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Impacts of sulfidation of silver nanowires on the degradation of bisphenol A in water
Ecotoxicology and Environmental Safety 185 (2019) 109739
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Impacts of sulfidation of silver nanowires on the degradation of bisphenol A in water
T
Yinqing Zhang, Kunkun Wang, Yi Yang, Jinliang Xu, Binbin Sun, Lingyan Zhu∗ Key Laboratory of Pollution Processes and Environmental Criteria (Ministry of Education), Tianjin Key Laboratory of Environmental Remediation and Pollution Control, College of Environmental Science and Engineering, Nankai University, Tianjin, 300350, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: AgNWs Sulfidation Ag2S Photocatalysis Bisphenol A
Silver nanowires (AgNWs) are widely produced in many electronic and optical products, and could be inevitably discharged into the aquatic environments. Sulfidation is one of the most important transformation processes of AgNWs, and could significantly affect their fate and interactions with other pollutants in aquatic environment. In the present study, the sulfidation products of AgNWs with different atomic ratio of Ag and S were prepared under environmentally relevant conditions. The crystal structure, elemental composition, morphology and size of the sulfidation products were comprehensively characterized by powder X-ray diffraction, UV–vis spectroscopy, Xray photoelectron spectroscopy and transmission electron microscope. The products were heterostructured nanowires and the Ag2S/Ag molar ratio increased with extension of the reaction time. The produced Ag2S–Ag nanowires displayed a good photocatalytic activity and facilitated the degradation of the copresent organic pollutant bisphenol A (BPA) under simulated sunlight irradiation. As sulfidation time increased, more Ag2S was generated and the Ag2S–Ag composites displayed high promotion effect on BPA degradation. This effect could be ascribed to the favorable synergistic effects between Ag2S and AgNWs, such as high electron-hole separation efficiency and low charge transfer resistance. The chemical scavenger experiments demonstrated that superoxide anion radicals and photogenerated holes in the sulfidation products of AgNWs could be the main reactive species for photocatalytic degradation.
1. Introduction Silver nanomaterials (nano-Ag) have been widely used in various industrial and commercial products, due to the unique catalytic, sensing properties, optical, and antibacterial effects (Tao and Yang, 2005; Anker et al., 2008; Ahamed et al., 2010). It was estimated that the world-wide production of nano-Ag was 550 tons/year in 2011 and about 435 products (24% of the total products using nanotechnology) contained nano-Ag in 2015 (Piccinno et al., 2012; Vance et al., 2015). As typical one-dimensional nano-Ag, silver nanowires (AgNWs) are deemed as potential building blocks for the next generation of electronic, optical, and sensing devices (Tang and Kotov, 2005). Largequantity production and increasing application will inevitably lead to the release of AgNWs into the environment, thus raising great concerns about their potential interactions with other pollutants present in natural water (Mitrano et al., 2014). Due to the reactive nature, nano-Ag may undergo different physical and chemical transformation processes once released to the environment, such as aggregation, dissolution, chlorination and sulfidation.
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The products of such transformations would in turn greatly affect the environmental behaviors and the interactions with other pollutants (Levard et al., 2012; Ellis et al., 2016). Regardless of the morphology, particle size and coating materials, nano-Ag is readily to react with sulfide to produce silver sulfide (Ag2S), forming a complex of Ag2S and nano-Ag (Liu et al., 2010). The sulfidation of nano-Ag and compositions of the complexes are dependent on the concentration of sulfide, other components in the aquatic environments as well as sunlight irradiation (Liu et al., 2011; Zhang et al., 2016a, 2018a). Sulfidation may not only reduce the overall bioavailability and toxicities of nano-Ag, but also produce Ag2S–Ag hybrid nanomaterials. Ag2S is an important sulfide semiconductor with a bandgap of about 1.0 eV, and Ag2S nanomaterials usually display high stability and strong light absorption in the solar irradiance spectrum (Jiang et al., 2015; Yu et al., 2016). Hybrid nanomaterials based on semiconductor-noble metal are usually promising photocatalysts under visible light arising from the synergistic interactions between each component (Yang et al., 2012; Kochuveedu et al., 2013; Huang et al., 2013; Fan et al., 2015). Thus, composites of Ag2S semiconductor and nano-Ag might be omnipresent in natural aquatic
Corresponding author. E-mail address: [email protected] (L. Zhu).
https://doi.org/10.1016/j.ecoenv.2019.109739 Received 10 June 2019; Received in revised form 17 September 2019; Accepted 27 September 2019 Available online 03 October 2019 0147-6513/ © 2019 Elsevier Inc. All rights reserved.
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dried in the vacuum oven at 90 °C for 1 h. The sulfidation products were named as Ag2S–Ag-1, Ag2S–Ag-4, Ag2S–Ag-8, Ag2S–Ag-12, Ag2S–Ag-16, respectively, representing the products obtained at the sulfidation time of 1, 4, 8, 12, 16 h.
environment (Zhang et al., 2016b), and they are hypothesized to display active photocatalytic activity under visible light (Pang et al., 2010). The photocatalytic activity of heterogeneous photocatalysts is highly related to their crystal structure, specific surface area and morphology (Xiao et al., 2015). Compared to the conventional metal oxide semiconductors, metal sulfides generally have narrow bandgaps and could utilize sunlight more efficiently (Li et al., 2010; Wang et al., 2016; Zhang et al., 2018b). Previous studies reported that many Ag2S–Ag hybrid structures, such as porous nanotubes (Yang et al., 2012), hollow heterodimers (Kumar et al., 2015), spherical nanoparticles (Jiang et al., 2011), heterostructured nanobowls (Li et al., 2015), hierarchical nanowires (Xiong et al., 2016), flower wires (Basu et al., 2017), displayed efficient photocatalytic activities to organic pollutant, such as methylene blue and tetracycline. However, these materials were prepared at strictly controlled reaction conditions, such as high temperature, using templates, or complicated procedures. It remains unclear if the Ag2S–Ag nanoscale heterostructures formed at environmentally relevant conditions would display similar photocatalytic activities, and then affect the fate of organic pollutants in aquatic environments. The purpose of this study was to investigate the impacts of the sulfidation of AgNWs on the abiotic transformation of organic pollutants in aquatic environment under sunlight irradiation, taking bisphenol A (BPA) as a model molecule. Because of the wide utilization, a large amount of BPA was released into the environment (Ohko et al., 2001; Vandenberg et al., 2010), and its ubiquitous presence in aquatic environments aroused great concerns on the ecological risk of BPA (Huang et al., 2012; Mizuta et al., 2017). The Ag2S–Ag hybrid nanowires could generate under environmentally relevant conditions through an in situ sulfidation and deposition method. The concentrations of AgNWs and BPA were set to be as low as possible to simulate their environmental levels but allowing to quantify them by the instruments accurately. To simulate the environmental process, Ag2S–Ag nanostructures with different contents of Ag2S were prepared by varying the period of sulfidation reaction, and monitored by UV–Vis spectrometry and inductively coupled plasma mass spectrometry (ICPMS). The possible photodegradation mechanisms were also investigated.
2.3. Characterization of the sulfidation products of AgNWs X-ray diffraction (XRD) patterns of AgNWs and the sulfidation products of AgNWs were analyzed on a D8-Advanced diffractometer (Bruker, German) with Cu Kα radiation. Ultraviolet–visible (UV–vis) absorption spectra were conducted on a UV-2600 spectrophotometer (Shimadzu, Japan) at room temperature. X-ray photoelectron spectroscopy (XPS) was obtained on a Axis Ultra DLD spectrometer (Kratos, Japan) equipped with a monochromated Al X-ray source. Gaussian curves were applied to fit the spectra. Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray spectroscopy (EDS) were obtained on a JEM-2800 transmission electron microscope (JEOL, Japan). The aqueous suspension of the sulfidation products of AgNWs was placed on the copper grids coated with ultrathin carbon film, and dried overnight in a dust-free box. Electron paramagnetic resonance (EPR) spectra were obtained using an EMX-10/12 EPR spectrometer (Bruker, Germany) at room temperature. 5,5-Dimethyl-1-pyrroline N-oxide (DMPO) was employed as the spin trapping agent.2.4. Photocatalytic degradation of BPA.
Photocatalytic degradation of BPA The photocatalytic activities of the sulfidation products of AgNWs were investigated by degrading BPA under simulated sunlight irradiation. The photocatalytic degradation experiments were performed in a photochemical reactor with 12 quartz test tubes surrounding an 800 W xenon lamp. The intensity of the simulated sunlight was 132 W/m2, and the emission spectrum is shown in Fig. S1. The lamp was cooled by a water jacketed condenser, and the reaction system was magnetically stirred and maintained at room temperature by circulating water. The total volume of the reaction solutions was 40 mL, the concentration of the sulfidation products of AgNWs was 1 μM which was as low as that of AgNWs during sulfidation process, and the dosages of BPA was 0.5 μM, given that the BPA level in natural environment could be as high as 0.6 μM (Fromme et al., 2002). Before irradiation, the reaction solutions were stirred for 0.5 h in dark to establish adsorption/desorption equilibrium for BPA on the sulfidized AgNWs. The photocatalytic experiment started after turning on the xenon lamp. Approximately 1 mL of the reaction solution was sampled at predetermined time, and then centrifuged under 2000g. The concentration of BPA was analyzed on a 1260 Infinity high performance liquid chromatograph (HPLC) instrument (Agilent, USA) with a fluorescence detector (excitation and emission wavelengths were 220 nm and 350 nm, respectively) and an Agilent ZORBAX Eclipse XDB-C18 column at 35 °C. The mobile-phase was methanol/water (65/35 in volume ratio), and the flow rate was 0.15 mL/min. The degradation products of BPA were analyzed by liquid chromatography-tandem mass spectrometry (LC-MS/MS) on a Xevo TQ-S system (Waters, USA) equipped with an Acquity UPLC C18 column. Detailed information of the LC-MS/MS analysis is provided in the supplementary material. The radical-trapping experiments were conducted with terephthalic acid (TPA), benzoquinone (BQ), and potassium iodide (KI), which were applied as the scavengers of hydroxyl radicals (·OH), superoxide anion radicals (·O2−), and photogenerated holes, respectively. 1 μM of each scavenger was added with the sulfidation product of AgNWs, and the other experimental steps and analytic methods are the same as described above.
2. Materials and methods 2.1. Chemicals and reagents AgNO3 (99.99%), ethylene glycol (EG, anhydrous, 99.9%), polyvinylpyrrolidone (PVP, K = 29–32), and Na2S·9H2O (> 99.9%) were obtained from Aladdin Chemistry Co. Ltd. (China). All the chemicals were applied without further purification. Ultrapure water was produced on Millipore Milli-Q Advantage A10 water purification system (US), and used throughout the experiments. AgNWs were synthesized by the modified solvothermal method (Zhang et al., 2016a, 2018a), and the detailed information was provided as supplementary material. The AgNWs stock suspension was stored in dark at 4 °C. 2.2. Sulfidation process of AgNWs The stock solution of Na2S was freshly prepared just before the sulfidation experiments. Predetermined volumes of Na2S solution and AgNWs suspension were added to water to obtain 20 mL of reaction suspension in a quartz test tube with a screw cap. The initial concentration of AgNWs was 1 μM, and the Ag/S mole ratio of the reaction mixture was 2. The test tubes containing the reaction solution were sealed tightly and rotated at 12 rpm facing an 800 W xenon lamp. The pH of the sulfidation solutions was constant in the range of 6.6–6.8, being measured on a Mettler Toledo MP120 pH meter. At each predetermined time, one tube was taken out, and the products were separated from the reaction mixture by centrifugal ultrafiltration. After being washed with ultrapure water and ethanol, the products were 2
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with different reaction time are shown in Fig. 1B. The pure AgNWs exhibit two absorbance peaks at 370 and 400 nm, which could be attributed to the longitudinal mode and the surface plasmon resonance (SPR) of AgNWs (Sun et al., 2002; Ramasamy et al., 2012). After 4 h sulfidation of AgNWs, the longitudinal mode almost disappeared and the SPR absorption band of AgNWs significantly decreased, redshifted and broadened, which was attributed to the transformation of AgNWs to generate Ag2S on the surface. In addition, the spectra exhibited a new absorption band centered at around 500 nm, which corresponded to the band gap of Ag2S. This is similar to the reported results of Ag2S nanocrystals (between 490 and 520 nm) (Zhao et al., 2007) and Ag2S–Ag heterostructures (495 and 514 nm) (Pang et al., 2010; Xiong et al., 2016). When the reaction time increased to 8 h, the SPR band of AgNWs almost disappeared because of further sulfidation, and the characteristic absorption band of Ag2S shifted to around 510 nm. The observed redshifts indicate the growth of Ag2S crystallite and enlarged Ag2S domain in the heterostructures. Their broad absorbance from UV to near-infrared region suggested that the sulfidation products of AgNWs were active under sunlight irradiation. To further investigate the compositions of the sulfidation products of AgNWs, XPS spectra of the Ag2S–Ag-8 were measured and the results are shown in Figs. 2 and S2. The XPS spectrum in Fig. S2 obviously exhibited the signals of silver and sulfur. As shown in Fig. 2A of the Ag 3d spectrum, the peaks at binding energy of 368.5 and 374.4 eV could be respectively assigned to Ag 3d5/2 and 3d3/2 of the metallic state Ag0 (Zhang et al., 2018b). The peaks at 367.9 and 373.8 eV fitted to those of Ag 3d5/2 and 3d3/2 of Ag+ ions. In Fig. 2B, the peaks at 161.3 and 162.5 eV corresponded to the binding energies of S 2p3/2 and 2p1/2 of S2− (Elechiguerra et al., 2005). These results suggested that the sulfidation product was consisted of both metallic Ag and Ag2S. The morphology structure and elemental distribution of the sulfidation product of AgNWs are elucidated by TEM in Fig. 3. The panoramic view of the sulfidation product showed uniform nanowires of ~50 nm in diameter and several micrometers in length. The surface of the product was much rough compared with AgNWs in Fig. S3, and there were many nanoscale grains appearing on the entire substrate surface. The HRTEM (Fig. 3B) clearly revealed that the distinctive lattice fringes were 0.247 and 0.236 nm, which could be respectively assigned to the distance of the (121) planes of monoclinic Ag2S and the distance of the (111) planes of metallic Ag. The lattice distortion between Ag2S and AgNWs was quite small. This suggested that there were less interfacial defects on the interface of Ag and Ag2S. The corresponding EDS elemental mapping analysis (Fig. 3C and D) was used to detect the chemical compositions in the sulfidation products of AgNWs. The results fortified the homogeneous distribution of silver and sulfur in the heterostructured nanowires. These results further confirmed that a large amount of metallic Ag0 transformed to Ag2S nanoparticles on the surface of initial AgNWs, forming well-defined Ag2S–Ag hybrid nanowires as a result of sulfidation.
Fig. 1. (A) XRD patterns of the sulfidation products of AgNWs obtained at different reaction time. (B) UV–vis absorption spectra of the sulfidation products of AgNWs obtained at different reaction time.
3. Results and discussion 3.1. Characterization of the sulfidation products of AgNWs The XRD results of AgNWs and the sulfidation products were shown in Fig. 1A. Diffraction patterns at 2θ of 38.1°, 44.3°, 64.5° and 77.4° corresponded to the (111), (200), (220) and (311) planes of face-centered cubic Ag (JCPDS card No. 87–0717). The AgNWs before sulfidation were Ag0 with high purity. After sulfidation, some diffraction peaks other than Ag, including those at 31.5°, 33.6°, 34.4°, 36.8°, 40.7°, 43.4°, 46.2°, 48.8°, 58.1° and 63.7°, corresponded to (−112), (120), (−121), (121), (031), (200), (−123), (014), (−141) and (−134) planes, respectively, of nanocrystalline monoclinic acanthite Ag2S structure (JCPDS card No. 14–0072). No additional peak was detected. The XRD patterns of the sulfidation products indicated the coexistence of Ag and Ag2S in the prepared hybrid materials. With the sulfidation time increasing, the intensities of Ag2S diffraction peaks increased gradually while those of Ag diffraction peaks decreased, suggesting that a portion of Ag was transformed to Ag2S, forming hybrid structures of Ag2S–Ag. The EDS results in Table S1 also demonstrated that the molar ratio of Ag2S/Ag increased when the sulfidation time increased. After sulfidation for 16 h, the content of metallic Ag became marginal, suggesting that most of the Ag was transformed to Ag2S. The UV–vis absorption spectra of the sulfidation products of AgNWs
3.2. Impacts of the sulfidation products of AgNWs on BPA degradation The impacts of the sulfidation products of AgNWs on BPA degradation were evaluated under simulated sunlight irradiation, and the results are illustrated in Fig. 4. The blank experiment showed that the degradation of BPA under sunlight irradiation was negligible in the absence of the sulfidation products of AgNWs. Pristine AgNWs displayed negligible effect on BPA degradation under sunlight irradiation. After 0.5 h adsorption-desorption equilibrium, less than 4.0% of BPA was adsorbed on the three sulfidation products of AgNWs. However, BPA degradation was promoted significantly in the presence of the sulfidation products of AgNWs under solar light irradiation. After 6 h irradiation, more than 80% of BPA was degraded. In addition, the photocatalytic activity of Ag2S–Ag-8 and Ag2S–Ag-12 was similar, which was distinctly greater than that of Ag2S–Ag-4. The degradation products of BPA were analyzed by LC-MS/MS. As shown in Fig. S4, only 3
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Fig. S6 shows the effect of pH on the photocatalytic degradation BPA in the presence of Ag2S–Ag-8. Specifically, the degradation reaction rate constant k increased gradually (0.32, 0.45, 0.59, and 0.91 h−1) with increasing the pH of reaction solution (3.0, 5.0, 6.8 as neutral pH and 11.0). The photocatalytic degradation rate in the presence of the sulfidation products of AgNWs could be affected by several factors. The pH of the reaction solution affected not only the interaction between the sulfidation products of AgNWs and BPA, but also the types of reactive radicals generated on the surface of Ag2S–Ag nanowires. At acidic pH, the Ag2S–Ag nanowires were less stable due to the dissolution of inner AgNWs, which could contribute to the lower BPA degradation rate. Considering the pKa of BPA is around 10, BPA exists as the ionic form with negatively charge at pH > 10. At pH 11.0, the ionic BPA could be more favorable to react with the photogenerated holes compared with the molecular BPA through the electrostatic attraction (Oh et al., 2018). In addition, the surface plasmon resonance of Ag was pH dependent, and basic condition was more favorable for plasmonic effect (Christopher et al., 2010; Csapó et al., 2012). 3.3. Mechanisms for the promoted degradation BPA by the sulfidation products of AgNWs During the sulfidation of AgNWs, the surface reactive Ag atoms reacted with sulfide to generate a thin uniform Ag2S layer at the early reaction stage (Zhang et al., 2016a, 2018a). The subsequent sulfidation of AgNWs led to the growth of convexity of nanoscale Ag2S grains and finally a formation of Ag2S–Ag nanowire heterostructures. The band gap of bulk Ag2S was reported to be 0.9, and 1.4–2.3 eV for Ag2S nanomaterials (Hong et al., 2014), while the work function of metallic Ag and the bottom of the conduction band of n-type Ag2S semiconductor was 4.26 and 4.42 eV, respectively, relative to the vacuum energy level (Jiang et al., 2011). When Ag2S generated on the surface of AgNWs, both Fermi levels shifted to be uniform to achieve equilibrium (Yang et al., 2012). Therefore, the energy bands of Ag2S would bend and downshift to generate ohmic contact at Ag2S-AgNWs interface, and the photoexcited electrons in the conduction band of Ag2S could easily transfer to AgNWs in the hybrid nanostructures. The induced charge separation suppressed the recombination of photogenerated holes and electrons, and hence increased the photocatalytic activity (Pang et al., 2010). The concentration of the sulfidation products of AgNWs used in the BPA degradation experiment might be higher than that in the real scenarios. However, due to the limits of detection of some instruments, such as UV–vis spectrophotometer and EPR spectrometer, the concentration was set as low as possible to simulate the real aquatic environments, but close to the concentrations in surface waters nearby point sources, such as the effluents from nanomaterials manufacturers or wastewater treatment plants. Generally, the photocatalytic process involves multiple reactive species, including hydroxyl radicals (•OH), superoxide anion radicals (•O2−), and photogenerated holes, which could react differently with the organic pollutants through radical addition, hydrogen abstraction, and electron transfer, leading to the formation of various degradation intermediates. In order to illustrate the photocatalytic degradation mechanisms of BPA by the sulfidation products of AgNWs under simulated sunlight irradiation, terephthalic acid (TPA), benzoquinone (BQ), and potassium iodide (KI) were added in the reaction solution as scavengers of hydroxyl radicals, superoxide anion radicals and photogenerated holes, respectively (Wu et al., 2016). As shown in Fig. 5, photocatalytic degradation of BPA in the presence of Ag2S–Ag-8 was negligibly affected by the addition of TPA, indicating that hydroxyl radicals were not the main active species in the reaction. On the contrary, the degradation of BPA was severely restrained when BQ or KI was added. Hence, photogenerated holes and superoxide radicals could be the dominant oxidizing species for BPA photodegradation in the presence of the sulfidation products of AgNWs under sunlight irradiation. Furthermore, DMPO spin-trapped EPR spectroscopy was also
Fig. 2. XPS spectra of the sulfidation products of AgNWs. (A) Ag 3d5/2 and Ag 3d3/2 spectra. (B) S 2p3/2 and S 2p1/2 spectra.
one intermediate product with m/z 133 was detected in the reaction system with Ag2S–Ag-8 as catalyst. This product was assigned as 4-vinylphenol, which was in agreement with some previous studies (Wu et al., 2016; Liu et al., 2018). The pseudo first-order kinetics Ct = C0 e-kt was applied to calculate the BPA degradation rates, where Ct and C0 are the concentration of BPA at time t and t = 0, respectively, and k is the photocatalytic reaction rate constant. The linear regression model of the degradation reaction kinetics between ln (Ct/C0) and time fitted the data very well (Fig. S5). The regression coefficient R2 was all greater than 0.98, indicating that the degradation of BPA catalyzed by the sulfidation products of AgNWs followed a pseudo first order kinetics. The degradation reaction rate constant k was 0.35, 0.59 and 0.61 h−1 for Ag2S–Ag-4, Ag2S–Ag-8 and Ag2S–Ag-12, respectively, suggesting that the content of Ag2S played an important role in promoting the photocatalytic activity of Ag2S–Ag nanowires. The results indicated that sulfidation of nano-Ag in water would significantly change the behaviors of pristine nano-Ag, and then affected the transformation of organic pollutants in the environment. Since Ag2S–Ag-8 gave similar photocatalytic performance as Ag2S–Ag-12 but with less sulfidation time, it was used for further photocatalytic study. 4
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Fig. 3. (A) TEM images of Ag2S–Ag-8. The scale bar of the image represents 200 nm. (B) HRTEM images of Ag2S–Ag-8 collected from the selected area in Fig. 3A. The scale bar of the image represents 5 nm. (C) and (D) EDS elemental mapping analysis of Ag2S–Ag-8 from the selected area in Fig. 3A. The scale bars of the images represent 50 nm.
Fig. 5. Photocatalytic degradation performance of Ag2S–Ag-8 nanowires in the presence of different scavengers under sunlight irradiation. Data points represent the average of three independent replicates.
Fig. 4. Photocatalytic degradation performance of different sulfidation products of AgNWs under sunlight irradiation. Data points represent the average of three independent replicates.
The prominent photocatalytic mechanism of the sulfidation products of AgNWs could be attributed to the synergistic effects of unique morphology and favorable interface charge transfer as shown in Fig. 6. In natural aquatic environment, AgNWs as well as other silver nanomaterials, would undergo chemical transformations, including oxidative dissolution, chlorination and sulfidation. Oxidative dissolution of
applied to analyze the active radicals generated in the reaction system. As shown in Fig. S7, six characteristic peaks of DMPO-•O2− adducts were detected in the reaction system using Ag2S–Ag-8 as catalyst, but they were not detected in the control test. The results indicated that superoxide radicals were generated on the surface of the sulfidation products of AgNWs. 5
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Fig. 6. Schematic illustration of the possible photocatalytic degradation mechanism of the sulfidation products of AgNWs under the sunlight irradiation.
21737003 and 41807491), Tianjin Municipal Science and Technology Commission , China (16PTSYJC00020 and 17JCYBJC23200), and Yangtze River scholar program of China, and 111 program, Ministry of Education, China (T2017002).
AgNWs is a relatively slow process, which was usually several orders of magnitude slower than the observed sulfidation (Zhang et al., 2016a). During the fast sulfidation reaction of AgNWs, Ag2S–Ag hybrid nanowires could be generated in the presence of many types of sulfide. Under sunlight irradiation, the nanoscale Ag2S convexity might be excited to generate holes and electrons on the surface, and the photogenerated electrons transferred feasibly from the conduction band Ag2S to metallic Ag nanowires because of the low interfacial charge transfer resistance. The dissolved O2 in water was reduced by electrons to generate superoxide radicals with the strong oxidizing activity, which would facilitate the abiotic degradation of the organic pollutants, such as BPA. Meanwhile, huge amounts of photogenerated holes accumulated in the valence band of Ag2S, which could work together with superoxide anion radicals to oxidatively degrade BPA. The photoinduced charge separation restrained the recombination of the holes and electrons, leading to apparently enhanced photocatalytic activity. Owing to the potential of •OH/H2O (2.32 V vs NHE) was more positive than the valence band of Ag2S (0.43 V vs NHE), stronger oxidative species such as hydroxyl radicals could not be produced by Ag2S–Ag hybrid products.
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4. Conclusions Sulfidation is one of the most important transformation processes of nano-Ag in aquatic environments. The present study provides insights into the impacts of sulfidation of AgNWs on the degradation of copresent organic pollutant in water. Under environmentally relevant conditions, AgNWs were sulfidized and generated Ag2S–Ag heterostructured nanowires, which displayed a good photocatalytic activity under simulated sunlight irradiation. As sulfidation time increased, more Ag2S was generated and the Ag2S–Ag composites significantly promoted BPA degradation. The results demonstrated that sulfidation of AgNWs, as well as other nano-Ag, have great potential in affecting the fates of pristine nano-Ag and the interactions with other pollutants copresent in aquatic environments. As nano-Ag materials enter aquatic environment, they would experience complex processes, which significantly change the properties, behaviors, and fates of the pristine materials. It is very important to understand the behaviors of nanomaterials in real environment, but not only the pristine ones. Acknowledgments The authors would like to thank the financial support of the National Natural Science Foundation of China (grants 21876088, 6
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Impacts of sulfidation of silver nanowires on the degradation of bisphenol A in water
T
Yinqing Zhang, Kunkun Wang, Yi Yang, Jinliang Xu, Binbin Sun, Lingyan Zhu∗ Key Laboratory of Pollution Processes and Environmental Criteria (Ministry of Education), Tianjin Key Laboratory of Environmental Remediation and Pollution Control, College of Environmental Science and Engineering, Nankai University, Tianjin, 300350, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: AgNWs Sulfidation Ag2S Photocatalysis Bisphenol A
Silver nanowires (AgNWs) are widely produced in many electronic and optical products, and could be inevitably discharged into the aquatic environments. Sulfidation is one of the most important transformation processes of AgNWs, and could significantly affect their fate and interactions with other pollutants in aquatic environment. In the present study, the sulfidation products of AgNWs with different atomic ratio of Ag and S were prepared under environmentally relevant conditions. The crystal structure, elemental composition, morphology and size of the sulfidation products were comprehensively characterized by powder X-ray diffraction, UV–vis spectroscopy, Xray photoelectron spectroscopy and transmission electron microscope. The products were heterostructured nanowires and the Ag2S/Ag molar ratio increased with extension of the reaction time. The produced Ag2S–Ag nanowires displayed a good photocatalytic activity and facilitated the degradation of the copresent organic pollutant bisphenol A (BPA) under simulated sunlight irradiation. As sulfidation time increased, more Ag2S was generated and the Ag2S–Ag composites displayed high promotion effect on BPA degradation. This effect could be ascribed to the favorable synergistic effects between Ag2S and AgNWs, such as high electron-hole separation efficiency and low charge transfer resistance. The chemical scavenger experiments demonstrated that superoxide anion radicals and photogenerated holes in the sulfidation products of AgNWs could be the main reactive species for photocatalytic degradation.
1. Introduction Silver nanomaterials (nano-Ag) have been widely used in various industrial and commercial products, due to the unique catalytic, sensing properties, optical, and antibacterial effects (Tao and Yang, 2005; Anker et al., 2008; Ahamed et al., 2010). It was estimated that the world-wide production of nano-Ag was 550 tons/year in 2011 and about 435 products (24% of the total products using nanotechnology) contained nano-Ag in 2015 (Piccinno et al., 2012; Vance et al., 2015). As typical one-dimensional nano-Ag, silver nanowires (AgNWs) are deemed as potential building blocks for the next generation of electronic, optical, and sensing devices (Tang and Kotov, 2005). Largequantity production and increasing application will inevitably lead to the release of AgNWs into the environment, thus raising great concerns about their potential interactions with other pollutants present in natural water (Mitrano et al., 2014). Due to the reactive nature, nano-Ag may undergo different physical and chemical transformation processes once released to the environment, such as aggregation, dissolution, chlorination and sulfidation.
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The products of such transformations would in turn greatly affect the environmental behaviors and the interactions with other pollutants (Levard et al., 2012; Ellis et al., 2016). Regardless of the morphology, particle size and coating materials, nano-Ag is readily to react with sulfide to produce silver sulfide (Ag2S), forming a complex of Ag2S and nano-Ag (Liu et al., 2010). The sulfidation of nano-Ag and compositions of the complexes are dependent on the concentration of sulfide, other components in the aquatic environments as well as sunlight irradiation (Liu et al., 2011; Zhang et al., 2016a, 2018a). Sulfidation may not only reduce the overall bioavailability and toxicities of nano-Ag, but also produce Ag2S–Ag hybrid nanomaterials. Ag2S is an important sulfide semiconductor with a bandgap of about 1.0 eV, and Ag2S nanomaterials usually display high stability and strong light absorption in the solar irradiance spectrum (Jiang et al., 2015; Yu et al., 2016). Hybrid nanomaterials based on semiconductor-noble metal are usually promising photocatalysts under visible light arising from the synergistic interactions between each component (Yang et al., 2012; Kochuveedu et al., 2013; Huang et al., 2013; Fan et al., 2015). Thus, composites of Ag2S semiconductor and nano-Ag might be omnipresent in natural aquatic
Corresponding author. E-mail address: [email protected] (L. Zhu).
https://doi.org/10.1016/j.ecoenv.2019.109739 Received 10 June 2019; Received in revised form 17 September 2019; Accepted 27 September 2019 Available online 03 October 2019 0147-6513/ © 2019 Elsevier Inc. All rights reserved.
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dried in the vacuum oven at 90 °C for 1 h. The sulfidation products were named as Ag2S–Ag-1, Ag2S–Ag-4, Ag2S–Ag-8, Ag2S–Ag-12, Ag2S–Ag-16, respectively, representing the products obtained at the sulfidation time of 1, 4, 8, 12, 16 h.
environment (Zhang et al., 2016b), and they are hypothesized to display active photocatalytic activity under visible light (Pang et al., 2010). The photocatalytic activity of heterogeneous photocatalysts is highly related to their crystal structure, specific surface area and morphology (Xiao et al., 2015). Compared to the conventional metal oxide semiconductors, metal sulfides generally have narrow bandgaps and could utilize sunlight more efficiently (Li et al., 2010; Wang et al., 2016; Zhang et al., 2018b). Previous studies reported that many Ag2S–Ag hybrid structures, such as porous nanotubes (Yang et al., 2012), hollow heterodimers (Kumar et al., 2015), spherical nanoparticles (Jiang et al., 2011), heterostructured nanobowls (Li et al., 2015), hierarchical nanowires (Xiong et al., 2016), flower wires (Basu et al., 2017), displayed efficient photocatalytic activities to organic pollutant, such as methylene blue and tetracycline. However, these materials were prepared at strictly controlled reaction conditions, such as high temperature, using templates, or complicated procedures. It remains unclear if the Ag2S–Ag nanoscale heterostructures formed at environmentally relevant conditions would display similar photocatalytic activities, and then affect the fate of organic pollutants in aquatic environments. The purpose of this study was to investigate the impacts of the sulfidation of AgNWs on the abiotic transformation of organic pollutants in aquatic environment under sunlight irradiation, taking bisphenol A (BPA) as a model molecule. Because of the wide utilization, a large amount of BPA was released into the environment (Ohko et al., 2001; Vandenberg et al., 2010), and its ubiquitous presence in aquatic environments aroused great concerns on the ecological risk of BPA (Huang et al., 2012; Mizuta et al., 2017). The Ag2S–Ag hybrid nanowires could generate under environmentally relevant conditions through an in situ sulfidation and deposition method. The concentrations of AgNWs and BPA were set to be as low as possible to simulate their environmental levels but allowing to quantify them by the instruments accurately. To simulate the environmental process, Ag2S–Ag nanostructures with different contents of Ag2S were prepared by varying the period of sulfidation reaction, and monitored by UV–Vis spectrometry and inductively coupled plasma mass spectrometry (ICPMS). The possible photodegradation mechanisms were also investigated.
2.3. Characterization of the sulfidation products of AgNWs X-ray diffraction (XRD) patterns of AgNWs and the sulfidation products of AgNWs were analyzed on a D8-Advanced diffractometer (Bruker, German) with Cu Kα radiation. Ultraviolet–visible (UV–vis) absorption spectra were conducted on a UV-2600 spectrophotometer (Shimadzu, Japan) at room temperature. X-ray photoelectron spectroscopy (XPS) was obtained on a Axis Ultra DLD spectrometer (Kratos, Japan) equipped with a monochromated Al X-ray source. Gaussian curves were applied to fit the spectra. Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray spectroscopy (EDS) were obtained on a JEM-2800 transmission electron microscope (JEOL, Japan). The aqueous suspension of the sulfidation products of AgNWs was placed on the copper grids coated with ultrathin carbon film, and dried overnight in a dust-free box. Electron paramagnetic resonance (EPR) spectra were obtained using an EMX-10/12 EPR spectrometer (Bruker, Germany) at room temperature. 5,5-Dimethyl-1-pyrroline N-oxide (DMPO) was employed as the spin trapping agent.2.4. Photocatalytic degradation of BPA.
Photocatalytic degradation of BPA The photocatalytic activities of the sulfidation products of AgNWs were investigated by degrading BPA under simulated sunlight irradiation. The photocatalytic degradation experiments were performed in a photochemical reactor with 12 quartz test tubes surrounding an 800 W xenon lamp. The intensity of the simulated sunlight was 132 W/m2, and the emission spectrum is shown in Fig. S1. The lamp was cooled by a water jacketed condenser, and the reaction system was magnetically stirred and maintained at room temperature by circulating water. The total volume of the reaction solutions was 40 mL, the concentration of the sulfidation products of AgNWs was 1 μM which was as low as that of AgNWs during sulfidation process, and the dosages of BPA was 0.5 μM, given that the BPA level in natural environment could be as high as 0.6 μM (Fromme et al., 2002). Before irradiation, the reaction solutions were stirred for 0.5 h in dark to establish adsorption/desorption equilibrium for BPA on the sulfidized AgNWs. The photocatalytic experiment started after turning on the xenon lamp. Approximately 1 mL of the reaction solution was sampled at predetermined time, and then centrifuged under 2000g. The concentration of BPA was analyzed on a 1260 Infinity high performance liquid chromatograph (HPLC) instrument (Agilent, USA) with a fluorescence detector (excitation and emission wavelengths were 220 nm and 350 nm, respectively) and an Agilent ZORBAX Eclipse XDB-C18 column at 35 °C. The mobile-phase was methanol/water (65/35 in volume ratio), and the flow rate was 0.15 mL/min. The degradation products of BPA were analyzed by liquid chromatography-tandem mass spectrometry (LC-MS/MS) on a Xevo TQ-S system (Waters, USA) equipped with an Acquity UPLC C18 column. Detailed information of the LC-MS/MS analysis is provided in the supplementary material. The radical-trapping experiments were conducted with terephthalic acid (TPA), benzoquinone (BQ), and potassium iodide (KI), which were applied as the scavengers of hydroxyl radicals (·OH), superoxide anion radicals (·O2−), and photogenerated holes, respectively. 1 μM of each scavenger was added with the sulfidation product of AgNWs, and the other experimental steps and analytic methods are the same as described above.
2. Materials and methods 2.1. Chemicals and reagents AgNO3 (99.99%), ethylene glycol (EG, anhydrous, 99.9%), polyvinylpyrrolidone (PVP, K = 29–32), and Na2S·9H2O (> 99.9%) were obtained from Aladdin Chemistry Co. Ltd. (China). All the chemicals were applied without further purification. Ultrapure water was produced on Millipore Milli-Q Advantage A10 water purification system (US), and used throughout the experiments. AgNWs were synthesized by the modified solvothermal method (Zhang et al., 2016a, 2018a), and the detailed information was provided as supplementary material. The AgNWs stock suspension was stored in dark at 4 °C. 2.2. Sulfidation process of AgNWs The stock solution of Na2S was freshly prepared just before the sulfidation experiments. Predetermined volumes of Na2S solution and AgNWs suspension were added to water to obtain 20 mL of reaction suspension in a quartz test tube with a screw cap. The initial concentration of AgNWs was 1 μM, and the Ag/S mole ratio of the reaction mixture was 2. The test tubes containing the reaction solution were sealed tightly and rotated at 12 rpm facing an 800 W xenon lamp. The pH of the sulfidation solutions was constant in the range of 6.6–6.8, being measured on a Mettler Toledo MP120 pH meter. At each predetermined time, one tube was taken out, and the products were separated from the reaction mixture by centrifugal ultrafiltration. After being washed with ultrapure water and ethanol, the products were 2
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with different reaction time are shown in Fig. 1B. The pure AgNWs exhibit two absorbance peaks at 370 and 400 nm, which could be attributed to the longitudinal mode and the surface plasmon resonance (SPR) of AgNWs (Sun et al., 2002; Ramasamy et al., 2012). After 4 h sulfidation of AgNWs, the longitudinal mode almost disappeared and the SPR absorption band of AgNWs significantly decreased, redshifted and broadened, which was attributed to the transformation of AgNWs to generate Ag2S on the surface. In addition, the spectra exhibited a new absorption band centered at around 500 nm, which corresponded to the band gap of Ag2S. This is similar to the reported results of Ag2S nanocrystals (between 490 and 520 nm) (Zhao et al., 2007) and Ag2S–Ag heterostructures (495 and 514 nm) (Pang et al., 2010; Xiong et al., 2016). When the reaction time increased to 8 h, the SPR band of AgNWs almost disappeared because of further sulfidation, and the characteristic absorption band of Ag2S shifted to around 510 nm. The observed redshifts indicate the growth of Ag2S crystallite and enlarged Ag2S domain in the heterostructures. Their broad absorbance from UV to near-infrared region suggested that the sulfidation products of AgNWs were active under sunlight irradiation. To further investigate the compositions of the sulfidation products of AgNWs, XPS spectra of the Ag2S–Ag-8 were measured and the results are shown in Figs. 2 and S2. The XPS spectrum in Fig. S2 obviously exhibited the signals of silver and sulfur. As shown in Fig. 2A of the Ag 3d spectrum, the peaks at binding energy of 368.5 and 374.4 eV could be respectively assigned to Ag 3d5/2 and 3d3/2 of the metallic state Ag0 (Zhang et al., 2018b). The peaks at 367.9 and 373.8 eV fitted to those of Ag 3d5/2 and 3d3/2 of Ag+ ions. In Fig. 2B, the peaks at 161.3 and 162.5 eV corresponded to the binding energies of S 2p3/2 and 2p1/2 of S2− (Elechiguerra et al., 2005). These results suggested that the sulfidation product was consisted of both metallic Ag and Ag2S. The morphology structure and elemental distribution of the sulfidation product of AgNWs are elucidated by TEM in Fig. 3. The panoramic view of the sulfidation product showed uniform nanowires of ~50 nm in diameter and several micrometers in length. The surface of the product was much rough compared with AgNWs in Fig. S3, and there were many nanoscale grains appearing on the entire substrate surface. The HRTEM (Fig. 3B) clearly revealed that the distinctive lattice fringes were 0.247 and 0.236 nm, which could be respectively assigned to the distance of the (121) planes of monoclinic Ag2S and the distance of the (111) planes of metallic Ag. The lattice distortion between Ag2S and AgNWs was quite small. This suggested that there were less interfacial defects on the interface of Ag and Ag2S. The corresponding EDS elemental mapping analysis (Fig. 3C and D) was used to detect the chemical compositions in the sulfidation products of AgNWs. The results fortified the homogeneous distribution of silver and sulfur in the heterostructured nanowires. These results further confirmed that a large amount of metallic Ag0 transformed to Ag2S nanoparticles on the surface of initial AgNWs, forming well-defined Ag2S–Ag hybrid nanowires as a result of sulfidation.
Fig. 1. (A) XRD patterns of the sulfidation products of AgNWs obtained at different reaction time. (B) UV–vis absorption spectra of the sulfidation products of AgNWs obtained at different reaction time.
3. Results and discussion 3.1. Characterization of the sulfidation products of AgNWs The XRD results of AgNWs and the sulfidation products were shown in Fig. 1A. Diffraction patterns at 2θ of 38.1°, 44.3°, 64.5° and 77.4° corresponded to the (111), (200), (220) and (311) planes of face-centered cubic Ag (JCPDS card No. 87–0717). The AgNWs before sulfidation were Ag0 with high purity. After sulfidation, some diffraction peaks other than Ag, including those at 31.5°, 33.6°, 34.4°, 36.8°, 40.7°, 43.4°, 46.2°, 48.8°, 58.1° and 63.7°, corresponded to (−112), (120), (−121), (121), (031), (200), (−123), (014), (−141) and (−134) planes, respectively, of nanocrystalline monoclinic acanthite Ag2S structure (JCPDS card No. 14–0072). No additional peak was detected. The XRD patterns of the sulfidation products indicated the coexistence of Ag and Ag2S in the prepared hybrid materials. With the sulfidation time increasing, the intensities of Ag2S diffraction peaks increased gradually while those of Ag diffraction peaks decreased, suggesting that a portion of Ag was transformed to Ag2S, forming hybrid structures of Ag2S–Ag. The EDS results in Table S1 also demonstrated that the molar ratio of Ag2S/Ag increased when the sulfidation time increased. After sulfidation for 16 h, the content of metallic Ag became marginal, suggesting that most of the Ag was transformed to Ag2S. The UV–vis absorption spectra of the sulfidation products of AgNWs
3.2. Impacts of the sulfidation products of AgNWs on BPA degradation The impacts of the sulfidation products of AgNWs on BPA degradation were evaluated under simulated sunlight irradiation, and the results are illustrated in Fig. 4. The blank experiment showed that the degradation of BPA under sunlight irradiation was negligible in the absence of the sulfidation products of AgNWs. Pristine AgNWs displayed negligible effect on BPA degradation under sunlight irradiation. After 0.5 h adsorption-desorption equilibrium, less than 4.0% of BPA was adsorbed on the three sulfidation products of AgNWs. However, BPA degradation was promoted significantly in the presence of the sulfidation products of AgNWs under solar light irradiation. After 6 h irradiation, more than 80% of BPA was degraded. In addition, the photocatalytic activity of Ag2S–Ag-8 and Ag2S–Ag-12 was similar, which was distinctly greater than that of Ag2S–Ag-4. The degradation products of BPA were analyzed by LC-MS/MS. As shown in Fig. S4, only 3
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Fig. S6 shows the effect of pH on the photocatalytic degradation BPA in the presence of Ag2S–Ag-8. Specifically, the degradation reaction rate constant k increased gradually (0.32, 0.45, 0.59, and 0.91 h−1) with increasing the pH of reaction solution (3.0, 5.0, 6.8 as neutral pH and 11.0). The photocatalytic degradation rate in the presence of the sulfidation products of AgNWs could be affected by several factors. The pH of the reaction solution affected not only the interaction between the sulfidation products of AgNWs and BPA, but also the types of reactive radicals generated on the surface of Ag2S–Ag nanowires. At acidic pH, the Ag2S–Ag nanowires were less stable due to the dissolution of inner AgNWs, which could contribute to the lower BPA degradation rate. Considering the pKa of BPA is around 10, BPA exists as the ionic form with negatively charge at pH > 10. At pH 11.0, the ionic BPA could be more favorable to react with the photogenerated holes compared with the molecular BPA through the electrostatic attraction (Oh et al., 2018). In addition, the surface plasmon resonance of Ag was pH dependent, and basic condition was more favorable for plasmonic effect (Christopher et al., 2010; Csapó et al., 2012). 3.3. Mechanisms for the promoted degradation BPA by the sulfidation products of AgNWs During the sulfidation of AgNWs, the surface reactive Ag atoms reacted with sulfide to generate a thin uniform Ag2S layer at the early reaction stage (Zhang et al., 2016a, 2018a). The subsequent sulfidation of AgNWs led to the growth of convexity of nanoscale Ag2S grains and finally a formation of Ag2S–Ag nanowire heterostructures. The band gap of bulk Ag2S was reported to be 0.9, and 1.4–2.3 eV for Ag2S nanomaterials (Hong et al., 2014), while the work function of metallic Ag and the bottom of the conduction band of n-type Ag2S semiconductor was 4.26 and 4.42 eV, respectively, relative to the vacuum energy level (Jiang et al., 2011). When Ag2S generated on the surface of AgNWs, both Fermi levels shifted to be uniform to achieve equilibrium (Yang et al., 2012). Therefore, the energy bands of Ag2S would bend and downshift to generate ohmic contact at Ag2S-AgNWs interface, and the photoexcited electrons in the conduction band of Ag2S could easily transfer to AgNWs in the hybrid nanostructures. The induced charge separation suppressed the recombination of photogenerated holes and electrons, and hence increased the photocatalytic activity (Pang et al., 2010). The concentration of the sulfidation products of AgNWs used in the BPA degradation experiment might be higher than that in the real scenarios. However, due to the limits of detection of some instruments, such as UV–vis spectrophotometer and EPR spectrometer, the concentration was set as low as possible to simulate the real aquatic environments, but close to the concentrations in surface waters nearby point sources, such as the effluents from nanomaterials manufacturers or wastewater treatment plants. Generally, the photocatalytic process involves multiple reactive species, including hydroxyl radicals (•OH), superoxide anion radicals (•O2−), and photogenerated holes, which could react differently with the organic pollutants through radical addition, hydrogen abstraction, and electron transfer, leading to the formation of various degradation intermediates. In order to illustrate the photocatalytic degradation mechanisms of BPA by the sulfidation products of AgNWs under simulated sunlight irradiation, terephthalic acid (TPA), benzoquinone (BQ), and potassium iodide (KI) were added in the reaction solution as scavengers of hydroxyl radicals, superoxide anion radicals and photogenerated holes, respectively (Wu et al., 2016). As shown in Fig. 5, photocatalytic degradation of BPA in the presence of Ag2S–Ag-8 was negligibly affected by the addition of TPA, indicating that hydroxyl radicals were not the main active species in the reaction. On the contrary, the degradation of BPA was severely restrained when BQ or KI was added. Hence, photogenerated holes and superoxide radicals could be the dominant oxidizing species for BPA photodegradation in the presence of the sulfidation products of AgNWs under sunlight irradiation. Furthermore, DMPO spin-trapped EPR spectroscopy was also
Fig. 2. XPS spectra of the sulfidation products of AgNWs. (A) Ag 3d5/2 and Ag 3d3/2 spectra. (B) S 2p3/2 and S 2p1/2 spectra.
one intermediate product with m/z 133 was detected in the reaction system with Ag2S–Ag-8 as catalyst. This product was assigned as 4-vinylphenol, which was in agreement with some previous studies (Wu et al., 2016; Liu et al., 2018). The pseudo first-order kinetics Ct = C0 e-kt was applied to calculate the BPA degradation rates, where Ct and C0 are the concentration of BPA at time t and t = 0, respectively, and k is the photocatalytic reaction rate constant. The linear regression model of the degradation reaction kinetics between ln (Ct/C0) and time fitted the data very well (Fig. S5). The regression coefficient R2 was all greater than 0.98, indicating that the degradation of BPA catalyzed by the sulfidation products of AgNWs followed a pseudo first order kinetics. The degradation reaction rate constant k was 0.35, 0.59 and 0.61 h−1 for Ag2S–Ag-4, Ag2S–Ag-8 and Ag2S–Ag-12, respectively, suggesting that the content of Ag2S played an important role in promoting the photocatalytic activity of Ag2S–Ag nanowires. The results indicated that sulfidation of nano-Ag in water would significantly change the behaviors of pristine nano-Ag, and then affected the transformation of organic pollutants in the environment. Since Ag2S–Ag-8 gave similar photocatalytic performance as Ag2S–Ag-12 but with less sulfidation time, it was used for further photocatalytic study. 4
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Fig. 3. (A) TEM images of Ag2S–Ag-8. The scale bar of the image represents 200 nm. (B) HRTEM images of Ag2S–Ag-8 collected from the selected area in Fig. 3A. The scale bar of the image represents 5 nm. (C) and (D) EDS elemental mapping analysis of Ag2S–Ag-8 from the selected area in Fig. 3A. The scale bars of the images represent 50 nm.
Fig. 5. Photocatalytic degradation performance of Ag2S–Ag-8 nanowires in the presence of different scavengers under sunlight irradiation. Data points represent the average of three independent replicates.
Fig. 4. Photocatalytic degradation performance of different sulfidation products of AgNWs under sunlight irradiation. Data points represent the average of three independent replicates.
The prominent photocatalytic mechanism of the sulfidation products of AgNWs could be attributed to the synergistic effects of unique morphology and favorable interface charge transfer as shown in Fig. 6. In natural aquatic environment, AgNWs as well as other silver nanomaterials, would undergo chemical transformations, including oxidative dissolution, chlorination and sulfidation. Oxidative dissolution of
applied to analyze the active radicals generated in the reaction system. As shown in Fig. S7, six characteristic peaks of DMPO-•O2− adducts were detected in the reaction system using Ag2S–Ag-8 as catalyst, but they were not detected in the control test. The results indicated that superoxide radicals were generated on the surface of the sulfidation products of AgNWs. 5
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Fig. 6. Schematic illustration of the possible photocatalytic degradation mechanism of the sulfidation products of AgNWs under the sunlight irradiation.
21737003 and 41807491), Tianjin Municipal Science and Technology Commission , China (16PTSYJC00020 and 17JCYBJC23200), and Yangtze River scholar program of China, and 111 program, Ministry of Education, China (T2017002).
AgNWs is a relatively slow process, which was usually several orders of magnitude slower than the observed sulfidation (Zhang et al., 2016a). During the fast sulfidation reaction of AgNWs, Ag2S–Ag hybrid nanowires could be generated in the presence of many types of sulfide. Under sunlight irradiation, the nanoscale Ag2S convexity might be excited to generate holes and electrons on the surface, and the photogenerated electrons transferred feasibly from the conduction band Ag2S to metallic Ag nanowires because of the low interfacial charge transfer resistance. The dissolved O2 in water was reduced by electrons to generate superoxide radicals with the strong oxidizing activity, which would facilitate the abiotic degradation of the organic pollutants, such as BPA. Meanwhile, huge amounts of photogenerated holes accumulated in the valence band of Ag2S, which could work together with superoxide anion radicals to oxidatively degrade BPA. The photoinduced charge separation restrained the recombination of the holes and electrons, leading to apparently enhanced photocatalytic activity. Owing to the potential of •OH/H2O (2.32 V vs NHE) was more positive than the valence band of Ag2S (0.43 V vs NHE), stronger oxidative species such as hydroxyl radicals could not be produced by Ag2S–Ag hybrid products.
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4. Conclusions Sulfidation is one of the most important transformation processes of nano-Ag in aquatic environments. The present study provides insights into the impacts of sulfidation of AgNWs on the degradation of copresent organic pollutant in water. Under environmentally relevant conditions, AgNWs were sulfidized and generated Ag2S–Ag heterostructured nanowires, which displayed a good photocatalytic activity under simulated sunlight irradiation. As sulfidation time increased, more Ag2S was generated and the Ag2S–Ag composites significantly promoted BPA degradation. The results demonstrated that sulfidation of AgNWs, as well as other nano-Ag, have great potential in affecting the fates of pristine nano-Ag and the interactions with other pollutants copresent in aquatic environments. As nano-Ag materials enter aquatic environment, they would experience complex processes, which significantly change the properties, behaviors, and fates of the pristine materials. It is very important to understand the behaviors of nanomaterials in real environment, but not only the pristine ones. Acknowledgments The authors would like to thank the financial support of the National Natural Science Foundation of China (grants 21876088, 6
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