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Enhanced visible light-driven photocatalytic performance and stability of Ag3PO4 by simultaneously loading AgCl and Fe(III)
Applied Surface Science 507 (2020) 145067
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Enhanced visible light-driven photocatalytic performance and stability of Ag3PO4 by simultaneously loading AgCl and Fe(III)
T
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Ying Wang, Hongbin Yu , Bin Zhao, Weichao Qin, Ying Lu, Suiyi Zhu, Mingxin Huo aEngineering Lab for Water Pollution Control and Resources Recovery, School of Environment, Northeast Normal University, Changchun 130117, China bScience and Technology Innovation Center for Municipal Wastewater Treatment and Water Quality Protection, Northeast Normal University, Changchun 130117, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Photocatalysis Ag3PO4/AgCl/Fe(III) Tetracycline Visible light
Silver phosphate (Ag3PO4) is widely used in the field of photocatalysis due to its excellent quantum efficiency and high photocatalytic activity. With Ag3PO4 as the precursor, we have prepared a ternary composite of Ag3PO4/AgCl/Fe(III) through an in-situ anion-exchange and impregnation method by using ferric chloride as the sources of chlorine and iron. The characterization results indicated that AgCl and Fe(III) were successfully loaded onto the surface of Ag3PO4 without any changes in its morphology and crystal structure. A significantly enhanced photocatalytic performance for the degradation of tetracycline (TC) was obtained under visible light irradiation (> 420 nm). The degradation rate of TC over Ag3PO4/AgCl/Fe(III) (0.458 min−1) was 2.32 times and 2.94 times that of Ag3PO4/AgCl (0.197 min−1) and the pure Ag3PO4 (0.156 min−1), respectively. Furthermore, the stability of the photocatalyst was also improved. On one hand, the presence of AgCl could prevent Ag3PO4 from dissolution due to the smaller solubility of AgCl. On the other hand, the heterostructure and Fe(III) in Ag3PO4/AgCl/Fe(III) could promote the transfer and the separation of photoexcited electrons and holes, thus enhancing the photocatalytic activity. Our investigation might give some new insights for the facile preparation of photocatalysts with high visible light-driven photocatalytic performance.
1. Introduction
catalysts (e.g. Pt [7], Au [8], Ag [9]) were usually used as electron transfer mediators or charge recombination centers in all-solid-state catalysts. Addionally, modification with small amount of AgX (X = Cl, Br, I) could improve the photocatalytic properties and recyclability owing to the decreased solubility (solubility: Ag3PO4: 0.02 g L−1, Ksp = 1.4 × 10−16; AgCl: 0.0019 g L−1, Ksp = 1.8 × 10−10; AgBr: 0.00014 g L−1 L, Ksp = 5 × 10−13; AgI: 0.00003 g L−1, Ksp = 8.3 × 10−17; 25 °C) and the proper energy band structure [10]. Moreover, the localized surface plasmon resonance (LSPR) of Ag could promote the absorption capability of composites for visible light. Recently, in many investigations, grafting with metal ions or metal compounds as co-catalysts, e.g., Cu(II) [11,12], Cr(III) [13], Ce(III) [14] and PdS [15], was believed as an effective method to enhance the photocatalytic activity of composites. Especially, the amorphous Fe(III) (low-cost and abundant on earth) was proved to have excellent capability to capture photoelectrons to improve the photocatalytic activity and the stability of photocatalysts [16,17]. Herein, we have investigated a facile method to synthesize a ternary photocatalyst of Ag3PO4/AgCl/Fe(III). It was found that the hybrid photocatalyst simultaneously exhibited a better recyclability and a
Semiconductor photocatalysis over Ag3PO4 has caught great attention since 2010 due to its efficient utilization of solar energy, and high quantum efficiency of 90% in photooxidative capability for water splitting and excellent performance for the decomposition of organic compounds [1,2]. Nevertheless, the silver ions (Ag+) in Ag3PO4 could easily trap the photogenerated electrons to form Ag0 when there are no scavengers added under visible light irradiation [2]. This phenomenon leads to the unstable structure and decreased photocatalytic performance. In addition, the slightly soluble property of Ag3PO4 (Ksp = 1.4 × 10−16, 0.0200 g L−1, 25 °C) in aqueous solution results in the limitation of application on the degradation of organic pollutant. The heterostructure in composites plays an important role in the photocatalytic degradation process. The hybrid photocatalysts (e.g. NCDs/g-C3N4/Ag3PO4 [3], ANP@MQD/FL-MN [4], TiO2TNTs/Ag3PO4 [5], ZnO/Fe2O3/g-C3N4 [6]) would normally exhibit an enhanced photocatalytic activity. This was because the photogenerated electronhole pairs could be effectively separated with the help of the heterostructure in photocatalysts. In these hybrid composites, noble metal co-
⁎ Corresponding author at: Engineering Lab for Water Pollution Control and Resources Recovery, School of Environment, Northeast Normal University, Changchun 130117, China. E-mail address: [email protected] (H. Yu).
https://doi.org/10.1016/j.apsusc.2019.145067 Received 5 October 2019; Received in revised form 5 December 2019; Accepted 12 December 2019 Available online 16 December 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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and the surface was relatively rough. It could be seen from Fig. 1b that, as compared with the pure Ag3PO4, the modification of FeCl3 gave rise to no obvious changes in mophologies. This was mainly because the amount of FeCl3 modified on the surface of Ag3PO4 was rare. The EDS analysis was performed to identify the components of the modified sample Ag3PO4/AgCl/Fe(III), and the results were demonstrated in Fig. 1c. It was found that, besides the elements of Ag, P and O corresponding to Ag3PO4, the elements Cl and Fe was also detected, suggesting the existence of AgCl and the component containing Fe. To further identify the distribution of different elements, the element mapping of Ag3PO4/AgCl/Fe(III) (5 wt%-110 min) was recorded. As displayed in Fig. 1d, the elements of Cl and Fe distributed evenly, and was similar to the the elements of Ag, P, and O, indicating that the elements of Cl and Fe should be modified mainly on the surface of Ag3PO4. The TEM characterization was conducted. The TEM and HR-TEM images of Ag3PO4/AgCl/Fe(III) are shown in Fig. S1a and b. As illustrated, clear lattice fringe information of the cubic phase Ag3PO4 could be seen and the interplanar spacing of 0.247 nm corresponded to the (2 2 0) crystal plane of Ag3PO4. In addition, the other indistinct lattice fringe of 0.196 nm could be assigned to the (2 1 1) crystal plane of AgCl. The SAED patterns (Fig. S1c) display the polycrystalline structure of the composite. Besides the distinct signal of Ag3PO4, a faint diffraction ring signal of AgCl corresponding to the (2 1 1) crystal plane was observed, which was in accordance with the result of HR-TEM. However, neither the obvious lattice fringe in HR-TEM nor diffraction ring signals in SAED patterns were detected for the Fe components, possibly due to a very limited amount or the amorphous property of the Fe components as result of the low-temperature of synthesis. The crystal structures of the Ag-based photocatalysts before and after AgCl and Fe(III) modification were revealed by the XRD (Fig. 2). It can be seen that most diffraction peaks for Ag3PO4, Ag3PO4/AgCl and Ag3PO4/AgCl/Fe(III) were attributed to the cubic phase of Ag3PO4 (JCPDS No. 06-0505). Moreover, all the samples showed similar diffraction peak intensity and full width at half-maximum, proving that the phase structure of Ag3PO4 was well maintained. The lattice constants of Ag3PO4 and Ag3PO4/AgCl/Fe(III) were calculated as 0.6008 nm and 0.6011 nm, respectively, which were almost the same as the standard value of 0.6013 nm listed in the JCPDS card (No. 060505). This indicated that the modification of AgCl and Fe(III) did not change the crystal structure of Ag3PO4, and thus Fe ions most likely deposited on the surface but not doped in the crystal lattice of catalysts. Considering quite little AgCl on the Ag3PO4 surface, an enlarged image from 30° to 38° was presented in the inset of Fig. 2 to demonstrate the diffraction peak of AgCl. It was found that both Ag3PO4/AgCl and Ag3PO4/AgCl/Fe(III) showed a diffraction peak at ca. 32.3° (〈2 0 0〉 ) with very weak intensity, which could be ascribed to the low concentration and relatively weak crystallinity of AgCl (JCPDS No. 311238) in the hybrids. There was no diffraction peaks of Fe(III)-containing component in the XRD patterns, to some extent, indicating that the Fe(III)-containing component was either fairly little or in an amorphous phase. Nevertheless, the existence of Fe(III) could be confirmed in the following XPS analysis. Chemical states of the surface elements of Ag3PO4/AgCl/Fe(III) have been investigated by the XPS analysis. As shown in Fig. 3, six elements including C, Ag, P, O, Cl, and Fe were observed in the survey spectrum (Fig. 3a). The peak at 284.6 eV was assigned to the residual carbon as a reference to determine the binding energy of other elements. In the case of Ag 3d spectra (Fig. 3b), the peaks at 367.7 eV and 373.7 eV corresponded to Ag 3d5/2 and Ag 3d3/2 of Ag+, while the weak peaks at 368.5 eV and 374.5 eV were ascribed to Ag 3d5/2 and Ag 3d3/2 of Ag0 [18,19]. These results verified the existence of metallic Ag0 in the Ag3PO4/AgCl/Fe(III) particles. In the high resolution spectrum of Cl 2p (Fig. 3c), the binding energy at 198.2 eV was ascribed to Cl 2p3/2 [20]. Fig. 3d shows the spectrum of Fe 2p, broad peaks at 711.5 eV and 725.2 eV corresponding to Fe 2p3/2 and Fe 2p1/2 proved the existence of
higher photocatalytic activity due to the presence of the heterostructure and Fe(III), which changed the pathway of photoelectrons and decreased the recombination of charge carriers, thus relieving the photocorrosion and improving the photocatalytic performance. 2. Experimental section 2.1. Preparation of photocatalysts The photocatalyst Ag3PO4 was synthesized through a precipitation method. AgNO3 (1.700 g) was dissolved into 200 mL deionized water under stirring for 20 min, and then appropriate amount of 0.005 M Na2HPO4 was added drop by drop into the AgNO3 aqueous solution under stirring for 12 h. Finally, the precipitation was collected by centrifugation, washed with deionized water, and dried at 60 °C in a vacuum oven. The ternary composite of Ag3PO4/AgCl/Fe(III) was synthesized by a facile method. That is, Ag3PO4 was used as the template and the silver ion source, while FeCl3 was added to provide chloride ion and Fe(III). Briefly, Ag3PO4 powers (0.200 g) were dispersed into 200 mL deionized water under stirring for 30 min. Different amount of freshly prepared FeCl3 solutions (0.47 mM), i.e. 2 wt%, 4 wt%, 5 wt%, and 6 wt%, were added dropwise into the suspension and then the suspension was transferred into a water bath at 55 °C for a period of time, i.e. 90, 110, and 130 min. The ternary material of Ag3PO4/AgCl/Fe(III) (X wt%-T min) was obtained by centrifugation, washed with deionized water, and dried at 60 °C in a vacuum oven. The term Ag3PO4/AgCl/Fe(III) stood for Ag3PO4/AgCl/Fe(III) (5%-110 min) throughout the paper unless specified. 2.2. Photocatalytic experiments The photocatalytic performance has been evaluated by the photodegradation of tetracycline hydrochloride (TC, 20 mg L−1) containing 0.50 g L−1 photocatalysts. Before irradiation, the mixed solutions were stirred for 30 min in dark to achieve an adsorption-desorption equilibrium of TC. A Xenon lamp (300 W) with a UV cut-off filter of 420 nm was used as the visible light source (170 mW cm−2). A certain volume of suspension was taken out at given time intervals for analysis at 358 nm wavelength on a UV–vis spectrophotometer. 2.3. Characterization The morphologies of photocatalysts were characterized by a scanning electron microscope (SEM, JSM-7500F, Japan) equipped with energy-dispersive X-ray (EDS). The crystal structure, element, optical absorption property were analyzed by powder X-ray diffractometer (XRD) with Cu Kα irradiation (D/maxZ200PC, Rigaku, Japan), X-ray photoelectron spectroscopy (XPS) with Al Kα radiation (ESCALAB MK II, VG Scientific, England) and UV–vis diffuse reflectance spectrophotometer (DRS, Varian, Cary 500, America). The electrochemical characterizations (transient photocurrent response and electrochemical impedance spectroscopy (EIS)) were carried out on an electrochemical working station (CHI-760E, China) in a three-electrode system with Na2SO4 solution (0.1 M) as the electrolyte. The prepared catalyst, a Pt foil and a saturated calomel electrode (SCE) were used as the working electrode, counter electrode, and reference electrode, respectively. 3. Results and discussion 3.1. Characterization Fig. 1 shows the typical SEM images of samples and the EDS information of Ag3PO4/AgCl/Fe(III) (5 wt%-110 min). Fig. 1a indicated that the average size of pure Ag3PO4 was estimated to be 200–350 nm 2
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Fig. 1. SEM of the pure Ag3PO4 (a), and the SEM image (b), EDS (c) and the mapping analysis (d) of Ag3PO4/AgCl/Fe(III).
the stretching vibration and deformation bands of eOH, which might be the eOH groups combined with Ag3PO4 [23] or the component of FeOOH [21]. Based on the discussions above, it was believed that Fe ions might deposit on the surface of catalysts most likely as amorphous FeOOH but not Fe2O3 due to the low temperature of synthesis. As reported in the literature [16,17,25,26], the modification of Fe ions by an impregnation technique at a relatively low temperature would usually result in the formation of FeOOH. In this work, the temperatures of preparation (55 °C) and drying process (60 °C) were not high. H.G. Yu et al. investigated the structural features of the grafted Fe ions by X-ray absorption fine structure analysis [25], and found that the structural parameters of the photocatalyst more closely resembled the parameters of FeOOH than those of Fe2O3. Additionally, the XPS results obtained here were similar to that reported in the literature, and the peaks at 711.5 and 725.2 eV corresponding to Fe 2p3/2 and Fe 2p1/2 were believed to stem from the Fe iron in FeOOH [16,17,25,26]. The UV–vis DRS spectra were detected to characterize the optical absorption properties of the photocatalysts. As shown in Fig. 4, all of the samples exhibited strong absorbance at the wavelength shorter than 530 nm, and the absorption edge was similar. The absorption intensities of Ag3PO4/AgCl and Ag3PO4/AgCl/Fe(III) especially in the UV light region were higher than that of pure Ag3PO4, which might be due to the epitaxial growth of AgCl on the surface of Ag3PO4 [24,27]. The existence of metallic Ag might favor the visible light absorption due to its localized surface plasmon resonance (LSPR) effect [28,29]. The band gap energy of samples could be calculated by Tauc’s equation [30]: (αhv)1/n = B(hv – Eg), where α, h, ν, Eg and B represent the absorption coefficient, plank constant, light frequency, band gap energy and constant, respectively. The parameter n is determined by
Fig. 2. XRD patterns of different photocatalysts, and the inset is the corresponding enlarged view (30–38°).
ferric iron in FeOOH [16,21,22]. The peak at around 718.8 eV might be the satellite peak due to the presence of Fe(III) [22], and result from Ag 3s as Yu et al. reported [16]. Fig. 3e presented the XPS of O 1s spectrum. The peaks at 530.4 e V and 531.4 e V corresponded to the O 1s in Ag3PO4 [3,10]. The peaks at 529.8 e V and 532.8 e V were attributed to the Fe-O and eOH group, respectively [3,21,23]. FT-IR analysis of Ag3PO4/AgCl/Fe(III) was performed, and the results are shown in Fig. S2. It can be seen that a peak appeared at around 1010 cm−1, corresponding to the asymmetric stretching vibration of PO43−, and the peak at about 550 cm−1 was ascribed to the symmetric vibration and asymmetric bending vibration of P-O [4,23,24]. Additionally, the peaks at around 1380 and 1630 cm−1 were assigned to 3
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Fig. 3. XPS survey spectrum of Ag3PO4/AgCl/Fe(III) (a), high resolution spectra of Ag 3d (b), Cl 2p (c), Fe 2p (d), and O 1s (e).
shown in Fig. 5a, the photocatalytic activity increased firstly, and then decreased with the increasing amount of FeCl3. Excessively modifying FeCl3 would lead to the formation of more AgCl on the surface of Ag3PO4, and attenuate the visible light absorption. The corresponding first-order rate constants of different catalysts were shown in Fig. 5b. The sample with the mass ratio of 5 wt% exhibited the highest photocatalytic activity, and the degradation rate constant was about 0.302 min−1 which was 1.64 times that of the sample with the mass ratio of 2 wt% (0.184 min−1). Therefore, the mass ratio of 5 wt% was used in the following experiments. The effect of heating time on the photocatalytic performance of Ag3PO4/AgCl/Fe(III) was investigated by photocatalytically degrading TC. It could be seen from Fig. 6a that all the hybrid photocatalysts exhibited better photocatalytic performance for the degradation of TC than the pure Ag3PO4. Especially, when the heating time increased to
the kind of optical transition of semiconductors (the value of n is 1/2 and 2 for direct transition and indirect transition, respectively). Therefore, the value of n is selected as 1/2 here because of the direct band gap of Ag3PO4 [2]. Fig. 4b shows the Tauc’s plots of (αhv)2 with the function of the energy hv. The Eg values of Ag3PO4, Ag3PO4/AgCl and Ag3PO4/AgCl/Fe(III) were estimated as 2.42, 2.40 and 2.43 eV, respectively, indicating that there was no obvious change in Eg after modification.
3.2. Photocatalytic properties In order to explore the effect of the amount of FeCl3 modified on the photocatalytic performance of Ag3PO4, the photocatalytic degradation of TC under visible light irradiation was carried out. A 300 W Xenon lamp with a UV cut-off filter of 420 nm was used as the light source. As 4
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Fig. 4. UV–vis DRS spectra of different photocatalysts (a), and the corresponding Tauc’s plots (b).
Fig. 5. Photocatalytic performance of Ag3PO4/AgCl/Fe(III) (110 min) with different amount of FeCl3 (wt%) (a), and the corresponding degradation rate constants (b).
Fe(III)-cocatalyst used in the photocatalysts should be optimized in order to inhibit the recombination of photogenerated charge carriers, and improve the photocatalytic activity. However, excessive modification of co-catalyst would usually occupy more active sites, thus decreasing the photocatalytic performance. The stability of Ag3PO4/AgCl/Fe(III) for the degradation of TC was investigated through recycle experiments under visible light, and the results were shown in Fig. 7a. It could be seen that the photocatalytic performance reduced slightly after five runs, and the degradation efficiency decreased from 77% to 70%, indicating the excellent stability of
110 min, the photocatalyst Ag3PO4/AgCl/Fe(III) showed the highest photocatalytic activity with the rate constant of 0.458 min−1, which was 2.32 times and 2.94 times that of Ag3PO4/AgCl (0.197 min−1) and the pure Ag3PO4 (0.156 min−1), respectively (Fig. 6b). However, the corresponding TOC removal was only 5.1%, indicating that most of the target pollutant of TC could be photocatalytically decomposed in 8 min, but more time should be needed for complete mineralization. Further extending the heating time leaded to the decrease in the degradation of TC, which was in agreement with other cocatalysts doped catalysts like Fe(III)/AgBr [31] and Fe(III)/TiO2 [25]. Consequently, the amount of
Fig. 6. Effect of heating time on the photocatalytic performance of Ag3PO4/AgCl/Fe(III) (5%) (a), and the corresponding degradation rate constants (b). 5
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Fig. 7. Recycle experiments for Ag3PO4/AgCl/Fe(III) (a), the XRD patterns of the used composite (b), and the inset is the corresponding enlarged view (37.0–39.0°).
much higher than (about two times) that of the Ag3PO4 electrode under either Vis light or UV–Vis light. Generally, the photocurrent could be mainly affected by the separation and migration of photogenerated carriers. The more efficient the separation of photogenerated electrons and holes, the higher the photocurrent, which would usually give rise to a higher photocatalytic activity [3,4,6,20,32]. Therefore, the enhanced transient photocurrent response for the Ag3PO4/AgCl/Fe(III) electrode implied its better photocatalytic performance, which was consistent with the results of photocatalytic degradation experiments. The improved photoelectrochemical properties of Ag3PO4/AgCl/Fe(III) might be mainly attributed to the heterojunction structure of Ag3PO4/AgCl and the deposition of Fe(III). Efficient separation of photogenerated electrons and holes could be further testified by the electrochemical characterization of EIS. The Nyquist plots of Ag3PO4 and Ag3PO4/AgCl/Fe(III) under different irradiation conditions are presented in Fig. S3b. It is well known that the radius of the arc on Nyquist plots is associated with the charge transfer at the electrode interface. The smaller the arc radius, the smaller the charge transfer resistance, which means the better electron-hole separation and higher charge transfer efficiency [3,6,20,32]. As illustrated in Fig. S3b, both Ag3PO4 and Ag3PO4/AgCl/Fe(III) exhibited large arc radii in the dark, indicating their large interfacial electron transfer resistance. However, charge carriers could be effectively emitted for both electrodes even under the visible light irradiation to reduce the interfacial electron transfer resistance, thus resulting in their smaller arc radii. Interestingly, it can be also seen that the arc radius of the Ag3PO4/AgCl/Fe(III) electrode decreased more markedly than that of the Ag3PO4 electrode when the light was switched on. This phenomenon indicated that the photoelectrochemical properties of
the composite in the photocatalytic process. The crystalline structure of the used Ag3PO4/AgCl/Fe(III) was detected. It can be seen from Fig. 7b that, compared with the as-prepared Ag3PO4/AgCl/Fe(III) (Fig. 2), the used photocatalyst had no significant change in the patterns of Ag3PO4 and AgCl. However, a weak peak at around 38° corresponding to Ag0 appeared. This result demonstrated that a little photo-corrosion of Ag+ still occurred in this work, and thus the photocatalytic activity decreased slightly in the recycle experiments, as shown in Fig. 7a. Additionally, we have comparatively explored the photocatalytic performance of Ag3PO4, Ag3PO4/AgCl and Ag3PO4/AgCl/Fe(III) under visible light and solar irradiation. The conditions of sunlight: position (Longitude 125.324°, Latitude 43.886°); time (12:00–15:00); intensity (70 mW cm−2). As shown in Fig. 8, a significantly enhanced photocatalytic activity was observed for Ag3PO4/AgCl/Fe(III) whether it was under visible light or solar irradiation. 3.3. Electrochemical characterization The electrochemical characterizations (transient photocurrent response and electrochemical impedance spectroscopy (EIS)) were carried out to investigate the interfacial charge transfer and the recombination of photogenerated carriers. As shown in Fig. S3a, evident photocurrent responses were produced for both Ag3PO4 and Ag3PO4/ AgCl/Fe(III) electrodes upon light illumination no matter with a UV cut-off or not, and the current signals would quickly went back to the original level in the dark when the light was switched off. This demonstrated that both Ag3PO4 and Ag3PO4/AgCl/Fe(III) electrodes had good photoresponse even under the visible irradiation. Additionally, the responded photocurrent for the Ag3PO4/AgCl/Fe(III) electrode was
Fig. 8. Photocatalytic performance of different photocatalysts under visible light (a) and solar irradiation (b). 6
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− Ec + Eg/2 (XAg3PO4 = 5.96 eV, XAgCl = 6.07 eV, Ec = 4.5 eV). The band gap energies of Ag3PO4 and AgCl were 2.42 eV and 3.25 eV, respectively. Consequently, the CB potentials of Ag3PO4 and AgCl were estimated as 0.250 and −0.055 eV vs. SHE, respectively, and the VB potentials were 2.670 eV and 3.195 eV vs. SHE, respectively. The band energy structure of Ag3PO4/AgCl/Fe(III) was schematically described in Fig. 10. Owning to the photocorrosion of Ag3PO4, it was nearly inevitable to introduce Ag0 in Ag3PO4/AgCl/Fe(III) to form Ag0/Ag3PO4/ AgCl/Fe(III) in the photocatalytic processes, which could be proved by Figs. 3b and 7b in this work. Under the visible light irradiation, the LSPR-induced electrons on Ag0 could transfer to the CB of AgCl due to the Schottky barrier between Ag0 and AgCl [28,29,35], and then trapped by O2 to generate the active species of O2−% (O2/ O2−% = −0.046 eV vs. SHE) [32]. Meanwhile, the positive charge remained on Ag0 would recombine with the electrons photogenerated on the CB of Ag3PO4. In addition, due to the more positive potential of Fe3+/Fe2+ (0.771 V vs. SHE) than the CB of Ag3PO4 (0.250 V vs. SHE), the photogenerated electrons in the CB of Ag3PO4 could possibly transfer to the Fe(III) co-catalyst. This electron trapping by Fe(III) would result in the formation of Fe(II) and a rapid separation of the photogenerated charge carriers, leaving the holes in the VB of Ag3PO4 to oxidize the target molecule of TC more efficiently. Considering that Fe(II) was unstable under ambient conditions, it inclined to be oxidized by O2 to Fe(III) again accompanied by the reduction of O2 via multielectron transfer routes to H2O2 (O2/H2O2 = 0.68 eV, vs. SHE) or H2O (O2/H2O = 1.23 eV, vs. SHE) [26,31]. As a promising co-catalyst, Fe (III) might have a broad prospect in the application of photocatalysis because it was abundant, inexpensive, and easily available, etc.
Fig. 9. Photocatalytic degradation of TC over Ag3PO4/AgCl/Fe(III) by adding different scavengers.
4. Conclusions In conclusion, a ternary photocatalyst Ag3PO4/AgCl/Fe(III) was synthesized by a facile method. With the help of the heterostructure, the formation of metallic Ag and the deposition of Fe(III), the photoexcited electron and hole pairs in the composite Ag3PO4/AgCl/Fe(III) separated efficiently. Consequently, both the photocatalytic activity and the stability of this hybrid were greatly enhanced. This work provided an innovative viewpoint for constructing visible light-driven photocatalysts modified with Fe(III) cocatalyst.
Fig. 10. Photocatalytic mechanism over Ag3PO4/AgCl/Fe(III).
photocatalysts were determined not only by the emission of charge carriers, but also by the separation of photogenerated electrons and holes. In this work, with the help of the heterojunction structure and the deposition of Fe(III), the photoexcited electrons and holes in the Ag3PO4/AgCl/Fe(III) electrode separated more efficiently. This was well in agreement with the result of photocurrent response.
CRediT authorship contribution statement Ying Wang: Investigation, Writing - original draft. Hongbin Yu: Supervision, Writing - review & editing. Bin Zhao: Validation. Weichao Qin: Resources. Ying Lu: Methodology. Suiyi Zhu: Data curation. Mingxin Huo: Resources.
3.4. Photocatalytic mechanism Acknowledgments A series of trapping experiments were carried out to investigate the active species by adding different scavengers under visible light (Fig. 9). The scavengers were benzoquinone (BQ, 3 mM), Na2C2O4 (10 mM) and isopropanol (IPA, 10 mM), which were used as O2−%, h+ and %OH quenchers, respectively. With no scavenger added, the photocatalytic degradation of TC was 77% in 8 min, while it was 75% once IPA was added. This suggested that the active species of %OH had little impact on the photocatalytic process. By contrast, an obvious inhibition effect was obtained by adding BQ and Na2C2O4, implying that the active species of O2−% and h+ played crucial roles in the photocatalytic process over Ag3PO4/AgCl/Fe(III). The stability of Ag3PO4 could be improved by depositing AgCl due to their different solubility (0.0019 g L−1 (AgCl) < 0.020 g L−1 (Ag3PO4)). However, AgCl cannot be excited under visible light because of its wide band gap. So there should be an efficient way by which the photocatalytic activity of Ag3PO4/AgCl/Fe(III) was enhanced. The energy band structure of Ag3PO4/AgCl/Fe(III) was calculated according to the Tauc’s plots (Fig. 4) and the equation as follows [10,33,34]: Evb = X
This work was supported by the National Natural Science Foundation of China (Nos. 51878133, 51778117, 51578118). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2019.145067. References [1] H. Yu, Y. Xue, Y. Lu, X. Wang, S. Zhu, W. Qin, M. Huo, Novel application of a Zscheme photocatalyst of Ag3PO4@g-C3N4 for photocatalytic fuel cells, J. Environ. Manage. 254 (2020) 109738. [2] Z. Yi, J. Ye, N. Kikugawa, T. Kako, S. Ouyang, H. Stuart-Williams, H. Yang, J. Cao, W. Luo, Z. Li, Y. Liu, R.L. Withers, An orthophosphate semiconductor with photooxidation properties under visible-lightirradiation, Nat. Mater. 9 (2010) 559–564. [3] X. Miao, X. Yue, Z. Ji, X. Shen, H. Zhou, M. Liu, K. Xu, J. Zhu, G. Zhu, L. Kong, S.A. Shah, Nitrogen-doped carbon dots decorated on g-C3N4/Ag3PO4 photocatalyst
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Full Length Article
Enhanced visible light-driven photocatalytic performance and stability of Ag3PO4 by simultaneously loading AgCl and Fe(III)
T
⁎
Ying Wang, Hongbin Yu , Bin Zhao, Weichao Qin, Ying Lu, Suiyi Zhu, Mingxin Huo aEngineering Lab for Water Pollution Control and Resources Recovery, School of Environment, Northeast Normal University, Changchun 130117, China bScience and Technology Innovation Center for Municipal Wastewater Treatment and Water Quality Protection, Northeast Normal University, Changchun 130117, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Photocatalysis Ag3PO4/AgCl/Fe(III) Tetracycline Visible light
Silver phosphate (Ag3PO4) is widely used in the field of photocatalysis due to its excellent quantum efficiency and high photocatalytic activity. With Ag3PO4 as the precursor, we have prepared a ternary composite of Ag3PO4/AgCl/Fe(III) through an in-situ anion-exchange and impregnation method by using ferric chloride as the sources of chlorine and iron. The characterization results indicated that AgCl and Fe(III) were successfully loaded onto the surface of Ag3PO4 without any changes in its morphology and crystal structure. A significantly enhanced photocatalytic performance for the degradation of tetracycline (TC) was obtained under visible light irradiation (> 420 nm). The degradation rate of TC over Ag3PO4/AgCl/Fe(III) (0.458 min−1) was 2.32 times and 2.94 times that of Ag3PO4/AgCl (0.197 min−1) and the pure Ag3PO4 (0.156 min−1), respectively. Furthermore, the stability of the photocatalyst was also improved. On one hand, the presence of AgCl could prevent Ag3PO4 from dissolution due to the smaller solubility of AgCl. On the other hand, the heterostructure and Fe(III) in Ag3PO4/AgCl/Fe(III) could promote the transfer and the separation of photoexcited electrons and holes, thus enhancing the photocatalytic activity. Our investigation might give some new insights for the facile preparation of photocatalysts with high visible light-driven photocatalytic performance.
1. Introduction
catalysts (e.g. Pt [7], Au [8], Ag [9]) were usually used as electron transfer mediators or charge recombination centers in all-solid-state catalysts. Addionally, modification with small amount of AgX (X = Cl, Br, I) could improve the photocatalytic properties and recyclability owing to the decreased solubility (solubility: Ag3PO4: 0.02 g L−1, Ksp = 1.4 × 10−16; AgCl: 0.0019 g L−1, Ksp = 1.8 × 10−10; AgBr: 0.00014 g L−1 L, Ksp = 5 × 10−13; AgI: 0.00003 g L−1, Ksp = 8.3 × 10−17; 25 °C) and the proper energy band structure [10]. Moreover, the localized surface plasmon resonance (LSPR) of Ag could promote the absorption capability of composites for visible light. Recently, in many investigations, grafting with metal ions or metal compounds as co-catalysts, e.g., Cu(II) [11,12], Cr(III) [13], Ce(III) [14] and PdS [15], was believed as an effective method to enhance the photocatalytic activity of composites. Especially, the amorphous Fe(III) (low-cost and abundant on earth) was proved to have excellent capability to capture photoelectrons to improve the photocatalytic activity and the stability of photocatalysts [16,17]. Herein, we have investigated a facile method to synthesize a ternary photocatalyst of Ag3PO4/AgCl/Fe(III). It was found that the hybrid photocatalyst simultaneously exhibited a better recyclability and a
Semiconductor photocatalysis over Ag3PO4 has caught great attention since 2010 due to its efficient utilization of solar energy, and high quantum efficiency of 90% in photooxidative capability for water splitting and excellent performance for the decomposition of organic compounds [1,2]. Nevertheless, the silver ions (Ag+) in Ag3PO4 could easily trap the photogenerated electrons to form Ag0 when there are no scavengers added under visible light irradiation [2]. This phenomenon leads to the unstable structure and decreased photocatalytic performance. In addition, the slightly soluble property of Ag3PO4 (Ksp = 1.4 × 10−16, 0.0200 g L−1, 25 °C) in aqueous solution results in the limitation of application on the degradation of organic pollutant. The heterostructure in composites plays an important role in the photocatalytic degradation process. The hybrid photocatalysts (e.g. NCDs/g-C3N4/Ag3PO4 [3], ANP@MQD/FL-MN [4], TiO2TNTs/Ag3PO4 [5], ZnO/Fe2O3/g-C3N4 [6]) would normally exhibit an enhanced photocatalytic activity. This was because the photogenerated electronhole pairs could be effectively separated with the help of the heterostructure in photocatalysts. In these hybrid composites, noble metal co-
⁎ Corresponding author at: Engineering Lab for Water Pollution Control and Resources Recovery, School of Environment, Northeast Normal University, Changchun 130117, China. E-mail address: [email protected] (H. Yu).
https://doi.org/10.1016/j.apsusc.2019.145067 Received 5 October 2019; Received in revised form 5 December 2019; Accepted 12 December 2019 Available online 16 December 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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and the surface was relatively rough. It could be seen from Fig. 1b that, as compared with the pure Ag3PO4, the modification of FeCl3 gave rise to no obvious changes in mophologies. This was mainly because the amount of FeCl3 modified on the surface of Ag3PO4 was rare. The EDS analysis was performed to identify the components of the modified sample Ag3PO4/AgCl/Fe(III), and the results were demonstrated in Fig. 1c. It was found that, besides the elements of Ag, P and O corresponding to Ag3PO4, the elements Cl and Fe was also detected, suggesting the existence of AgCl and the component containing Fe. To further identify the distribution of different elements, the element mapping of Ag3PO4/AgCl/Fe(III) (5 wt%-110 min) was recorded. As displayed in Fig. 1d, the elements of Cl and Fe distributed evenly, and was similar to the the elements of Ag, P, and O, indicating that the elements of Cl and Fe should be modified mainly on the surface of Ag3PO4. The TEM characterization was conducted. The TEM and HR-TEM images of Ag3PO4/AgCl/Fe(III) are shown in Fig. S1a and b. As illustrated, clear lattice fringe information of the cubic phase Ag3PO4 could be seen and the interplanar spacing of 0.247 nm corresponded to the (2 2 0) crystal plane of Ag3PO4. In addition, the other indistinct lattice fringe of 0.196 nm could be assigned to the (2 1 1) crystal plane of AgCl. The SAED patterns (Fig. S1c) display the polycrystalline structure of the composite. Besides the distinct signal of Ag3PO4, a faint diffraction ring signal of AgCl corresponding to the (2 1 1) crystal plane was observed, which was in accordance with the result of HR-TEM. However, neither the obvious lattice fringe in HR-TEM nor diffraction ring signals in SAED patterns were detected for the Fe components, possibly due to a very limited amount or the amorphous property of the Fe components as result of the low-temperature of synthesis. The crystal structures of the Ag-based photocatalysts before and after AgCl and Fe(III) modification were revealed by the XRD (Fig. 2). It can be seen that most diffraction peaks for Ag3PO4, Ag3PO4/AgCl and Ag3PO4/AgCl/Fe(III) were attributed to the cubic phase of Ag3PO4 (JCPDS No. 06-0505). Moreover, all the samples showed similar diffraction peak intensity and full width at half-maximum, proving that the phase structure of Ag3PO4 was well maintained. The lattice constants of Ag3PO4 and Ag3PO4/AgCl/Fe(III) were calculated as 0.6008 nm and 0.6011 nm, respectively, which were almost the same as the standard value of 0.6013 nm listed in the JCPDS card (No. 060505). This indicated that the modification of AgCl and Fe(III) did not change the crystal structure of Ag3PO4, and thus Fe ions most likely deposited on the surface but not doped in the crystal lattice of catalysts. Considering quite little AgCl on the Ag3PO4 surface, an enlarged image from 30° to 38° was presented in the inset of Fig. 2 to demonstrate the diffraction peak of AgCl. It was found that both Ag3PO4/AgCl and Ag3PO4/AgCl/Fe(III) showed a diffraction peak at ca. 32.3° (〈2 0 0〉 ) with very weak intensity, which could be ascribed to the low concentration and relatively weak crystallinity of AgCl (JCPDS No. 311238) in the hybrids. There was no diffraction peaks of Fe(III)-containing component in the XRD patterns, to some extent, indicating that the Fe(III)-containing component was either fairly little or in an amorphous phase. Nevertheless, the existence of Fe(III) could be confirmed in the following XPS analysis. Chemical states of the surface elements of Ag3PO4/AgCl/Fe(III) have been investigated by the XPS analysis. As shown in Fig. 3, six elements including C, Ag, P, O, Cl, and Fe were observed in the survey spectrum (Fig. 3a). The peak at 284.6 eV was assigned to the residual carbon as a reference to determine the binding energy of other elements. In the case of Ag 3d spectra (Fig. 3b), the peaks at 367.7 eV and 373.7 eV corresponded to Ag 3d5/2 and Ag 3d3/2 of Ag+, while the weak peaks at 368.5 eV and 374.5 eV were ascribed to Ag 3d5/2 and Ag 3d3/2 of Ag0 [18,19]. These results verified the existence of metallic Ag0 in the Ag3PO4/AgCl/Fe(III) particles. In the high resolution spectrum of Cl 2p (Fig. 3c), the binding energy at 198.2 eV was ascribed to Cl 2p3/2 [20]. Fig. 3d shows the spectrum of Fe 2p, broad peaks at 711.5 eV and 725.2 eV corresponding to Fe 2p3/2 and Fe 2p1/2 proved the existence of
higher photocatalytic activity due to the presence of the heterostructure and Fe(III), which changed the pathway of photoelectrons and decreased the recombination of charge carriers, thus relieving the photocorrosion and improving the photocatalytic performance. 2. Experimental section 2.1. Preparation of photocatalysts The photocatalyst Ag3PO4 was synthesized through a precipitation method. AgNO3 (1.700 g) was dissolved into 200 mL deionized water under stirring for 20 min, and then appropriate amount of 0.005 M Na2HPO4 was added drop by drop into the AgNO3 aqueous solution under stirring for 12 h. Finally, the precipitation was collected by centrifugation, washed with deionized water, and dried at 60 °C in a vacuum oven. The ternary composite of Ag3PO4/AgCl/Fe(III) was synthesized by a facile method. That is, Ag3PO4 was used as the template and the silver ion source, while FeCl3 was added to provide chloride ion and Fe(III). Briefly, Ag3PO4 powers (0.200 g) were dispersed into 200 mL deionized water under stirring for 30 min. Different amount of freshly prepared FeCl3 solutions (0.47 mM), i.e. 2 wt%, 4 wt%, 5 wt%, and 6 wt%, were added dropwise into the suspension and then the suspension was transferred into a water bath at 55 °C for a period of time, i.e. 90, 110, and 130 min. The ternary material of Ag3PO4/AgCl/Fe(III) (X wt%-T min) was obtained by centrifugation, washed with deionized water, and dried at 60 °C in a vacuum oven. The term Ag3PO4/AgCl/Fe(III) stood for Ag3PO4/AgCl/Fe(III) (5%-110 min) throughout the paper unless specified. 2.2. Photocatalytic experiments The photocatalytic performance has been evaluated by the photodegradation of tetracycline hydrochloride (TC, 20 mg L−1) containing 0.50 g L−1 photocatalysts. Before irradiation, the mixed solutions were stirred for 30 min in dark to achieve an adsorption-desorption equilibrium of TC. A Xenon lamp (300 W) with a UV cut-off filter of 420 nm was used as the visible light source (170 mW cm−2). A certain volume of suspension was taken out at given time intervals for analysis at 358 nm wavelength on a UV–vis spectrophotometer. 2.3. Characterization The morphologies of photocatalysts were characterized by a scanning electron microscope (SEM, JSM-7500F, Japan) equipped with energy-dispersive X-ray (EDS). The crystal structure, element, optical absorption property were analyzed by powder X-ray diffractometer (XRD) with Cu Kα irradiation (D/maxZ200PC, Rigaku, Japan), X-ray photoelectron spectroscopy (XPS) with Al Kα radiation (ESCALAB MK II, VG Scientific, England) and UV–vis diffuse reflectance spectrophotometer (DRS, Varian, Cary 500, America). The electrochemical characterizations (transient photocurrent response and electrochemical impedance spectroscopy (EIS)) were carried out on an electrochemical working station (CHI-760E, China) in a three-electrode system with Na2SO4 solution (0.1 M) as the electrolyte. The prepared catalyst, a Pt foil and a saturated calomel electrode (SCE) were used as the working electrode, counter electrode, and reference electrode, respectively. 3. Results and discussion 3.1. Characterization Fig. 1 shows the typical SEM images of samples and the EDS information of Ag3PO4/AgCl/Fe(III) (5 wt%-110 min). Fig. 1a indicated that the average size of pure Ag3PO4 was estimated to be 200–350 nm 2
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Fig. 1. SEM of the pure Ag3PO4 (a), and the SEM image (b), EDS (c) and the mapping analysis (d) of Ag3PO4/AgCl/Fe(III).
the stretching vibration and deformation bands of eOH, which might be the eOH groups combined with Ag3PO4 [23] or the component of FeOOH [21]. Based on the discussions above, it was believed that Fe ions might deposit on the surface of catalysts most likely as amorphous FeOOH but not Fe2O3 due to the low temperature of synthesis. As reported in the literature [16,17,25,26], the modification of Fe ions by an impregnation technique at a relatively low temperature would usually result in the formation of FeOOH. In this work, the temperatures of preparation (55 °C) and drying process (60 °C) were not high. H.G. Yu et al. investigated the structural features of the grafted Fe ions by X-ray absorption fine structure analysis [25], and found that the structural parameters of the photocatalyst more closely resembled the parameters of FeOOH than those of Fe2O3. Additionally, the XPS results obtained here were similar to that reported in the literature, and the peaks at 711.5 and 725.2 eV corresponding to Fe 2p3/2 and Fe 2p1/2 were believed to stem from the Fe iron in FeOOH [16,17,25,26]. The UV–vis DRS spectra were detected to characterize the optical absorption properties of the photocatalysts. As shown in Fig. 4, all of the samples exhibited strong absorbance at the wavelength shorter than 530 nm, and the absorption edge was similar. The absorption intensities of Ag3PO4/AgCl and Ag3PO4/AgCl/Fe(III) especially in the UV light region were higher than that of pure Ag3PO4, which might be due to the epitaxial growth of AgCl on the surface of Ag3PO4 [24,27]. The existence of metallic Ag might favor the visible light absorption due to its localized surface plasmon resonance (LSPR) effect [28,29]. The band gap energy of samples could be calculated by Tauc’s equation [30]: (αhv)1/n = B(hv – Eg), where α, h, ν, Eg and B represent the absorption coefficient, plank constant, light frequency, band gap energy and constant, respectively. The parameter n is determined by
Fig. 2. XRD patterns of different photocatalysts, and the inset is the corresponding enlarged view (30–38°).
ferric iron in FeOOH [16,21,22]. The peak at around 718.8 eV might be the satellite peak due to the presence of Fe(III) [22], and result from Ag 3s as Yu et al. reported [16]. Fig. 3e presented the XPS of O 1s spectrum. The peaks at 530.4 e V and 531.4 e V corresponded to the O 1s in Ag3PO4 [3,10]. The peaks at 529.8 e V and 532.8 e V were attributed to the Fe-O and eOH group, respectively [3,21,23]. FT-IR analysis of Ag3PO4/AgCl/Fe(III) was performed, and the results are shown in Fig. S2. It can be seen that a peak appeared at around 1010 cm−1, corresponding to the asymmetric stretching vibration of PO43−, and the peak at about 550 cm−1 was ascribed to the symmetric vibration and asymmetric bending vibration of P-O [4,23,24]. Additionally, the peaks at around 1380 and 1630 cm−1 were assigned to 3
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Fig. 3. XPS survey spectrum of Ag3PO4/AgCl/Fe(III) (a), high resolution spectra of Ag 3d (b), Cl 2p (c), Fe 2p (d), and O 1s (e).
shown in Fig. 5a, the photocatalytic activity increased firstly, and then decreased with the increasing amount of FeCl3. Excessively modifying FeCl3 would lead to the formation of more AgCl on the surface of Ag3PO4, and attenuate the visible light absorption. The corresponding first-order rate constants of different catalysts were shown in Fig. 5b. The sample with the mass ratio of 5 wt% exhibited the highest photocatalytic activity, and the degradation rate constant was about 0.302 min−1 which was 1.64 times that of the sample with the mass ratio of 2 wt% (0.184 min−1). Therefore, the mass ratio of 5 wt% was used in the following experiments. The effect of heating time on the photocatalytic performance of Ag3PO4/AgCl/Fe(III) was investigated by photocatalytically degrading TC. It could be seen from Fig. 6a that all the hybrid photocatalysts exhibited better photocatalytic performance for the degradation of TC than the pure Ag3PO4. Especially, when the heating time increased to
the kind of optical transition of semiconductors (the value of n is 1/2 and 2 for direct transition and indirect transition, respectively). Therefore, the value of n is selected as 1/2 here because of the direct band gap of Ag3PO4 [2]. Fig. 4b shows the Tauc’s plots of (αhv)2 with the function of the energy hv. The Eg values of Ag3PO4, Ag3PO4/AgCl and Ag3PO4/AgCl/Fe(III) were estimated as 2.42, 2.40 and 2.43 eV, respectively, indicating that there was no obvious change in Eg after modification.
3.2. Photocatalytic properties In order to explore the effect of the amount of FeCl3 modified on the photocatalytic performance of Ag3PO4, the photocatalytic degradation of TC under visible light irradiation was carried out. A 300 W Xenon lamp with a UV cut-off filter of 420 nm was used as the light source. As 4
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Fig. 4. UV–vis DRS spectra of different photocatalysts (a), and the corresponding Tauc’s plots (b).
Fig. 5. Photocatalytic performance of Ag3PO4/AgCl/Fe(III) (110 min) with different amount of FeCl3 (wt%) (a), and the corresponding degradation rate constants (b).
Fe(III)-cocatalyst used in the photocatalysts should be optimized in order to inhibit the recombination of photogenerated charge carriers, and improve the photocatalytic activity. However, excessive modification of co-catalyst would usually occupy more active sites, thus decreasing the photocatalytic performance. The stability of Ag3PO4/AgCl/Fe(III) for the degradation of TC was investigated through recycle experiments under visible light, and the results were shown in Fig. 7a. It could be seen that the photocatalytic performance reduced slightly after five runs, and the degradation efficiency decreased from 77% to 70%, indicating the excellent stability of
110 min, the photocatalyst Ag3PO4/AgCl/Fe(III) showed the highest photocatalytic activity with the rate constant of 0.458 min−1, which was 2.32 times and 2.94 times that of Ag3PO4/AgCl (0.197 min−1) and the pure Ag3PO4 (0.156 min−1), respectively (Fig. 6b). However, the corresponding TOC removal was only 5.1%, indicating that most of the target pollutant of TC could be photocatalytically decomposed in 8 min, but more time should be needed for complete mineralization. Further extending the heating time leaded to the decrease in the degradation of TC, which was in agreement with other cocatalysts doped catalysts like Fe(III)/AgBr [31] and Fe(III)/TiO2 [25]. Consequently, the amount of
Fig. 6. Effect of heating time on the photocatalytic performance of Ag3PO4/AgCl/Fe(III) (5%) (a), and the corresponding degradation rate constants (b). 5
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Fig. 7. Recycle experiments for Ag3PO4/AgCl/Fe(III) (a), the XRD patterns of the used composite (b), and the inset is the corresponding enlarged view (37.0–39.0°).
much higher than (about two times) that of the Ag3PO4 electrode under either Vis light or UV–Vis light. Generally, the photocurrent could be mainly affected by the separation and migration of photogenerated carriers. The more efficient the separation of photogenerated electrons and holes, the higher the photocurrent, which would usually give rise to a higher photocatalytic activity [3,4,6,20,32]. Therefore, the enhanced transient photocurrent response for the Ag3PO4/AgCl/Fe(III) electrode implied its better photocatalytic performance, which was consistent with the results of photocatalytic degradation experiments. The improved photoelectrochemical properties of Ag3PO4/AgCl/Fe(III) might be mainly attributed to the heterojunction structure of Ag3PO4/AgCl and the deposition of Fe(III). Efficient separation of photogenerated electrons and holes could be further testified by the electrochemical characterization of EIS. The Nyquist plots of Ag3PO4 and Ag3PO4/AgCl/Fe(III) under different irradiation conditions are presented in Fig. S3b. It is well known that the radius of the arc on Nyquist plots is associated with the charge transfer at the electrode interface. The smaller the arc radius, the smaller the charge transfer resistance, which means the better electron-hole separation and higher charge transfer efficiency [3,6,20,32]. As illustrated in Fig. S3b, both Ag3PO4 and Ag3PO4/AgCl/Fe(III) exhibited large arc radii in the dark, indicating their large interfacial electron transfer resistance. However, charge carriers could be effectively emitted for both electrodes even under the visible light irradiation to reduce the interfacial electron transfer resistance, thus resulting in their smaller arc radii. Interestingly, it can be also seen that the arc radius of the Ag3PO4/AgCl/Fe(III) electrode decreased more markedly than that of the Ag3PO4 electrode when the light was switched on. This phenomenon indicated that the photoelectrochemical properties of
the composite in the photocatalytic process. The crystalline structure of the used Ag3PO4/AgCl/Fe(III) was detected. It can be seen from Fig. 7b that, compared with the as-prepared Ag3PO4/AgCl/Fe(III) (Fig. 2), the used photocatalyst had no significant change in the patterns of Ag3PO4 and AgCl. However, a weak peak at around 38° corresponding to Ag0 appeared. This result demonstrated that a little photo-corrosion of Ag+ still occurred in this work, and thus the photocatalytic activity decreased slightly in the recycle experiments, as shown in Fig. 7a. Additionally, we have comparatively explored the photocatalytic performance of Ag3PO4, Ag3PO4/AgCl and Ag3PO4/AgCl/Fe(III) under visible light and solar irradiation. The conditions of sunlight: position (Longitude 125.324°, Latitude 43.886°); time (12:00–15:00); intensity (70 mW cm−2). As shown in Fig. 8, a significantly enhanced photocatalytic activity was observed for Ag3PO4/AgCl/Fe(III) whether it was under visible light or solar irradiation. 3.3. Electrochemical characterization The electrochemical characterizations (transient photocurrent response and electrochemical impedance spectroscopy (EIS)) were carried out to investigate the interfacial charge transfer and the recombination of photogenerated carriers. As shown in Fig. S3a, evident photocurrent responses were produced for both Ag3PO4 and Ag3PO4/ AgCl/Fe(III) electrodes upon light illumination no matter with a UV cut-off or not, and the current signals would quickly went back to the original level in the dark when the light was switched off. This demonstrated that both Ag3PO4 and Ag3PO4/AgCl/Fe(III) electrodes had good photoresponse even under the visible irradiation. Additionally, the responded photocurrent for the Ag3PO4/AgCl/Fe(III) electrode was
Fig. 8. Photocatalytic performance of different photocatalysts under visible light (a) and solar irradiation (b). 6
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− Ec + Eg/2 (XAg3PO4 = 5.96 eV, XAgCl = 6.07 eV, Ec = 4.5 eV). The band gap energies of Ag3PO4 and AgCl were 2.42 eV and 3.25 eV, respectively. Consequently, the CB potentials of Ag3PO4 and AgCl were estimated as 0.250 and −0.055 eV vs. SHE, respectively, and the VB potentials were 2.670 eV and 3.195 eV vs. SHE, respectively. The band energy structure of Ag3PO4/AgCl/Fe(III) was schematically described in Fig. 10. Owning to the photocorrosion of Ag3PO4, it was nearly inevitable to introduce Ag0 in Ag3PO4/AgCl/Fe(III) to form Ag0/Ag3PO4/ AgCl/Fe(III) in the photocatalytic processes, which could be proved by Figs. 3b and 7b in this work. Under the visible light irradiation, the LSPR-induced electrons on Ag0 could transfer to the CB of AgCl due to the Schottky barrier between Ag0 and AgCl [28,29,35], and then trapped by O2 to generate the active species of O2−% (O2/ O2−% = −0.046 eV vs. SHE) [32]. Meanwhile, the positive charge remained on Ag0 would recombine with the electrons photogenerated on the CB of Ag3PO4. In addition, due to the more positive potential of Fe3+/Fe2+ (0.771 V vs. SHE) than the CB of Ag3PO4 (0.250 V vs. SHE), the photogenerated electrons in the CB of Ag3PO4 could possibly transfer to the Fe(III) co-catalyst. This electron trapping by Fe(III) would result in the formation of Fe(II) and a rapid separation of the photogenerated charge carriers, leaving the holes in the VB of Ag3PO4 to oxidize the target molecule of TC more efficiently. Considering that Fe(II) was unstable under ambient conditions, it inclined to be oxidized by O2 to Fe(III) again accompanied by the reduction of O2 via multielectron transfer routes to H2O2 (O2/H2O2 = 0.68 eV, vs. SHE) or H2O (O2/H2O = 1.23 eV, vs. SHE) [26,31]. As a promising co-catalyst, Fe (III) might have a broad prospect in the application of photocatalysis because it was abundant, inexpensive, and easily available, etc.
Fig. 9. Photocatalytic degradation of TC over Ag3PO4/AgCl/Fe(III) by adding different scavengers.
4. Conclusions In conclusion, a ternary photocatalyst Ag3PO4/AgCl/Fe(III) was synthesized by a facile method. With the help of the heterostructure, the formation of metallic Ag and the deposition of Fe(III), the photoexcited electron and hole pairs in the composite Ag3PO4/AgCl/Fe(III) separated efficiently. Consequently, both the photocatalytic activity and the stability of this hybrid were greatly enhanced. This work provided an innovative viewpoint for constructing visible light-driven photocatalysts modified with Fe(III) cocatalyst.
Fig. 10. Photocatalytic mechanism over Ag3PO4/AgCl/Fe(III).
photocatalysts were determined not only by the emission of charge carriers, but also by the separation of photogenerated electrons and holes. In this work, with the help of the heterojunction structure and the deposition of Fe(III), the photoexcited electrons and holes in the Ag3PO4/AgCl/Fe(III) electrode separated more efficiently. This was well in agreement with the result of photocurrent response.
CRediT authorship contribution statement Ying Wang: Investigation, Writing - original draft. Hongbin Yu: Supervision, Writing - review & editing. Bin Zhao: Validation. Weichao Qin: Resources. Ying Lu: Methodology. Suiyi Zhu: Data curation. Mingxin Huo: Resources.
3.4. Photocatalytic mechanism Acknowledgments A series of trapping experiments were carried out to investigate the active species by adding different scavengers under visible light (Fig. 9). The scavengers were benzoquinone (BQ, 3 mM), Na2C2O4 (10 mM) and isopropanol (IPA, 10 mM), which were used as O2−%, h+ and %OH quenchers, respectively. With no scavenger added, the photocatalytic degradation of TC was 77% in 8 min, while it was 75% once IPA was added. This suggested that the active species of %OH had little impact on the photocatalytic process. By contrast, an obvious inhibition effect was obtained by adding BQ and Na2C2O4, implying that the active species of O2−% and h+ played crucial roles in the photocatalytic process over Ag3PO4/AgCl/Fe(III). The stability of Ag3PO4 could be improved by depositing AgCl due to their different solubility (0.0019 g L−1 (AgCl) < 0.020 g L−1 (Ag3PO4)). However, AgCl cannot be excited under visible light because of its wide band gap. So there should be an efficient way by which the photocatalytic activity of Ag3PO4/AgCl/Fe(III) was enhanced. The energy band structure of Ag3PO4/AgCl/Fe(III) was calculated according to the Tauc’s plots (Fig. 4) and the equation as follows [10,33,34]: Evb = X
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