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Article

The Roles of Graphene and Ag in the Hybrid Ag@Ag2O-Graphene for Sulfamethoxazole Degradation

by
Li Zhou
1,2,
Guoyan Zou
1,2,* and
Huiping Deng
3,*
1
Institute of Eco-environment and Plant Protection, Shanghai Academy of Agricultural Sciences, Shanghai 201403, China
2
Shanghai Engineering Research Centre of Low-Carbon Agriculture (SERCLA), Shanghai 201415, China
3
College of Environmental State Key Laboratory of Pollution Control and Resource Reuse (Tongji University), Shanghai 200092, China
*
Authors to whom correspondence should be addressed.
Catalysts 2018, 8(7), 272; https://doi.org/10.3390/catal8070272
Submission received: 24 May 2018 / Revised: 2 July 2018 / Accepted: 2 July 2018 / Published: 4 July 2018
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Abstract

:
Ag@Ag2O-graphene (Ag@Ag2O-G) with different concentrations of graphene was synthesized using a facile in situ precipitation method. The photocatalysts were characterized by field emission scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS), UV-vis diffuse reflectance spectra (DRS). The antibioticsulfamethoxazole (SMX) degradationunder simulated solar light and visible light irradiationwas investigated to evaluate photocatalytic performance. The composite photocatalyst Ag@Ag2O-G with 2.5 wt% graphene presented the highest activity among all the prepared composite photocatalysts. The coupling of graphene and Ag0 increased the photocatalyticactivity and stability of pure Ag2O. Under higher SMX concentrations, the adsorption, not the photocatalytic ability, playeda crucial role during the SMX removal process. On the basis of the characterization and reactive oxygen species (ROS) scavenging experiments, a separation and transfer mechanism of photogenerated carriers was proposed. In the photocatalytic degradation of SMX, the major active species wereidentified as photogenerated holes; photogenerated electrons in the conduction band (CB) of Ag2O could not transfer to graphene through Ag0due to the more negative reduction potential of graphene. This is an important result regardinggraphene and Ag0 roles which isdifferent from that for the photocatalytic degradation of dyes. This researchmay provide new insights into photocatalytic processes for the degradation of non-dye pollutants bycomposite materials to guidethe design of highly efficient reaction systems.

Graphical Abstract

1. Introduction

Semiconductor photocatalysis, one of the most promising technologies for solar energy utilization and environmental remediation, has attracted tremendous attention since the semiconductor TiO2 was applied to the decomposition of organic contaminants [1,2]. One of the most important objectives in semiconductor photocatalysis research is the development of efficient photocatalysts that can harvest visible light under sunlight. Although TiO2-based photocatalysts have been demonstrated to be efficient photocatalytic materials under visible light irradiation [3], the variety of visible light photocatalysts is still very limited, and the exploration of new and efficient photocatalytic materials sensitive to visible light remains challenging. Presently, some Ag-based semiconductors, such as Ag3PO4 [4], AgCl [5], and Ag2 O have gained attention because of their characteristics of easy visible light excitation and low mammalian toxicity. Among them, Ag2O, a brown powder with a simple cubic structure anda lattice distance of 0.472 nm, has a narrow band-gap energy that can be easily excited by incident visible light [6]. However, after absorbing incident light, photons in Ag2O, the photogenerated electrons in the conduction band (CB) are captured by surface lattice Ag+ ions to form Ag0 clusters, whereas photogenerated holes in the valence band (VB) oxidize lattice O2− to release O2, which results in the photodecomposition of Ag2O and the formation of Ag0. Therefore, pure Ag2O has been rarely used as a visible light-driven photocatalyst because of its high photosensitivity and low-stability properties under light irradiation. There have been a few attempts to use Ag2O-based materials as a photocatalyst under visible light irradiation. Ag@Ag2O, AgBr/Ag2O, Ag2O/TiO2 heterostructure were introduced to decompose methyl orange (MO), and such attempts have exhibited high efficiency and stability [7,8,9]. These results indicate that Ag2O-based materials have greater potential as stable and highly efficient photocatalysts than pure Ag2O for photocatalytic decomposition of organic contaminants under visible light irradiation.
It is worthwhile to note that once a certain amount of Ag0 forms on the surface of Ag2O, the following photogenerated electrons tend to transfer to the Ag0 sites and are captured by O2 [10]. To inhibit Ag2O corrosion, it is expected that the photogenerated holes could transfer to other electron donors (such as organic substances), and the photogenerated electrons could transfer to other electron captures (such as O2) before reacting with lattice elements. It is possible that the produced Ag0 works as an electron pool and transfers the photogenerated electrons to improve photocatalytic activities. Therefore, the produced Ag0 could bring about an effective segregation capability of photo-excited electron-hole pairs and suppress Ag2O photo-reduction. In order to addressAg2O photocatalyst instability, a strategy of implementing hybrid material has been undertaken to determine an effective method to improve the catalytic activity and stability. Graphene, owing to its large specific area, flexible structure, high thermal and electrical conductivity, high chemical stability, and excellent electron capture and transport properties [11], could provide a supporting matrix to suppress the aggregation of nanoparticles and promote the effective separation of photogenerated carriers as an electron mediator for shuttling electrons [12]. Recently, combing graphene (G) with photocatalysts to enhance their photocatalytic performance has attracted considerable attention [13,14]. For example, the coupling of graphene with TiO2 could extend the light absorption from UV wavelengths into the visible region [15] and improve adsorption abilities and photocatalytic activities [16]. Graphene has been one of the most promising materials to enhance the photocatalytic performance in the new generation of photocatalysts [17]. However, in previous reports about Ag@Ag3PO4-G composite materials, the coupling graphene and Ag0 could not enhance its photocatalytic activity in organic pollutants degradation [4]. Therefore, the effects of graphene and Ag0 in composite photocatalysts remain unclear and deserve further research.
It should be noted that most previous studies of Ag2O-based composite photocatalysts have focused on the decomposition of dyes, such as methyl orange (MO) [7], rhodamine B (RhB) [14], or methylene blue (MB) [18]. However, dyes used as photosensitization compounds could be excited by some wavelength lights to release electrons affecting the transfer of photogenerated carriers [19]. Other appropriate target pollutants should also be explored. The trace-level water contaminant sulfamethoxazole (SMX) can only absorb light at wavelengths lower than 320 nm [20], which makes SMX an ideal candidate to analyze photocatalytic activities and mechanisms of visible light-driven photocatalysts.
Herein, hybrid Ag@Ag2O-G photocatalysts with different graphene concentrations were synthesized through a precipitation method. Photocatalytic activity of Ag@Ag2O-G was evaluated by monitoring the degradation of SMX under simulated solar light (λ > 280 nm) and visible light irradiation (λ > 400 nm). In addition to photodegradation and adsorption abilities of Ag@Ag2O-G on SMX, the stability of one of the photocatalysts after several recycles was also investigated. A possible photocatalytic degradation mechanism was proposed through investigation of the influence of reactive oxygen species (ROS) scavengers on Ag@Ag2O-G photocatalytic activities. Through this research, the roles of graphene and Ag0 in the composite Ag2O-based materials were investigated systemically using SMX (non-photosensitizer, as target pollutant). This will provide new insights into photocatalytic mechanism analysis for composite photocatalysts.

2. Results and Discussions

2.1. Catalyst Characterization

Morphology of the as-prepared Ag@Ag2O-G2 was elucidated by SEM (scanning electron microscopy) and TEM (transmission electron microscopy), as shown in Figure 1a,b. It can be clearly seen that the Ag2O nanoparticles were close to spherical in shape with an average diameter of approximately 100 nm and covered by graphene with a folder surface. From the different colors of Figure 1b, the Ag2O particles in the dark color and graphene in the light color are clearly identified. The production of Ag on the surface of Ag2O particles was confirmed by the following XRD (X-ray diffraction) and XPS analysis.
XRD patterns of the pure Ag2O, Ag@Ag2O-G1, Ag@Ag2O-G2, and Ag@Ag2O-G3 are showed in Figure 2a. The diffraction peaks at 2θ = 32.8°, 38.1°, 54.9°, 65.4°, and 68.8° can be attributed to the respective (111), (200), (220), (311), and (222) planes of the cubic Ag2O (JCPDS No. 41-1104). No characteristic peak of graphene was observed in the patterns of composites, which might be ascribed to the high graphene dispersion. Compared with the pure Ag2O, the diffraction peaks of composite Ag@Ag2O-G2 at 2θ = 44.3°, 64.4° and 77.5° can be attributed to the cubic Ag (JCPDS No. 04-0783), which confirmed the formation of Ag. As shown in Figure 2b, the XPS peaks of Ag 3d also confirm the formation of Ag. The peaks centered at 368, and 374 eV corresponded to Ag 3d5/2 and Ag 3d3/2, respectively. The Ag 3d5/2 peak is divided into two different peaks at 368.6 and 368.1 eV, and the Ag 3d3/2 peak is divided into 374.6 and 374.1 eV, respectively. The peaks at 368.6 and 374.6 eV are attributed to Ag, whereas the peaks at 368.1 and 374.1 eV are attributed to Ag+ in Ag2O, and the two pairs of separated silver peaks show a difference of D = 6 eV [21]. The molar content of the surface metallic Ag is calculated to be 11.2 wt% of the total silver element in Ag@Ag2O-G2 according to the areas of divided peaks (as shown in Figure 2b). The XPS peak corresponded highly to the XRD analysis, proving the co-existence of Ag and Ag2O in the composite.

2.2. Photocatalytic Performance

As shown in Figure 3a–c, after adsorption in the dark for 30 min, the adsorption capacity of Ag2O, Ag@Ag2O-G1, Ag@Ag2O-G2, and Ag@Ag2O-G3 were 2.77, 4.77, 5.17, and 5.48 mg/g respectively. The addition of graphene in Ag@Ag2O-G1, Ag@Ag2O-G2, and Ag@Ag2O-G3 increased 72.2%, 86.6%, and 97.8% of adsorption capacity than pure Ag2O, respectively. Under a different light source, Ag2O-based photocatalysts showed the same trend in SMX degradation. The SMX concentration in the solution increased rapidly after irradiation, thus indicating that desorption had occurred. For Ag2O, Ag@Ag2O-G1, Ag@Ag2O-G2, and Ag@Ag2O-G3, the desorption percentages were 71.59%, 43.69%, 37.30%, and 35.94% under simulated solar light and 69.83%, 40.81%, 36.48%, and 33.85% under visible light, respectively. It was concluded that the light of the UV part ofthe simulated solar light could result in more Ag2O corrosion, which desorbsthe SMX more than visible light [9]. The relationship between ln(C5/CT-5) (C5, the initial concentration of SMX after 5 min irradiation and CT-5, the concentration of SMX after (T-5) min irradiation) and irradiation time followedfirst-order kinetics. The basic kinetic parameters for the Ag2O seriesphotocatalysts under different light irradiations areshowed in Table S1. Obtained values of the apparent rate constants were 0.016, 0.0353, 0.045, and 0.027 min−1 under simulated solar light and 0.012, 0.0340, 0.038, and 0.028 min−1 under visible light (R2 > 0.96). The difference between photocatalytic activities under simulated solar light and visible light was not significant because of the similar light intensity (37.7 mW/cm2 for simulated solar light and 34.2 mW/cm2 for visible light). The adsorption capacity increased and the desorption percentages decreased with the additional graphene and the Ag0 produced in the photocatalysts. However, the photocatalytic activities were not in linear relation with the graphene addition, and the Ag@Ag2O-G2 with 2.5 wt% of graphene content showed the highest degradation efficiency for SMX removal. The photocatalytic performance deteriorated when the graphene dosage was increased above its optimal value. This phenomenon could be because graphene absorbs some light and, thus, might cause a light harvesting competition between Ag2O and graphene with the increasing graphene concentration [22], which was consistent with light absorption abilities of photocatalysts (as shown in Figure S1). On the other hand, thephotogenerated Ag0 on the surface of Ag2O enhanced its structural stability due to a self-stability mechanism [10] that decreased the photocatalyst corrosion and SMX desorption percentages. In order to avoid photocatalyst corrosion, visible light was adopted to conduct further experiments.
The results of adsorption and photocatalytic degradation in different SMX concentrations are shown in Figure 4. In the higher concentrations of 2.5, 5, and 10 mg/L, the SMX was mostly removed by adsorption. The adsorption capacities of Ag@Ag2O-G2 for 1, 2.5, 5, and 10 mg/L SMX were 5.0, 23.6, 47.4, and 98.4 mg/L, respectively (as shown in Figure 4a). As the SMX concentrations were increased, the adsorption capacity of the photocatalyst also obviously increased, and almost no degradation products were detected after 10 mg/L SMX was adsorbed and photocatalytically degraded (as shown in Figure 5b). However, with the exception of an SMX substance peak at 4.11 min for 1 mg/L SMX, there were some other peaks for degradation products identified by high-performance liquid chromatography (HPLC) (as shown in Figure 5a). The results indicate that the photocatalyst adsorption ability was not positively related to the photocatalytic activity, which is consistent with the results of previous research [23]. This may be ascribed to the competition of light absorption between the photocatalyst and pollutants. The higher SMX concentration beyond the value of 2.5 mg/L with multilayer SMX coverage on the photocatalyst weakens the radiant flux necessary for the Ag2O [24], which explains the phenomenon that no obvious desorption occurred after 5 min irradiation under a higher SMX concentration (≥2.5 mg/L).

2.3. Photocatalyst Stability

As shown in Figure 6, in the first two successive experimental runs (a total of four hours), SMX removal by adsorption and photocatalytic degradation of Ag@Ag2O-G2 remained almost the same. However, the SMX removal percentagedecreased from almost 100% to approximately 40% in the third run and almost all of it was removed by adsorption. When the SMX concentration was 2.5 mg/L, the adsorption ability was crucial to the removal of SMX. After the photocatalyst was recycled twice, the SMX removal efficiency decreased rapidly in the third recycle due to adsorption capacity saturation and more photocorrosion occurred in the photocatalyst (as shown in Figure S2). It could be concluded that the Ag@Ag2O-G2 has the ability to photocatalytically oxidize low-concentration SMX (≤1 mg/L) and adsorb high-concentration SMX (≥2.5 mg/L).

2.4. Mechanism Analysis

In the photocatalytic system (CSMX = 1 mg/L, CAg@Ag2O-G2 = 0.05 g/L), the oxidative species were probed by scavenger addition into the solution. The results in Figure 7 show that benzoquinone and sodium azide addition had negligible effects, while isopropanol addition had a slight effect on SMX photodegradation. The results indicatedthat the main reactive oxygen species were not •OH, O2•− and 1O2 in this photocatalytic process. A rapid termination of photocatalysis was investigated after Na2-EDTA addition and only 5.2% of SMX was removed after 90 min of irradiation. One possible explanation is that, on the surface of the photocatalyst, the deposited EDTA anion as a hole trap blocked the reaction sites. This implies that the SMX oxidation process occurred on the surface of photocatalyst, and photogenerated holes were the main reactive species.
The schematics of the possible charge transfer and oxidation processes under visible light irradiation are illustrated in Figure 8. Ag2O was excited to photogenerated electrons and holes in its CB and VB. The CB and VB potentials of Ag2O were +0.20 and +1.50 V [25] (as shown in Figure 8) versus a normal hydrogen electrode (NHE), respectively. The holes in the VB of the Ag2O (+1.50 V/NHE) could degrade the SMX (+0.9 V/NHE [26]) to CO2 or to other intermediates, but they could not oxidize the OH− to •OH(+1.89 V/NHE [27]) or H2O to •OH (+2.68 V/NHE [28]). The electrons in the CB of the Ag2O (+0.20 V/NHE) could not reduce the O2 to O2•− (−0.33 V/NHE) and HO2• (−0.55 V/NHE) [29]. However, the electrons could reduce the O2 to H2O2 (+0.695 V/NHE) [29] through a two-electron reduction [30] and then continue to react with electrons to produce active •OH [31], as verified by the scavenging experiments. The Ag0 produced in the catalyst preparation process could act as electron capture centers [32], and transfer electrons in the CB of the Ag2O to oxygen through multi-electron-transfer routes. In other studies, graphene has been considered effective in reducing the combination rate of photogenerated electron-hole pairs when coupling with semiconductors owing to the electron shuttle from the conjugated carbon network [22]. However, the electrons in the CB of the Ag2O could not transfer to the graphene through Ag0 because of the more negative graphene reduction potential (−0.11 V) [19,33], which indicated that graphene could increase photocatalytic activities only through higheradsorption capacity and more active sites to anchor the SMX. Therefore, in the Ag@Ag2O-G photocatalytic system, the major active species for photocatalytic degradation were identified as the holes, while graphene was identified as the adsorbent and Ag0 as the electron capture center, which increased the Ag2O photocatalytic activities to remove the SMX and the stability of photocatalysts.

3. Materials and Methods

3.1. Catalyst Preparation

Silver nitrate (AgNO3), sodium hydroxide (NaOH), ammonia solution (25–28%), ethanol (>99.7%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Graphene was purchased from Nanjing XFNANO Materials Tech Co., Ltd. (Nanjing, China). All chemicals used in this study were of analytical grade without further purification. In a typical synthesis of the Ag@Ag2O-G composite, solutions of 50 mL of 0.2 M AgNO3, 100 mL of 0.2 M ammonia water, and 100 mL of 2 M NaOH were prepared in advance. The ammonia water was added to the AgNO3 solution slowly, changing the color of the solution from yellowish-brown to transparent—thus, creating a silver-ammonia solution. A certain amount of graphene powder was then dispersed into the silver-ammonia solution and sonicated for 2 h. Afterwards, the NaOH solution was dropped into the silver-ammonia solution with added graphene to form black Ag2O-G. To synthesize the Ag@Ag2O-G, the Ag2O-G suspension was irradiated for 3 h using a 300 W Xe lamp equipped with an optical cut-off filter (λ > 400 nm). The resultantAg@Ag2O-G was separated by centrifugation (1000 rpm for 30 min), and then washed with Milli-Q water and ethanol several times and freeze-dried under a vacuum. According to the graphene concentrations of 1.7, 2.5, and 3.4 wt%, the obtained samples were labeled as Ag@Ag2O-G1, Ag@Ag2O-G2, and Ag@Ag2O-G3. For comparison, the same procedure without graphene and irradiation was carried out to obtain pure Ag2O.

3.2. Catalyst Characterization

Sample morphology was examined by transmission electron microscopy (TEM) (CM200FEG, Philips, Amsterdam, The Netherlands) and scanning electron microscopy (SEM) (XL30FEG, Philips, The Netherlands). Analyses by X-ray diffraction (XRD) were performed with a Bruker D8 Advance diffractometer with Cu-Kα radiation (Optik GmbH, Ettlingen, Germany). Experiments using an X-ray photoelectron spectroscopy (XPS) (PHI, Chanhassen, MN, USA) were carried out on an RBD (RBD Instruments, Bend, OR, USA) upgraded PHI-5000C ESCA system (Perkin Elmer, Waltham, MA, USA) with Mg-Kα radiation (h = 1253.6 eV). UV-vis diffuse reflectance spectra (DRS) were measured by a Shimadzu UV-2550 spectrometer (Shimadzu Corporation, Kyoto, Japan), while BaSO4 was used as the reference.

3.3. Analysis Method

The SMX concentration was quantified by high-performance liquid chromatography (HPLC) supplied by Agilent with a Teknokroma C-18 Tracer Extrasil ODS2 (Teknokroma, Barcelona, Spain) (250 mm × 4.6 mm). The mobile phase was a mixture of 40% acetonitrile and 60% water which was adjusted at pH = 3 with phosphoric acid. The analysis was performed under isocratic conditions at a flow rate of 1 mL/min, with an injection volume of 10 μL and an oven temperature of 25 °C. The wavelength of the UV detector was set at 270 nm.

3.4. Photocatalytic Degradation of SMX

The photocatalytic experiments for SMX removal were conducted with a 300 W xenon lamp with a cut-off filter (λ > 280 nm) as a simulated solar light source and a cut-off filter (λ > 400 nm) as the visible light source. The light intensity on the surface of the suspension was kept at 37.7 mW/cm2 for simulated solar light and 34.2 mW/cm2 for visible light. SMX photocatalytic oxidation was completed in a 250 mL beaker with a cooling system on the bottom to maintain a constant solution temperature of 25 ± 2 °C. In a typical experiment, 5 mg of photocatalysts were suspended in 100 mL of 1 mg/L SMX solution by ultrasonication for 5 min. Before irradiation, the mixture suspension was stirred thoroughly in the dark for 30 min to ensure an adsorption-desorption equilibrium between the SMX and the photocatalyst. The lamp was then switched on to initiate the photocatalytic reaction. 3 mL of solution was sampled at certain time intervals and filtered with 0.45 μm syringe filters to remove suspended photocatalysts for the SMX analysis. Experiments for different SMX concentrations were conducted (Ccatalyst = 0.05 g/L, CSMX = 1, 2.5, 5, 10 mg/L) using the same procedures. Investigation of the reactive oxidative species was similar to the photodegradation process, while the scavengers (10 mM of iso-propanol, 0.5 μM of benzoquinone, 0.5 μM of sodium azide (NaN3) and 5 mM of disodium ethylene diamin tetraacetate (Na2-EDTA)) were added to the SMX solution prior to addition of the photocatalyst. In the recycling experiments, higher concentrations of photocatalysts and SMX (Ccatalyst = 0.25 g/L, CSMX = 5 mg/L) were used. The used photocatalysts were separated from the solution and then washed with Milli-Q water thoroughly before being re-dispersed in the fresh SMX solution for another cycle.

4. Conclusions

A hybrid Ag@Ag2O-graphene photocatalyst was synthesized with a facile chemical precipitation approach and reduction procedure under visible light. The Ag@Ag2O-G2 with 2.5 wt% of graphene content possessed a higher activity than the other prepared composite photocatalysts with 1.7 and 3.4 wt% of graphene content. The addition of graphene and produced Ag0 not only increased the photocatalyst adsorption capacities, but also decreased desorption percentages after irradiation. Under a higher SMX concentration, the adsorption, but not the photocatalytic ability, played a crucial role during the SMX removal process. The Ag@Ag2O-G2 showed stable photocatalytic activities in at least two successive experimental runs (a total of four hours). During the photocatalytic processes of Ag@Ag2O-G2, the generated holes were dominant reactive oxygen species for the photocatalytic degradation of SMX and Ag0, as the electron capture center could not transfer electrons in the CB of the Ag2O to graphene due to the negative graphene reduction potential.

Supplementary Materials

The following are available online at https://www.mdpi.com/2073-4344/8/7/272/s1, Figure S1: UV-vis spectra of Ag2O, Ag@Ag2O-G1, Ag@Ag2O-G2 and Ag@Ag2O-G3, Table S1: The rate constants and R2 of first order kinetic reaction for Ag2O, Ag@Ag2O-G1, Ag@Ag2O-G2 and Ag@Ag2O-G3 to degrade SMX, Figure S2: XPS spectra of Ag 3d in Ag@Ag2O-G2 before and after recycle use.

Author Contributions

L.Z., G.Z. and H.D. conceived and designed the experiments; L.Z. performed the experiments; L.Z. and G.Z. analyzedthe data; H.D. contributed reagents/materials/analysis tools; L.Z. wrote the paper.

Acknowledgments

This work was financially supported by the Chinese Ministry of Construction Foundation for Major Special projects of Water Pollution Control and Management of Science & Technology (No. 2017ZX07201-003 and No. 2017ZX07202004-004).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) SEM and (b) TEM images of Ag@Ag2O-G2 (with 2.5 wt% graphene).
Figure 1. (a) SEM and (b) TEM images of Ag@Ag2O-G2 (with 2.5 wt% graphene).
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Figure 2. (a) XRD patternsof Ag2O, Ag@Ag2O-G1, Ag@Ag2O-G2, Ag@Ag2O-G3 and XPS peaks of (b) Ag 3d in Ag@Ag2O-G2.
Figure 2. (a) XRD patternsof Ag2O, Ag@Ag2O-G1, Ag@Ag2O-G2, Ag@Ag2O-G3 and XPS peaks of (b) Ag 3d in Ag@Ag2O-G2.
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Figure 3. Adsorption abilities and photocatalytic activities of Ag2O, Ag@Ag2O-G1, Ag@Ag2O-G2, Ag@Ag2O-G3 for SMX removal under (a) simulated solar light (λ > 280 nm), (b) visible light (λ > 400 nm) irradiation (CSMX = 1 mg/L, Ccatalysts = 0.05 g/L), and (c) the adsorption capacity of photocatalysts and a comparison of the desorption percentage and pseudo-first-order reaction rate under different light irradiation.
Figure 3. Adsorption abilities and photocatalytic activities of Ag2O, Ag@Ag2O-G1, Ag@Ag2O-G2, Ag@Ag2O-G3 for SMX removal under (a) simulated solar light (λ > 280 nm), (b) visible light (λ > 400 nm) irradiation (CSMX = 1 mg/L, Ccatalysts = 0.05 g/L), and (c) the adsorption capacity of photocatalysts and a comparison of the desorption percentage and pseudo-first-order reaction rate under different light irradiation.
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Figure 4. (a) Adsorption capacity of Ag@Ag2O-G2 for different SMX concentrations in the darkand (b) adsorption capacity and the photocatalytic activities of Ag@Ag2O-G2 for different SMX concentrations removal under visible light (Ccatalyst = 0.05 g/L).
Figure 4. (a) Adsorption capacity of Ag@Ag2O-G2 for different SMX concentrations in the darkand (b) adsorption capacity and the photocatalytic activities of Ag@Ag2O-G2 for different SMX concentrations removal under visible light (Ccatalyst = 0.05 g/L).
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Figure 5. HPLC tests of (a) 1 mg/L, and (b) 10 mg/L SMX after adsorption and degradation by Ag@Ag2O-G2 under visible light.
Figure 5. HPLC tests of (a) 1 mg/L, and (b) 10 mg/L SMX after adsorption and degradation by Ag@Ag2O-G2 under visible light.
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Figure 6. Repeated adsorption and photocatalytic degradation of SMX under visible light by Ag@Ag2O-G2 (Ccatalyst = 0.25 g/L, CSMX = 2.5 mg/L).
Figure 6. Repeated adsorption and photocatalytic degradation of SMX under visible light by Ag@Ag2O-G2 (Ccatalyst = 0.25 g/L, CSMX = 2.5 mg/L).
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Figure 7. Photocatalytic activities of Ag@Ag2O-G2 in the reactive species scavenging experiments under visible light.
Figure 7. Photocatalytic activities of Ag@Ag2O-G2 in the reactive species scavenging experiments under visible light.
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Figure 8. The charge separation and transfer in SMX photodegradation over the Ag@Ag2O-G catalysts under visible light.
Figure 8. The charge separation and transfer in SMX photodegradation over the Ag@Ag2O-G catalysts under visible light.
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MDPI and ACS Style

Zhou, L.; Zou, G.; Deng, H. The Roles of Graphene and Ag in the Hybrid Ag@Ag2O-Graphene for Sulfamethoxazole Degradation. Catalysts 2018, 8, 272. https://doi.org/10.3390/catal8070272

AMA Style

Zhou L, Zou G, Deng H. The Roles of Graphene and Ag in the Hybrid Ag@Ag2O-Graphene for Sulfamethoxazole Degradation. Catalysts. 2018; 8(7):272. https://doi.org/10.3390/catal8070272

Chicago/Turabian Style

Zhou, Li, Guoyan Zou, and Huiping Deng. 2018. "The Roles of Graphene and Ag in the Hybrid Ag@Ag2O-Graphene for Sulfamethoxazole Degradation" Catalysts 8, no. 7: 272. https://doi.org/10.3390/catal8070272

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