Ag decorated ZnAl-layered double hydroxide with enhanced visible light photocatalytic activity for tetracycline degradation

Ecotoxicology and Environmental Safety 172 (2019) 423–431

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Fabrication of Ag2O/Ag decorated ZnAl-layered double hydroxide with enhanced visible light photocatalytic activity for tetracycline degradation

T

Chao-Rong Chena, Hong-Yan Zenga, , Mo-Yu Yia, Gao-Fei Xiaoa, Run-Liang Zhub, Xiao-Jv Caoa, Shi-Gen Shena, Jia-Wen Penga ⁎

a

College of Chemical Engineering, Xiangtan University, Xiangtan, Hunan, 411105, China Key Laboratory of Mineralogy and Metallogeny, Guangdong Provincial Key Laboratory of Mineral Physics and Material Research & Development, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China

b

ARTICLE INFO

ABSTRACT

Keywords: Hydrotalcite Surface plasmon resonance Photodegradation Visible light photocatalysis

The photocatalytic performance of layered double hydroxides (LDH) is usually confined to the slow interface mobility and high recombination rate of photogenerated electron-hole pairs in material. To overcome the low photocatalytic efficiency, novel Ag2O/Ag decorated LDH (LDH-Ag2O/Ag) was successfully synthesized by depositing Ag2O on the surface of LDH and then converted to Ag° nanoparticles in the right position after heat treatment. The as-synthesized LDH-Ag2O/Ag composites were characterized by Powder X-ray diffraction (XRD), Scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), UV–visible diffuse reflectance spectra (UV–vis DRS), photoluminescence spectra (PL) and transient photocurrent (TPC) analysis. Compared with virgin LDH, the photocatalytic activities of LDH-Ag2O/Ag composites were enhanced significantly. The optimum photocatalytic efficiency of LDH-Ag10 (0.0184 min−1) was nearly 46 times higher than that of virgin LDH (0.0004 min−1). The result of active species trapping experiments indicated that •OH, h+, and •O2− have an effect on the TC degradation, where •OH played the predominant role during the photocatalytic process. The possible photocatalytic mechanisms involving the charge transfer pathway and reactive species generation during the process of TC degradation were also discussed. The improved photocatalytic activity of LDH-Ag2O/Ag could be attributed to the synergetic effect between LDH and Ag2O/Ag that extended visible light range and reduced photogenerated charge carriers recombination.

1. Introduction Tetracycline (TC), as one of the most widely used antibiotics in the world, is frequently used in therapeutic medicine and feed supplements (Wang et al., 2017; Hong et al., 2018). However, TC molecules are hard to be metabolized by human and animals (Murphy et al., 2012; Shi et al., 2017), which are considered to be a potential threat to ecosystem and human health due to the ecotoxicity and antimicrobial resistance (Khan et al., 2010; Hoa et al., 2011; Xue et al., 2015; Huo et al., 2017). In recent years, numerous methods such as adsorption (Peng et al., 2014), electrolysis (Liang et al., 2018), microbial degradation (Yang et al., 2017a), and photodegradation (Xie et al., 2018) have been devoted into the removal of TC from wastewater. Among these methods, photocatalysis is one of the most promising technologies due to the high efficiency, ecofriendly and easy to operate (Saleh and Gupta, 2012; Saravanan et al., 2013; Saravanan et al., 2015; Darwish et al., 2016; Huo et al., 2017; Yan et al., 2017).

⁎

Layered double hydroxides (LDH) are typical two-dimensional anionic clays that made up of positively charged brucite-like layers and negatively charged interlayer anions (Huang et al., 2014). Their great stability, low cost, flexible structures, effective supports, and narrow band gap semiconductor properties have attracted great concern in visible light photocatalysis (Zhao et al., 2009; Mohapatra and Parida, 2016). However, the poor visible light response and high recombination of photoinduced electron-hole pairs generally resulted in low photocatalytic efficiency of virgin LDH (Kumar et al., 2017; Yuan and Li, 2017). Till now, a number of studies have tried to accelerate charge separation efficiency and thus improve the photocatalytic efficiency of LDH (Suárez-Quezada et al., 2016; Fu et al., 2016; Boppella et al., 2018; Tokudome et al., 2018). For example, coupling with other semiconductors (e.g., BiOCl, g-C3N4, Nb2O5, Cu2O) (Huang et al., 2016; Wu et al., 2016; Liu et al., 2017a; Yuan and Li, 2017) and doping elements (e.g., Tb and Ce) (Morales-Mendoza et al., 2015; Suárez-Quezada et al., 2016) can effectively accelerate the separation of photogenerated

Corresponding author. E-mail address: [email protected] (H.-Y. Zeng).

https://doi.org/10.1016/j.ecoenv.2019.01.080 Received 22 October 2018; Received in revised form 17 January 2019; Accepted 19 January 2019 0147-6513/ © 2019 Elsevier Inc. All rights reserved.

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electron-hole pairs and result in an comparatively improved photocatalytic activity. Recently, several studies showed that noble metal (e.g., Ag and Au) modified LDH can reduce the charge carrier recombination effectively (Mikami et al., 2016; Li et al., 2018). Despite the improving of photocatalytic activity of LDH is obvious, more effective approaches are still needed to carry on at present. As it is known that Ag2O/Ag composite can promote photogenerated charge carriers separation and transfer effectively for the following two reasons: (i) Ag2O possesses a band gap of 1.2 eV with excellent absorption capacity of visible light (Yang et al., 2017b); (ii) the surface plasmon resonance (SPR) of Ag° can not only extend more visible light range, but can reduce charge carriers recombination (Chen et al., 2016). Thus, considering the remarkable properties of Ag2O/Ag, we expect that the photocatalytic activity of LDH could be improved efficiently by combining with Ag2O/Ag, which not only can accelerate the electron transfer from Ag2O/Ag to LDH, but also can suppress the aggregation of Ag2O/Ag nanoparticles due to the net trap confinement effect of LDH (Jin et al., 2015). To the best of our knowledge, LDHAg2O/Ag composite as photocatalyst for organic pollutants degradation has not been investigated. In this study, Ag2O/Ag decorated ZnAl LDH (LDH-Ag2O/Ag) visible light responsive photocatalysts were synthesized by depositing different contents of Ag2O/Ag on the surface of LDH via precipitation and thermal decomposition method. The structures, morphologies, chemical compositions, optical and photo-electrochemical properties of LDH-Ag2O/Ag composites were investigated by XRD, SEM, TEM, XPS, UV–vis DRS, PL, and TPC analysis. The photocatalytic performances of the photocatalysts for TC degradation were evaluated under visible light irradiation. Furthermore, the role of active species on the reaction of TC degradation was studied, and a possible mechanism involve the electron transfer pathways and the enhancement of photocatalytic activity was also proposed.

heat-drying process at 80 °C, Ag° nanoparticles were formed from the partial thermally decomposition of Ag2O. The Ag content (wt%) represent the calculated mass percentage of Ag+ to LDH in reaction solution. For convenience, the final products with different Ag contents (2.5, 5.0, 10.0 and 15.0 wt%) were designated as LDH-Ag2.5, LDH-Ag5.0, LDH-Ag10 and LDH-Ag15, respectively.

2. Experiments

2.4. Photocatalytic experiments

2.1. Materials

The TC degradation activity of the as-prepared materials was tested using a 300 W Xe lamp with a 420 nm cutoff filter as the visible light source. The temperature of reaction solution was maintained by circulating water jacket. Typically, 50 mg photocatalyst was dispersed in 50 mL aqueous solution containing TC with an initial concentration of 40 mg L-1. Before irradiation, the mixed solution was vigorously stirred for 30 min to establish an adsorption-desorption equilibrium between catalyst and TC. At given time intervals, 2 mL of the reaction solution was withdrawn and filtered immediately with 0.22 µm membrane filters to remove the residual photocatalyst. Subsequently, the TC concentration was determined by UV–vis spectrophotometer at absorbance wavelength of 356 nm.

2.3. Characterizations Powder X-ray diffraction (XRD) measurements were performed on a Japan Rigaku D/max 2550PC (λ = 1.5405 Å) with Cu Kα irradiation. The scan step was 0.02 (2θ) with a filament intensity of 30 mA and a voltage of 40 kV. Scanning electron microscopy (SEM, JEOL JSM6700F) and transmission electron microscopy (TEM, FEI Talos F200S) were used to observe the morphologies and microstructure of the samples. The elemental mappings were carried out on a scanning transmission electron microscope (STEM) unit with high-angle annular dark-field (HAADF) detector. X-ray photoelectron spectroscopy (XPS) analyses were carried out using a Thermo Fisher Scientific K-Alpha spectrometer. The C1s peak from the adventitious carbon based contaminant with a binding energy of 284.8 eV was used as the reference for calibration. UV–visible diffuse reflectance spectra (UV–vis DRS) were measured by using a UV–visible spectrophotometer (Shimadzu UV-2550, Kyoto, Japan). The analysis of photoluminescence (PL) spectra was carried out at room temperature on a fluorescence spectrophotometer (F-4600, Hitachi, Japan) with an excitation wavelength of 400 nm. The scanning speed was 1200 nm/min and the width of the emission slit was 5.0 nm. The transient photocurrent (TPC) measurements were implemented on a VersaSTAT three-electrochemical system (Princeton) in a standard three-electrode system at room temperature. The electrolyte was 0.5 mol L–1 Na2SO4 solution, and a 300 W lamp was used as the light source.

Urea (CH4N2O, AR), silver nitrate (AgNO3, AR), zinc nitrate (Zn (NO3)2·6H2O, AR), aluminum nitrate (Al(NO3)3·9H2O, AR), sodium hydroxide (NaOH, AR), and hydrochloric acid (HCl, AR) were purchased from Tianjin Damao Chemical Reagent Factory, China. Tetracycline (TC) was purchased from Shanghai Shunbo Biological Engineering Co., Ltd, China. All the chemical reagents were used without further purification, and all the solutions were made with deionized water. 2.2. Preparation of photocatalysts ZnAl-CO32− hydrotalcite was prepared through urea method (Zhang et al., 2014). Typically, 2.68 g Zn(NO3)2·6H2O, 1.13 g Al (NO3)3·9H2O and 4.86 g urea were added into were 80 mL deionized water, after dissolved completely, the mixture were heated at 105 °C for 12 h under vigorous mechanic stirring and then aged at 80 °C for another 12 h. The white precipitate was gained after centrifugal and washed thoroughly with deionized water. For simplicity, the final product was named as LDH. LDH-Ag2O/Ag composite was synthesized through precipitation and thermal decomposition method (Hu et al., 2015b). 0.50 g LDH powder and appropriate AgNO3 were dissolved in 200 mL alcoholic solution (alcohol/water volume ratio of 1:1), and subsequently a 2.0 mol∙L−1 NaOH solution was added dropwise into the above mixture solution with continuous stirring until pH of the solution reached to 14. The Ag+ reacted with NaOH to form AgOH, which is unstable and dissociate to Ag2O. Then the suspension was stirred for 12 h to get a complete reaction of the AgOH dissociation. After reaction, the suspension was centrifuged and washed with deionized water for several times. In the

3. Results and discussion 3.1. Characterization of photocatalysts The XRD patterns of the samples were collected as shown in Fig. 1. The pattern of the virgin LDH was in consistent with the typical ZnAl LDH reported in previous investigations (Yuan and Li, 2017; RodriguezRivas et al., 2018), while the interlayer distance of the virgin LDH (0.763 nm) corresponded to CO32− intercalated hydrotalcite (JCPDS 38–0486) (Santos et al., 2017; Yuan and Li, 2017). For the LDH-Ag2O/ Ag samples, the diffraction peaks at about 2θ = 38.1° and 44.3° were indexed to the (111) and (200) facets of Ag° (JCPDS 65–2871) (Liu et al., 2015), and the additional peaks at 32.6°, 54.8°, and 65.4° can be assigned crystal facet of Ag2O phase (JCPDS 41–1104) (Chen et al., 2011; Cui et al., 2017). The results suggested the coexistence of Ag° and Ag2O in LDH-Ag2O/Ag composites, where Ag° was formed mainly due to the partial thermal decomposition of Ag2O particles during drying process (Hu et al., 2015b). All the LDH-Ag2O/Ag samples clearly 424

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LDH materials. At the same time, some tiny nanoparticles dispersed on the surface of the composites are belonged to Ag2O/Ag nanoparticles. The amounts of Ag2O/Ag nanoparticles increased along with the increase of Ag contents, while the Ag2O/Ag nanoparticles on the surface of the LDH-Ag15 aggregated to form bigger cluster compared to the LDH-Ag10. On the other hand, the EDS spectrum of the LDH-Ag10 (Fig. 2f) further confirmed the existence of Zn, Al, Ag, C and O elements, where the practical contents of Ag element (9.27%) is close to theoretical values. TEM and HRTEM were carried out to further investigate the morphology and structure information details of LDH-Ag10 (Fig. 3). The Ag species were all well dispersed on the surface of the LDH-Ag10 particles with average sizes ranging from 4 to 50 nm (Fig. 3a). The lattice fringes (Fig. 3b) showed the interplanar spacing of 0.23 nm corresponds to the (111) lattice plane of Ag°, while a spacing with 0.27 nm is indexed to the (111) plane of Ag2O (Yu et al., 2014; Zhu et al., 2018a). Besides, the interplanar spacing of 0.30 nm is in agreement with (110) plane of LDH (Zhao et al., 2015). The HAADF-STEM mapping (Fig. 3c~g) showed that Zn, Al, Ag, and O elements were well distributed in the corresponding region. These results further confirmed the existence of Ag° and Ag2O on the surface of the LDH-Ag10 particles. The XPS analysis was applied to study the surface element composition and chemical state of LDH-Ag10 (Fig. 4). The Ag 3d spectrum exhibited two distinct peaks, which could be further divided into four individual peaks at about 367.7, 368.2, 373.7, and 374.2 eV, respectively. The peaks located at 367.7 and 373.7 eV were ascribed to the Ag+ of Ag2O, while those at 368.2 and 374.2 eV were attributed to Ag° (Hu et al., 2015a). The XPS spectra of O 1 s indicated a broad and strong peak, which was fitted with two peaks located at 529.6 and 531.2 eV. The peak at 529.6 eV was attributed to the lattice oxygen of Ag2O, another peak at 531.2 eV correspond to hydroxyl and chemisorbed oxygen (Zhu et al., 2018a). The XPS analysis also confirmed the presence of Ag° and Ag2O on LDH-Ag10. The light absorption performances of the LDH-Ag2O/Ag photocatalysts were studied by UV–vis DRS spectra (Fig. 5a). For the virgin

Fig. 1. XRD patterns of virgin LDH, LDH-Ag2.5, LDH-Ag5, LDH-Ag10, and LDHAg15.

showed the typical layered double hydroxide structure reflections, indicating that the introduction of Ag species did not change the crystal structure of ZnAl hydrotalcite-like compounds. According to Scherrer's formula (Lucarelli et al., 2018), the average cation-cation distance (a) in brucite-like sheets of LDH can be calculated from the (110) reflection. The similarity a values (virgin LDH: 0.308 nm, LDH-Ag2O/Ag: about 0.305 nm) indicated that Ag° and Ag2O did not enter into the crystal lattice of LDH. SEM images were taken to demonstrate the morphology of the virgin LDH, LDH-Ag2.5, LDH-Ag5, LDH-Ag10 and LDH-Ag15 samples (Fig. 2). In Fig. 2a, the virgin LDH clearly exhibited irregular but very smooth surface, which was typical ZnAl hydrotalcite-like morphology (Li et al., 2017b). Thin lamellar structures was retained in the LDHAg2O/Ag composites (Fig. 2b~e), demonstrating that the incorporation of Ag species had no significant effect on the morphology structure of

Fig. 2. SEM images of virgin LDH, LDH-Ag2.5, LDH-Ag5, LDH-Ag10, and LDH-Ag15. 425

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Fig. 3. (a) TEM images, (b) HRTEM images, and HAADF-STEM mapping (c~g) of LDH-Ag10.

LDH, the weak absorption peak in the region between 200 nm and 400 nm can be resulted from the electronic transition in ZnAl LDH from the O 2p to the metal ns or np levels (n = 4 for Zn, and n = 3 for Al) (Mendoza-Damián et al., 2016). After introducing Ag2O/Ag, all of the LDH-Ag2O/Ag composites presented a broad and strong absorption at

400–800 nm, which should be attributed the SPR effect of Ag° deposited on the surface of the composites and the synergistic effect between Ag2O/Ag and ZnAl LDH (Seery et al., 2007; Zhu et al., 2018b). Especially, the LDH-Ag10 photocatalyst demonstrated the strongest visible light absorption among all of the LDH-Ag2O/Ag composites.

Fig. 4. XPS spectra of Ag 3d and O 1 s from the LDH-Ag10. 426

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Fig. 5. (a) UV–vis DRS spectra and (b) PL spectra of virgin LDH, LDH-Ag2.5, LDH-Ag5, LDH-Ag10, and LDH-Ag15 (c) Photocurrent responses of virgin LDH and LDHAg10.

Notably, the separation efficiency of photogenerated electron-hole pairs is crucial for the photocatalytic performance of photocatalysts. Therefore, photoluminescence (PL) spectra were collected to estimate the efficiency of interfacial charges transfer in photocatalysts (Jiang et al., 2015). As can be seen from Fig. 5b, the virgin LDH exhibited a strong and broad emission peak at around 355 nm, which can be ascribed to the surface defects of ZnAl-LDH nanocrystals (Zhang et al., 2013). As for the LDH-Ag2O/Ag composites, the PL maximum was blueshifted after incorporating the Ag species, and the PL intensity decreased obviously compared to that of the virgin LDH. This phenomenon could be due to the effectively transfer of photogenerated carriers between the Ag2O/Ag and ZnAl LDH. The Ag2O/Ag dispersed on the surface of LDH could effectively promote the separation of photogenerated electron-hole pairs. The intensity continuously reduced to the minimum value after reach at 10.0 wt% of Ag contents, followed by a raise up when the ratio of Ag increase to 15 wt%. A possible explanation was that the excess Ag species could act as a recombination center, and resulted in the decrease of charge separation efficiency (Luo et al., 2012; Xu et al., 2016; Zhu et al., 2018b). The analysis results indicated that moderate amounts of Ag species favoring the separation of photogenerated electron-hole pairs. The photogenerated electron-hole pairs separation efficiency of the virgin LDH and LDH-Ag10 was further investigated using transient photocurrent (TPC) response (Fig. 5c). The TPC curves evidently

showed the rise and fall of the photocurrent responses for each switchon and switch-off event. Upon irradiation, both of virgin LDH and LDHAg10 can be excited to produce photocurrent. The photocurrent response of LDH-Ag10 was significantly improved, and the photocurrent of LDH-Ag10 electrode was about 4.4 times higher than that of virgin LDH electrode. The remarkable photocurrent improvement of LDHAg10 indicated that the mobility and separation efficiency of the photogenerated charge carriers was obviously improved after decorating Ag2O/Ag nanoparticles on the surface of LDH. The results further confirmed that appropriate amounts of Ag species and good interfacial contact between Ag2O/Ag and LDH played an important role in charge separation and migration. 3.2. Photocatalytic degradation of TC In order to evaluate the photocatalytic activity of LDH and LDHAg2O/Ag composites, the degradation of TC using LDH, LDH-Ag2.5, LDH-Ag5.0, LDH-Ag10.0, and LDH-Ag15.0 were tested under visible light irradiation. As shown in Fig. 6a, lower than 14% TC was adsorbed onto the as-prepared photocatalysts in the dark, indicating relatively poor adsorption capacity of TC over these samples. Under visible light irradiation, the blank experiment test revealed that the self-degraded of TC is nearly negligible. The LDH-Ag2O/Ag composites displayed an obviously enhanced photocatalytic activity (65–92% for 90 min), while 427

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Fig. 6. (a) The photocatalytic performance of virgin LDH and LDH-Ag2O/Ag composites for TC degradation under visible-light irradiation (b) Rate constants (k) and pseudo-first order kinetics (inset) of virgin LDH and LDH-Ag2O/Ag composites for TC degradation (c) Recycling of the LDH-Ag10 photocatalyst for TC degradation (d) Trapping experiment of the active species for TC degradation over the LDH-Ag10.

the virgin LDH exhibited poor photocatalytic degradation efficiency (18.0% for 90 min) for TC degradation. Moreover, the TC degradation ratio increased with the Ag contents from 2.5 to 10.0 wt%, which could be related to the stronger visible light absorption ability and more LDHAg2O/Ag heterojunction. However, the catalytic activity decreased slightly when further increasing the Ag species content to 15 wt%, revealing that excess Ag species may become new recombination centers and against the photocatalytic reaction. Among those samples, LDHAg10 showed the highest photocatalytic efficiency (92%) after 90 min. To further gain a deep insight on the TC degradation properties of the photocatalysts, it is necessary to investigate the kinetic behaviors. The TC degradation kinetics is usually described by the pseudo-firstorder model (−ln(Ct/Co) = kt) due to their good representation of the experimental data (Liu et al., 2017b). Based on the data obtained in the present study (Fig. 6a), the apparent rate constant (k) were calculated by the model (Fig. 6b). The results showed that all of the correlation coefficients R2 were above 0.95, demonstrating the applicability of the model, and the k values of LDH-Ag2O/Ag composites are obviously higher than that of the virgin LDH. The k values were gradually increased with Ag species rising from 2.5 to 10.0 wt% but downwardly decreased at 15.0 wt%, where the LDH-Ag10 achieved the highest k value (up to 0.0184 min−1) and about 46 times higher than that of the virgin LDH (0.0004 min−1). Moreover, the TC degradation activities over difference photocatalysts reported in recent years are summarized in Table 1. Comparing with the reported photocatalysts, the LDH-Ag10 prepared in present work possessed excellent photocatalytic activity for TC degradation. Therefore, the results further demonstrated that the

Table 1 Comparison of photocatalytic activity with other previously reported photocatalysts for TC degradation in recent years. Sample

k (min−1)

TC degradation (%)

Ref.

Ag/Bi3TaO7 g-C3N4 powder

0.0162 0.0051

85.42 86.00

WO3 /K+Ca2Nb3O10− Nb2O5/g-C3N4 Ag2O/Ta3N5 g-C3N4/K+Ca2Nb3O10− Bi/α-Bi2O3/g-C3N4 LDH-Ag2O/Ag

0.0151 0.0096 0.0079 0.0137 0.0122 0.0184

85.50 76.20 78.30 81.00 91.20 92.00

Luo et al. (2015) Hernández-Uresti et al. (2016) Ma et al. (2017) Hong et al. (2016) Li et al. (2017a) Jiang et al. (2017) Chen et al. (2018) This work

Ag2O/Ag decorated LDH composite can be a promising photocatalyst for TC degradation. 3.3. Stability of the photocatalyst The stability of photocatalyst is equally as important as photocatalytic activity in practical application. Therefore, the recycling experiments of TC degradation over LDH-Ag10 were carried out (Fig. 6c). At the end of each cycle, the LDH-Ag10 catalyst was separated and reused in the next cycle. The photocatalytic activity decreased slightly after 5 cycles, which could be attributed to the loss of the photocatalysts during the recovery process. The TC degradation ratio still remained at 428

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Degradation products

O2

•O2–

e– e– e – e – e –

TC

•OH

CB

–

•OH

•O2

Degradation products

TC

TC

e– e– e– e– e–

Ag2O

O2

CB

VB h+ h+ h+ h+ h+

ZnAl-LDH

Degradation products

VB

Scheme 1. Possible photocatalytic mechanism for TC degradation.

above 80.0% after five recycles. The results confirmed that the assynthesized photocatalyst possessed high stability and reusability, which could be regarded as a qualified candidate for photodegradation organic pollutions in practical applications.

them. LDH-Ag2O/Ag + hv → e− + h+

(1)

O2 + e− → •O2− → •OH

(2)

•O2− + H2O → •OOH + OH−

3.4. Possible photocatalytic mechanism

•OOH + H2O + e H2O2 + e

+

In order to gain in-depth understanding on the active species (h , •OH and •O2−) in the photodegradation process of TC over the LDHAg10, disodium ethylenediaminetetraacetate (EDTA-Na2) was chosen as the hole (h+) scavenger, tert-butanol (t-BuOH) as the hydroxyl radical (•OH) scavenger, and benzoquinone (BZQ) as the superoxide radical (•O2−) scavenger. As shown in Fig. 6d, when t-BuOH was added, the TC degradation ratio significantly decreased from 92% to 14%, indicating that •OH was the major active species. At the same time, when EDTANa2 and BZQ were added, the TC degradation ratio decreased to 27% and 36%, respectively, suggesting that h+ and •O2− are also important in TC degradation reaction. On the foundation of the analysis and discussion above, the possible photocatalytic mechanism for the enhanced photocatalytic activity of TC degradation over LDH-Ag10 was illustrated in Scheme 1. The Ag2O on the surface of LDH-Ag10 can be excited under visible light irradiation and formed photogenerated electron-hole pairs, while the ZnAl LDH cannot be excited under visible light. At the same time, parts of the photogenerated electrons at the CB of Ag2O (+0.2 eV) (Ren et al., 2014) could transfer to the CB of ZnAl LDH due to the more negative CB position than that of ZnAl LDH (+0.55 eV) (Yuan and Li, 2017). Subsequently, ZnAl LDH acts as a reaction platform to promote the dissolved oxygen forming •O2− radicals and then further transformed to •OH by combining with H2O (Eqs. (2)–(5)). Specifically, the VB position of Ag2O (+1.4 eV) (Ren et al., 2014) is far above the standard redox potential of •OH/H2O (+2.68 eV) (Kumar et al., 2013), demonstrating that the holes on the VB of Ag2O cannot oxidize H2O to •OH (Ren et al., 2014; Yuan and Li, 2017). Therefore, the holes on the VB of Ag2O would react with TC directly. On the other hand, more visible light photons could be absorbed because of the SPR effect of Ag° species, causing the transfer of more photoinduced electrons from Ag° to the surface of Ag2O (Cui et al., 2017). The produced active species (•OH, h+ and •O2−) can efficiently degrade TC into intermediate products and finally into CO2 and H2O (Eq. (6)), and •OH played a major role among

+

h , •OH,

−

−

→ H2O2 + OH

(3) −

−

→ •OH + OH

•O2−

+ TC → Degradation products

(4) (5) (6)

4. Conclusions In the present work, a series of Ag2O/Ag decorated ZnAl LDH photocatalysts were synthesized successfully by precipitation and thermal decomposition method. The result of characterization analysis showed that the Ag species existed in the forms of Ag° and Ag2O. The as-synthesized LDH-Ag2O/Ag composite with 10 wt% content of Ag species showed the highest photocatalytic activity for TC degradation than that of virgin LDH. The apparent rate constant k of LDH-Ag10 (0.0184 min−1) was nearly 46 times higher than the virgin LDH (0.0004 min−1). The improved photocatalytic activity was attributed to the localized SPR of Ag0 and efficient separation of photogenerated carriers. Furthermore, the LDH-Ag10 composite possessed good stability under visible light irradiation after five cycles, which could be a promising photocatalyst for environmental remediation. Acknowledgments This work was financially supported by National Natural Science Foundation of China (41572031), National Youth Topnotch Talent Support Program, Joint Research Program of Hunan Provincial Natural Science Foundation (Xiangtan) of China (2016JJ5030), Xiangtan University undergraduate innovative experiment program (2018XTUXJ036), Hunan Provincial Key Research and Development Program (2018SK2010), and Hunan Provincial Innovation Foundation For Postgraduate (CX2018B043).

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