies were further examined with scanning electron microscopy (SEM, JSM-7001F) and transmission electron microscopy (TEM, JEM-2100F) operated at an accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) analysis was carried out by using a Kratos Analytical AXIS Ultra DLD spectrometer using a monochromatic Al Kα source. The BrunauerEmmett–Teller (BET) surface area and porosity were measured on a Quantachrome Autosorb-1 (USA) automated sorption system. Fourier transform infrared spectra (FTIR) were recorded from KBr pellet in a Thermo Nicolet 380 spectrophotometer. The optical properties of the samples were studied by the UV–visible diffuse reflectance spectroscopy (DRS) using a UV–vis spectrometer (UV2550, Shimadzu, Japan) in the range of 250–700 nm, in which BaSO4 was used as the reflectance standard material. The concentration of Ag ions in remaining solutions was measured by inductively coupled plasma atomic emission spectrometry (ICP-AES, Thermo iCAP6300, USA).
3.3. SEM and TEM images To further obtain the microscopic morphology and structure information, the SEM and TEM analysis of the as-prepared samples were investigated. As shown in Fig. 3a, the pure Ag-AgBr particles exhibited the irregular and aggregated grains with size of several micrometers. In contrast, when the GO was added to the reaction system, the Ag-AgBr particles were well dispersed on the surface of RGO sheets, and the average size of Ag-AgBr decreased to ca. 0.6 μm. This could be due to 279
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Fig. 1. Schematic illustration of the formation of the AgAgBr-RGO composite.
the interaction between the oxygen-functional groups and hydroxyl groups of RGO and that firmly bonded Ag-AgBr particles, thus preventing their aggregation. TEM images of Ag-AgBr-RGO composite further revealed the evidence of Ag-AgBr particles deposited on the surface of RGO, and exhibited a homogeneous distribution on the RGO sheets. The observed lattice spacing of 0.204 nm in Fig. 3d corresponded to the (2 0 0) crystallographic planes of Ag0. In view of the AgBr phase in Fig. 2 and Ag0 particles in Fig. 3d, it is clear that the asprepared photocatalyst can be referred to the Ag-AgBr particles that are well loaded on the RGO surface.
element composition and chemical status of elements. As shown in Fig. 4, the survey spectrum in Fig.4a indicates that the main elements on the surface of Ag-AgBr-RGO composite are Ag, Br, C and O. Furthermore, the typical high-resolution XPS spectra of Ag 3d of the AgBrRGO and the Ag-AgBr-RGO composite are shown in Fig. 4b. As observed in Fig. 4b, the binding energies of Ag 3d shift to the higher binding energies after irradiated by visible light (λ > 400 nm). The changes in the binding energy indicate the presence of Ag0 generated after photoreduction [21]. In addition, quantitative analysis was performed by utilizing the XPS peak area of individual element and its corresponding sensitivity factor according to the following equation [22]:
3.4. XPS analysis
n(E1)/n(E2) = [A(E1)/S(E1)]/[A(E2)/S(E2)]
The samples were further analyzed by XPS to investigate the surface
(2)
where n is the atomic number, E is the element and S is the elemental Fig. 3. SEM images of (a) pure Ag-AgBr, (b) Ag-AgBr-RGO; TEM images of Ag-AgBr-RGO (c) and (d).
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Fig. 2. XRD patterns of GO, Ag-AgBr and Ag-AgBr-RGO samples.
Fig. 5. FTIR spectra of Ag-AgBr-RGO and GO samples.
sensitivity factor. Based on the XPS results, the atomic ratio between the silver and the bromine species was determined as 1.2:1. This value is larger than the theoretic stoichiometric atomic ratio with the value of 1:1 between the silver and the bromine element of AgBr. The results indicated that the photoinduced reduction of metallic Ag0 occurred in the AgBr-RGO surface, which further implied the coexistence of Ag0
and Ag+ in the composite. The XPS spectra of the Ag-AgBr-RGO composite in the C1s region show three peaks at 284.6 eV, 286.6 eV and 288.6 eV, which can be assigned to the sp2-hybridized carbon (CeC), epoxy/hydroxyls carbon (CeO) and carboxyl (OeC]O), respectively [23,24]. Compared to
Fig. 4. The XPS survey spectra of (a) the Ag-AgBr-RGO, (b) high resolution XPS spectra of Ag 3d; XPS spectra of C1s of the (c) GO and (d) Ag-AgBr-RGO.
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unreduced GO (Fig. 4c), the intensity of the peaks ascribed to the CeO and OeC]O apparently decreased, indicating the successful deoxygenation of GO to RGO.
3.5. FTIR analysis In order to attain a better insight into the chemical structure of AgAgBr-RGO, FTIR analysis was carried out (Fig. 5). GO shows a broad peak around 3400 cm−1 which corresponds to OeH stretching vibration, and the two peaks at 1716 cm−1 and 1222 cm−1 correspond to the carboxylates or ketones C]O stretching and CeOeC stretching vibrations [25–27]. The peak around 1051 cm−1 for the CeO stretching vibration indicates the presence of the epoxide group in the GO layer, and peak at 1620 cm−1 is attributed to the vibrations of the adsorbed water molecules and unoxidize graphite [28]. After the solvothermal and photoreduction process, the intensity of characteristics bands corresponding to oxygen functional groups exhibit a significant decrease, suggesting the effective reduction of GO to RGO, which is in good agreement with the XPS results.
Fig. 7. UV–vis diffuse reflectance spectra of samples (a) Ag-AgBr-RGO, (b) Ag-AgBr and (c) AgBr.
3.7. UV–vis DRS analysis 3.6. N2 adsorption-desorption isotherms
The UV–vis diffuse reflectance spectra in the wavelength range of 250–700 nm for AgBr, Ag-AgBr and Ag-AgBr-RGO are depicted in Fig. 7. It can be clearly seen that the pure AgBr cannot strongly absorb visible light. However, the curve (b) reveals that Ag-AgBr sample exhibits the stronger visible light absorption ability to a certain extent based on the SPR of Ag0 nanoparticles, which derives from the enhancement consequence of the local electromagnetic field due to the collective response of the metal free electrons at specific wavelength [29–32]. Notably, the Ag-AgBr-RGO sample shows the strongest light absorption feature at the wavelength of 400–700 nm. This observation indicates that the introduction of RGO should generate an impurity band and narrow the band gap of AgBr [33]. Their corresponding band gap energy of was estimated by the equation [34]:
The Brunauer-Emmett-Teller (BET) specific surface area and porosity of the Ag-AgBr-RGO composite was investigated by the nitrogen adsorption-desorption isotherms. As shown in Fig. 6, the isotherms are identified as type IV with a distinct hysteresis loop observed in the range of (0–1.0) P/P0 (BDDT classification), which indicates the presence of mesopores in the size range of 2–50 nm. The BET specific surface area of Ag-AgBr-RGO was calculated to be 162.58 m2/g, which was much larger than that of Ag-AgBr (62.38 m2/g). This result can be attributed to the rougher surface of the investigated composite and the existence of RGO that had much larger specific surface area. The larger specific surface area of photocatalyst should offer more interface-reaction sites and thus enhancing the photocatalytic efficiency. In addition, the pore size distribution of Ag-AgBr-RGO was determined by using the (BJH) method (inset in Fig. 6), which indicates the presence of mesopores in the Ag-AgBr-RGO composite. The high specific surface area and porous structure should provide more active sites and allow the reactive molecules to easily diffuse during the photocatalytic degradation process.
E = 1240/ λ
(3)
which λ is the maximum absorption wavelength of photon. The calculated band gap of AgBr, Ag-AgBr and Ag-AgBr-RGO are 3.42 eV, 2.51 eV and 1.93 eV, respectively. In other words, due to the SPR effect of Ag0 NPs and the introduction of RGO, Ag-AgBr-RGO exhibits the enhanced absorption in the visible region compared with the pure AgBr and Ag-AgBr, which is in favor of the improved photocatalytic Fig. 6. N2 adsorption-desorption isotherms and (inset) pore size distribution of Ag-AgBr-RGO catalyst.
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Fig. 8. Photodegradation efficiencies of RhB (a) under different conditions, (b) under different content of RGO in Ag-AgBr-RGO.
performance. 3.8. Photocatalytic performance RhB, a chemically stable molecule, was chosen as a representative model pollutant to evaluate the photocatalytic performance of samples. As clearly shown in Fig. 8a, a blank test in the absence of photocatalyst under visible light irradiation shows that the photolysis of RhB was negligible. For comparison, degradation of RhB on pure Ag-AgBr was also performed under the same conditions. Fig. 8a shows the visible light photocatalytic activity of Ag-AgBr-RGO was obviously higher than that of Ag-AgBr. The remarkably enhanced visible photocatalytic activity of Ag-AgBr-RGO is mainly ascribed to the synergetic effect between Ag-AgBr and RGO. On one hand, the specific surface area of the RGO is very high, thus rendering Ag-AgBr-RGO with specific surface area and strong adsorption capacity. On another hand, the excellent electron-mobility of graphene could suppress the charge recombination, and consequently enhancing the photocatalytic activity. Notably, the photocatalytic activity was not gradually enhanced with the increasing of RGO content in the composite (seen in Fig. 8(b)). This phenomenon may be ascribed to the following reasons: (i) RGO may absorb and consume a part of visible light energy, thus resulting in a light competition between Ag-AgBr and RGO, which decreases the photocatalytic performance to a certain extent [26,35]. (ii) The excessive RGO can act as a kind of recombination center and then promote the recombination of e--h+ pairs in RGO surface [36]. The mineralization degree of RhB over Ag-AgBr-RGO composite was examined by TOC measurement. As shown in Fig. 9, the discoloration and TOC removal efficiency of RhB in 30 min reached 100% and 82%, respectively, indicating that the Ag-AgBr-RGO composite can effectively mineralize the organic contaminants during the photocatalysis process. To further study the photocatalytic degradation process of RhB, the variation of the UV–vis spectra of RhB degraded in the presence of AgAgBr-RGO was illustrated in Fig. 10. The maximum absorbance shifts from 554 nm to 498 nm along with the rapid decrease of RhB during the photodegradation process. The hypsochromic-shift should be ascribed to the de-ethylation process, suggesting a chemical change of RhB, other than an adsorption during the degradation process [37,38]. The rapid decrease of RhB is caused by the cleavage of the whole conjugated chromophore structure of RhB, which indicates that the RhB can be efficiently degraded to small molecules, and be finally mineralized to CO2. The result demonstrates that both the N-deethylation process and
Fig. 9. Discoloration and TOC removal efficiency of RhB over Ag-AgBr-RGO composite under visible light irradiation.
Fig. 10. UV–vis absorption spectra of RhB solution at different contact times with AgAgBr-RGO, inset: the color change of RhB during the process.
the cleavage of conjugated chromophore structure could occur simultaneously during the photocatalytic reaction [39]. It is well-known that PNP is a priority aromatic compound and has 283
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Fig. 11. Degradation and TOC removal of PNP and (inset) time-dependence absorption spectra during the photocatalytic process.
Fig. 12. Photocatalytic stability for the degradation of RhB over the as-prepared Ag-AgBr (a) and Ag-AgBr-RGO (b) samples.
Fig. 13. FTIR spectra of (a) RhB, (b) original Ag-AgBr-RGO and (c) Ag-AgBr-RGO after 5th run reaction.
Fig. 14. Ag ions concentration in solution during the photocatalytic process.
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Fig. 15. The mechanism of Ag+ concentration changed with time.
the relatively high toxicity and persistence in aqueous media [40]. In order to further investigate the catalytic activity of the as-prepared AgAgBr-RGO composite, the degradation of PNP was also carried out. The inset of Fig. 11 shows the time-dependent absorption spectra of PNP in the presence of Ag-AgBr-RGO composite. It can be seen that the characteristic peak of PNP at 317 nm diminished quickly with the increase of irradiation time. Besides, the mineralization degree of PNP was further examined by analyzing the decrease in TOC (Fig. 11). It is found that the degradation and TOC removal efficiency of PNP could reach 99.3% and 87% after 180 min visible light irradiation, indicating that the Ag-AgBr-RGO catalyst is efficient for the degradation of PNP as well. As the stability of photocatalysts is a general requirement for the practical application, the reusability of the Ag-AgBr-RGO composite was investigated. For comparison, the cycling experiments of Ag-AgBr were also carried out to evaluate its long-term serving life. The recycled Ag-AgBr-RGO and Ag-AgBr catalyst was separated easily from the reaction solution by a simple precipitation method and was used for the next run. As shown in Fig. 12b, Ag-AgBr-RGO composite exhibits enough photocatalytic stability during the repeated experiments without any significant loss of photocatalytic activity. In contrast, when using Ag-AgBr as catalyst, the degradation efficiency of RhB decreased apparently after five runs. Only ca. 37% RhB molecules were decomposed in the fifth run, which may be caused by inevitable photoreduction of Ag+ ions to metallic Ag0 under visible light irradiation. However, in the presence of RGO, the electrons derived from AgBr and Ag0 nanoparticles can be transferred to RGO, and thus prevent the photoreduction property during the light irradiation process. It is thus confirmed that the RGO greatly enhanced the stability of the Ag-AgBr catalyst. Also can be seen from Fig. 13, no changes of chemical bonds in the FTIR spectrum of the Ag-AgBr-RGO composite were observed after five cycles compared with the fresh one, indicating that the Ag-AgBr-RGO composite was stable and active in successive cycles. In addition, the Ag ions concentration in the RhB aqueous solution versus irradiation time in the photocatalytic system is shown in Fig. 14. The Ag ions concentration increased with increasing the irradiation time and attained a peak value at 10 min when the degradation of RhB reached the maximum. Notably, the Ag ions concentration in the RhB aqueous solution clearly decreased with the decrease of photodegradation efficiency. This phenomenon can be illustrated by Fig. 15. Photogenerated electrons and holes are produced under visible light
Fig. 16. The photocatalytic degradation of RhB with different quenchers under visible light irradiation.
Fig. 17. Proposed photocatalytic mechanism over Ag-AgBr-RGO under visible light irradiation.
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irradiation, and the electrons can be transferred to the RGO surface. On the other hand, the holes transfer to AgBr with its surface negatively charged by Br− to form Br0. Ag ions concentration in the RhB aqueous solution increased during this time. Br0 can oxidize RhB and be reduced to Br− again. The leaching Ag+ ions further react with Br- to form AgBr, maintaining the balance of between Ag0 and AgBr, thus ensuring the stability of Ag-AgBr-RGO composite.
Innovation Programs of Higher Education Institutions in Shanxi (STIP, 2016147), Natural Science Foundation of Shanxi (2015021062) and the Science Foundation of Taiyuan University of Technology for Young Teachers (1205-04020102). The authors thank Sehrina Eshon for the English polishing.
3.9. Visible light photodegradation mechanism
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In general, the photo-induced active species such as trapped holes (h+), hydroxyl radicals (HO·) and superoxide radical anions (O2·–) are well-known as the important active species in the photocatalytic process. To investigate the mechanism of the photocatalytic process, the main active species generated during the decomposition process of RhB were inspected by trapping experiments. As shown in Fig. 16, it is found that the addition of T-butyl alcohol, a well known scavenger of HO· radicals [41], will result in the decrease of photocatalytic degradation efficiency to a certain degree, implying that the free HO·radicals has effect on RhB degradation. Additionally, the photodegradation efficiency was evidently decreased in the presence of h+ radical scavenger (Ammonium oxalate, AO) [42], which suggest that the holes were the main active species in the photocatalytic reaction. Meanwhile, the degradation efficiency of RhB was depressed evidently with the addition of O2·– scavenger (1,4-Benzoquinone, BQ) [43,44], which suggests that the O2·– was an another important active species in the reaction. Based on the obtained results, a possible photocatalytic mechanism for the Ag-AgBr-RGO composite is shows in Fig. 17. AgBr and Ag0 can be excited by visible light due to the narrow band gap and surface plasmon resonance, respectively. Under visible light irradiation, the photocatalytic reaction is initiated by the absorption of visible light photons to fabricate the photogenerated electrons and holes. Generally, the majority of e−-h+ pairs rapidly recombines with each other and only a fraction of them participate in the photocatalytic degradation process [45]. In the presence of RGO, the photoinduced electrons can transfer to the RGO surface to produce O2·–, which is a strong oxidant species to degrade RhB and PNP molecules. As a consequence, the RGO nanosheets work as a highly efficient co-catalyst for the rapid transfer of photogenerated electrons to promote the charge separation and to enhance the photocatalytic activity [46]. In addition, the HO% radicals generated from the reaction of H2O and active holes can degrade organic pollutants effectively under visible light irradiation. Furthermore, reactive holes at the valence band are able to oxidize organic pollutants directly because of the strong oxidation potential. All in all, higher surface area, better optical absorption property, faster transferring rate of charges and lower recombination rate of electron-hole pairs contribute to the photocatalytic activity of Ag-AgBr-RGO composite. 4. Conclusions The well-monodispersed Ag-AgBr particles bonded on the RGO sheets have been successfully synthesized via a facile solvothermalphotoreduction method. The as-prepared Ag-AgBr-RGO composite, which integrates the synergetic effect of the nanosized plasmonic photocatalyst of Ag-AgBr and the support of RGO, displays much higher photocatalytic activity and more excellent stability for degradation of RhB and PNP than pure Ag-AgBr. Photocatalytic mechanism investigations demonstrate that the degradation of RhB over Ag-AgBrRGO composite under visible light irradiation is mainly via O2·– oxidation mechanism and direct h+ oxidation pathway. Such a composite photocatalyst is promising for water purification applications and environmental remediation. Acknowledgements This work was supported by the Scientific and Technological 286
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