Improved photocatalytic degradation of organic dye ... - Springer Link

Front. Mater. Sci. 2017, 11(4): 366–374 https://doi.org/10.1007/s11706-017-0404-x

RESEARCH ARTICLE

Improved photocatalytic degradation of organic dye using Ag3PO4/MoS2 nanocomposite Madhulika SHARMA1, Pranab Kishore MOHAPATRA2, and Dhirendra BAHADUR (✉)1 1 Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology, Bombay, Powai, Mumbai 400076, India 2 Department of Physics, Indian Institute of Technology, Bombay, Powai Mumbai 400076, India

© Higher Education Press and Springer-Verlag GmbH Germany 2017

ABSTRACT: Highly efficient Ag3PO4/MoS2 nanocomposite photocatalyst was synthesized using a wet chemical route with a low weight percentage of highly exfoliated MoS2 (0.1 wt.%) and monodispersed Ag3PO4 nanoparticles (~5.4 nm). The structural and optical properties of the nanocomposite were studied using various characterization techniques, such as XRD, TEM, Raman and absorption spectroscopy. The composite exhibits markedly enhanced photocatalytic activity with a low lamp power (60 W). Using this composite, a high kinetic rate constant (k) value of 0.244 min-1 was found. It was observed that ~97.6% of dye degrade over the surface of nanocomposite catalyst within 15 min of illumination. The improved photocatalytic activity of Ag3PO4/MoS2 nanocomposite is attributed to the efficient interfacial charge separation, which was supported by the PL results. Large surface area of MoS2 nanosheets incorporated with well dispersed Ag3PO4 nanoparticles further increases charge separation, contributing to enhanced degradation efficiency. A possible mechanism for charge separation is also discussed. KEYWORDS: Ag3PO4/MoS2 nanocomposite; methylene blue; degradation efficiency; photocatalysis

Contents 1 Introduction 2 Experimental 2.1 Synthesis procedure 2.2 Characterization techniques 3 Results and discussion 3.1 XRD, HRTEM and SEM 3.2 Raman spectroscopy 3.3 UV-Vis spectroscopy 3.4 Photocatalytic activity of decomposition of the MB dye 4 Conclusions Received September 6, 2017; accepted October 7, 2017 E-mail: [email protected]

Acknowledgements References

1

Introduction

Development and exploration of stable and efficient visible light active photocatalysts for degradation of organic contaminants is one of the significant strategies for solving environmental problems [1–3]. A number of semiconductor photocatalysts including AgX (X = Cl and Br), BiVO4 and ZnO have been explored for pollutant degradation applications [3–5]. Recently, silver orthophosphate (Ag3PO4) has emerged as a promising candidate for the degradation of organic contaminants [6–9]. The photocatalytic activity of Ag3PO4 is remarkably higher than those of other previously reported visible light driven

Madhulika SHARMA et al. Improved photocatalytic degradation of organic dye using Ag3PO4/MoS2 nanocomposite

semiconductor photocatalysts, with high (90%) quantum efficiency at longer wavelengths ( > 420 nm) [6]. However, one disadvantage of Ag3PO4 is its poor chemical stability, which restricts the photocatalytic efficiency [10]. Therefore, effective measures are essential to improve the photocatalytic performance of Ag3PO4. Designing heterogeneous photocatalysts is an effective approach for improving catalytic performance due to efficient charge separation in such systems. Recently, several reports demonstrate that the photocatalytic efficiency improves when Ag3PO4 is coupled with other semiconductors [11– 13]. Such heterostructures not only increase stability but also enhance photocatalytic activity of Ag3PO4 by inhibiting electron hole recombination and thereby improving effective charge separation. Thus, fabrication of novel composite photocatalysts with enhanced photocatalytic performance necessitates an extensive study. Among such complexes, two-dimensional (2D) materials based heterogeneous photocatalysts are proven to be a potential candidate [14]. In such systems, the large surface area could effectively transfer photogenerated carriers and thus enhance the photocatalytic activity. Moreover, 2D materials have been proven to be effective in preventing the agglomeration and controlling the size of particles in the composite [15–16]. Layered 2D MoS2 has shown great potential as a co-catalyst, promoting oxygen activation for generation of superoxide radical in the photocatalytic process [17–19]. Ag3PO4/MoS2 based nanocomposite shows high photo-

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catalytic efficiency [20–29]. However, most of the literatures discuss photocatalytic activity using a high power visible light source (200–800 W) [20–21,24–29] and larger sized Ag3PO4 nanoparticles in the composite along with a high degree of agglomeration [20–25,27–29]. Also, in many cases, MoS2 nanosheets present in the composite are found to be much thicker [21–23,25,27,29]. This could effectively reduce the surface area available for the dye degradation, and thus can decrease the efficiency. Herein, we report a facile wet chemical synthesis route to fabricate Ag3PO4/MoS2 nanocomposite for photocatalytic degradation of an organic dye, methylene blue (MB). It has been demonstrated that the synthesized Ag3PO4/MoS2 nanocomposite shows enhanced photocatalytic activity under a low power visible light source (60 W), using a low weight percent (0.1) of MoS2, and a monodispersed Ag3PO4 nanoparticles which is the highlight of our present work. The detail comparison (our work and reported literature) of various parameters affecting the efficiency of dye degradation is represented in Table 1. The kinetic rate constant for the nanocomposite is calculated to be 0.244 min–1, which is found to be larger than most of the reported literature [20–25,27]. The increase in photocatalytic activity is achieved by incorporating smaller sized (~5.4 nm) monodispersed Ag3PO4 nanoparticles in highly exfoliated MoS2 nanosheets. This can be attributed to the efficient charge separation of the photogenerated electrons and holes at the Ag3PO4/MoS2 interface under the irradiation of visible light.

Table 1 Comparison of the photodegradation performance of Ag3PO4/MoS2 based composites δAg3PO4 /nm

Ref.

Photocatalyst

MoS2 content

XL/XW

k /min–1

P/W

Cs /mg

C0 /(mg$L–1)

td /min

This work

Ag3PO4/MoS2

0.1 wt.%

–

0.244

60a)

20

20

15

5.4

[20]

3D Ag3PO4/MoS2

15 wt.%

150

0.133

300b)

25

10

30

30–40

[21]

Ag3PO4@MoS2

3.5 wt.%

35

0.139

800b)

250

2.5

16

–

[22]

Ag3PO4/MoS2

3 wt.%

30

0.142

–

50

10

30

15000

[23]

Ag3PO4/MoS2

0.648 wt.%

6.5

0.065

35c)

30

20

60

10–20

[24]

Ag3PO4/MoS2

0.5 wt.%

5

0.124

300b)

100

20

8

200–300

[25]

MoS2/Ag3PO4

0.1 wt.%

1

0.095

300b)

75

10

12

30–50

[26]

Ag3PO4@MoS2 QD/FL MoS2 nanosheet

6 mL MoS2

–

0.269

300b)

–

10

16

10.7

[27]

Fe3O4@MoS2/Ag3PO4

0.1 g (Fe3O4 MoS2)

–

0.192

500b)

0.2e)

20

10

30–50

[28]

Ag3PO4–0.02(MoS2/ 0.005 graphene)

2 wt.% [0.02(MoS2/ 0.005GR)]

–

–

500b)

20

20

60

500

[29]

MoS2/Ag3PO4

1:40 (MoS2:Ag3PO4)

–

–

150d)

100

20

16

250

c)

Notes: XL = the weight percentage value of MoS2 given in literature; XW = the weight percentage value of MoS2 calculated in our work; k = kinetic rate constant value of Ag3PO4/MoS2 nanocomposite; P = lamp power; Cs = sample concentration used to degrade dye; C0 = dye concentration; td = time taken for dye degradation; δAg3PO4 = size of Ag3PO4 nanoparticles. a) CFL; b) xenon; c) solar simulator; d) tungsten halogen; e) unit: mg/mL.

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2

Experimental

2.1

Synthesis procedure

Ag3PO4 nanoparticles were synthesized using a wet chemical route as reported by Thiyagarajan et al. [9]. Silver nitrate (AgNO3) and disodium hydrogen phosphate were used as precursors. The aqueous solution of AgNO3 (50 mg) was prepared under vigorous stirring in a magnetic stirrer. To this mixture, 20 mL solution of disodium hydrogen phosphate (72 mg) was added drop wise and kept for 3 h stirring followed by sonication at room temperature. The precipitate obtained was centrifuged and dried overnight at 60°C in a vacuum oven. The yellow color powder obtained was collected for further characterizations. A three-step modified Li intercalated exfoliation technique is followed to obtain layered MoS2 nanosheets [30– 31]. The first step involves the expansion of MoS2 where, 1.6 g of MoS2 powder was taken along with 20 mL of hydrazine hydrate in an autoclave and heated at 130°C for 48 h. The expanded MoS2 was centrifuged three times using deionized water at 10000 r/min for 10 min and dried at 120°C for 10 h. In the next step, lithium was intercalated in between layers of pre-expanded MoS2. Intercalation was done in an argon atmosphere using n-butyl lithium under constant stirring for 48 h. Subsequently, the sample was vacuum filtered using hexane to obtain the lithium intercalated dry MoS2 powder. The third and final step involves the exfoliation of MoS2 sample. The intercalated sample was sonicated for 1 h followed by centrifugation at 8000 r/min to remove excess lithium. Further centrifugation was performed at 1000 r/min to separate the unexfoliated flakes. The obtained supernatant was collected and subjected for characterization. Ag3PO4/MoS2 nanocomposite was prepared by adding 20 mg of Ag3PO4 nanoparticles into 1 mL of MoS2 dispersion. The mixture was then stirred for 2 h followed by a 10 min of sonication. 2.2

Characterization techniques

The prepared samples are subjected to various characterization techniques: such as UV-Vis absorption spectroscopy, photoluminescence (PL) and Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). XRD spectrum has been recorded using a Pan Analytical X-ray diffractometer between 10° and 80°. Crystallite size

and lattice spacing have been obtained using transmission electron microscope (JEOL JEM-2100 FEG TEM). For TEM analysis, the water dispersed sample was drop-casted on a copper grid followed by 15 min drying under IR lamp. The SEM images were recorded using JEOL JSM-7600F FEG-SEM (field emission gun-scanning electron microscopy). Raman experiments were performed at room temperature using the Horiba Jobin Yvon HR800 confocal Raman spectrometer with Ar– ion laser at an excitation wavelength of 514.5 nm. Absorption studies have been carried out to evaluate the photodegradation efficiency using UV-Visible spectrophotometer (Cecil-302). Room temperature PL measurements were carried out using a fluorescence spectrophotometer (Horiba-fluromax-4 spectrofluorometer). The photocatalytic activities were evaluated by measuring the degradation rate of dye under visible-light irradiation. A 60 W CFL (Compact fluorescent lamp) bulb was used as the light source. For dye degradation, 0.20 g/L of photocatalyst was dispersed in 100 mL aqueous solution of dye having a concentration of 20 ppm (0.02 g/L). To measure the photocatalytic activity in case of composite, 20 mg of Ag3PO4 along with 1 mL stock solution of MoS2 (0.1 wt.%) was used to degrade the dye. Before light irradiation, the suspension was stirred for 30 min in dark in order to attain adsorption/desorption equilibrium. About 1 mL of suspension was taken out after every 5 min time interval followed by centrifugation at 5000 r/min to remove the catalyst. The concentration of supernatant was determined by measuring the absorbance of solution at 664 nm using UV-visible spectrophotometer. Distance of the beaker from the light source was kept constant i.e. 10 cm in each experiment. MB was selected as a model pollutant for studying the photocatalytic properties of the prepared samples.

3

Results and discussion

3.1

XRD, HRTEM and SEM

The XRD patterns of Ag3PO4 nanoparticles and Ag3PO4/ MoS2 nanocomposite are shown in Fig. 1. All the characteristic peaks in the XRD pattern of bare Ag3PO4 are well indexed to the body-centered cubic structure of Ag3PO4 (JCPDS No. 06-0505). The XRD pattern of Ag3PO4/MoS2 composite matches with Ag3PO4 but no sign corresponding to hexagonal structure of MoS2 is


Fig. 1 XRD patterns of bare Ag3PO4 (a) and Ag3PO4/MoS2 nanocomposite (b).

visible which could be due to small amount of MoS2 (0.1%) in the composite. For reference, the XRD pattern of bulk MoS2 can be seen from Ref. [32].

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HRTEM images of the as-prepared Ag3PO4, MoS2 and Ag3PO4/MoS2 samples are shown in Fig. 2. Ag3PO4 shows spherical monodispersed nanoparticles having an average size of ~5.4 nm (Fig. 2(a)). The lattice spacing of 0.26 nm (inset in Fig. 2(a)) corresponds to the (210) plane of Ag3PO4 [33]. Layered structure of MoS2 sample can be clearly seen in Fig. 2(b). The corresponding SAED pattern of MoS2 sheet is also shown in the inset of Fig. 2(b), which indicates a hexagonal lattice structure of MoS2. The interlayer spacing of 0.27 nm, corresponds to the (100) crystallographic plane of the MoS2 (Fig. 2(c)) [34]. Figures 2(d) and 2(e) depict a uniform decoration of Ag3PO4 nanoparticles all over the surface of MoS2 sheet. The average size (5.4 nm) of Ag3PO4 nanoparticles has been determined by plotting a histogram as shown in the inset of Fig. 2(e). Lattice parameter corresponding to both Ag3PO4 nanoparticle and MoS2 nanosheet is observed in highresolution image indicating the presence of both components in the composite (Fig. 2(f)). SEM images in Fig. 3 show the surface morphology of Ag3PO4, MoS2 and Ag3PO4/MoS2 samples. Bare Ag3PO4

Fig. 2 HRTEM images of (a) Ag3PO4 nanoparticles (inset: the d210 plane of Ag3PO4) and (b) MoS2 nanosheets (inset: the SAED pattern showing the hexagonal crystal structure). (c) High resolution image of MoS2 showing the lattice structure. (d)(e) HRTEM images of Ag3PO4 nanoparticles decorated over MoS2 nanosheets (inset: a histogram representing the average size of nanoparticles). (f) The dspacing for Ag3PO4 and MoS2 in the nanocomposite.

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is formed as monodispersed spherical nanoparticles (Fig. 3 (a)). MoS2 has shown flaky sheet like structure with a lateral size of ~1 μm as shown in Fig. 3(b). Figure 3(c) shows the morphology of Ag3PO4/MoS2 composite representing the decoration of Ag3PO4 nanoparticles on MoS2 sheets which is in agreement with the TEM results. Distinct flakes visible on edges (Fig. 3(c)) confirm a high quality exfoliation in the sample. 3.2

Raman spectroscopy

Figure 4 shows Raman spectra of Ag3PO4, MoS2 and the composite sample of Ag3PO4/MoS2. The Raman spectrum of Ag3PO4 nanoparticles shows a sharp peak centered at 908 cm–1 along with a broad band at 1005 cm–1 (broad band marked with arrow in the graph) (Fig. 4(a)), which are assigned to the PO43– symmetric and antisymmetric stretching vibration respectively [35–36]. The Raman spectrum of MoS2 consists of two peaks, centered at 385 cm–1 (E2g) and 408 cm–1 (A1g) (Fig. 4(b)). The enlarge view of Raman spectra of MoS2 is shown in the inset of Fig. 4 (b). The E2g peak corresponds to the in-plane vibration of two sulphur atoms with respect to the molybdenum atoms while the A1g peak is attributed to the out-of-plane vibrations of sulphur atoms. The frequency difference between the two peaks is found to be 23 cm–1 indicating the presence of 3–4 layers of MoS2 (Eda et al. and Lee et al.) [31,37]. For the MoS2/Ag3PO4 composite as shown in Fig. 4(c), all of the Raman bands corresponding to bare Ag3PO4 and MoS2 can be observed. The Raman results support the presence of both MoS2 and Ag3PO4 in the composite. 3.3

UV-Vis spectroscopy

The UV-Vis absorption spectra of Ag3PO4, MoS2 and the nanocomposite are shown in Fig. 5. The UV spectrum of Ag3PO4 shows absorption edge at 520 nm (Curve a in Fig. 5). Curve b in Fig. 5 shows three absorption bands at 450,

Fig. 3

620 and 680 nm for MoS2. The broad band centered at 450 nm is attributed to the direct electronic transition between conduction and valence band energy level [38]. Another two characteristic bands observed at 620 and 680 nm (absorbance position marked with arrow in the figure) is attributed to the transitions that occur due to spin orbit splitting [31]. The absorption spectrum of the composite is mainly dominated by Ag3PO4 as shown by Curve c in Fig. 5. A clear red-shift in absorption edge of composite is seen which is attributed to the interfacial band alignment due to narrow and wide band gap MoS2 nanosheet and Ag3PO4 nanoparticles respectively [20]. 3.4

Photocatalytic activity of decomposition of the MB dye

The as-synthesized Ag3PO4 and the nanocomposite were used for dye degradation experiment. The photocatalytic degradation of the MB dye solution was done under visible-light irradiation. Figure 6 shows the photocatalytic degradation of MB dye using Ag3PO4 and Ag3PO4/MoS2 composite respectively. A blank test in the absence of catalysts confirmed that self degradation of dye is negligible. The composite photocatalyst exhibits higher photocatalytic activity than that of Ag3PO4. The kinetics of the photocatalytic degradation reactions was also studied. The ln(C0/C) versus time curve of Ag3PO4 and its composite are shown in the inset of Fig. 6. The linear relationship between ln(C0/C) with irradiation time plot indicates that the reaction kinetics can be described by using a pseudo first order reaction, which is given as under: lnðC0 =CÞ ¼ kt

(1)

where, C0 is initial concentration of dye, C is concentration of dye at time t and k is the degradation rate constant. The degradation curve of ln(C0/C) versus time is used to determine the rate constant. The rate constants k1 and k2 are calculated to be 0.071 and 0.244 min–1 for Ag3PO4 and Ag3PO4/MoS2 composite respectively. The ratio of k2 and

SEM images of (a) Ag3PO4 nanoparticles, (b) exfoliated MoS2 nanosheets and (c) Ag3PO4/MoS2 nanocomposite.


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Fig. 6 Photocatalytic activity of bare Ag3PO4 nanoparticles and Ag3PO4/MoS2 nanocomposite under visible-light irradiation. The inset shows ln(C0/C) versus time curve.

Fig. 4 Raman spectra of (a) pure Ag3PO4 nanoparticle, (b) MoS2 nanosheet and (c) Ag3PO4/MoS2 nanocomposite. The inset shows enlarged view of Raman spectrum of MoS2.

the valence band (VB) maxima lies at 2.62 eV [29]. The energy positions of conduction and valence bands can be calculated using the equation as follows: Eg 2

(2)

EVB ¼ ECB þ Eg

(3)

ECB ¼ χ – Ee –

Fig. 5 UV-Vis absorbance spectra of Ag3PO4 (a), MoS2 (b), and Ag3PO4/MoS2 nanocomposite (c).

k1, i.e. k2/k1, is calculated as 3.4 indicating degradation reaction with composite is ~3 times faster than with Ag3PO4 alone. So it can be concluded that the composite has a better degradation efficiency as compared to Ag3PO4 itself. Based on the results discussed above, the mechanism for the enhanced photocatalysis is analyzed. The schematic for the photocatalytic degradation of dye in the presence of Ag3PO4/MoS2 composite is illustrated in Fig. 7. The conduction and valence bands of MoS2 are positioned at – 0.13 and 1.77 eV, respectively [29] and for Ag3PO4, the conduction band (CB) minima occurs at 0.29 eV whereas

where, ECB and EVB are the CB and VB edge potentials respectively, χ is the absolute electro negativity of the semiconductor. The χ values of Ag3PO4 and MoS2 are calculated to be 5.96 and 5.32 eV respectively [29]. Ee is the free electron energy on the hydrogen scale (4.5 eV) and Eg is the band gap determined using fundamental absorption edge from UV spectrum of composite. The Eg values of Ag3PO4 and MoS2 are calculated to be 2.38 and 1.9 eV, respectively. Under visible-light irradiation, both for Ag3PO4 and MoS2, electrons are excited from the valence band to the conduction band forming holes in VB. Since the conduc-

Fig. 7 The mechanism of photocatalytic reduction of dye with Ag3PO4/MoS2 nanocomposite under the visible-light irradiation.

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tion band and valence band potentials of MoS2 are more negative than those of Ag3PO4, the photogenerated electrons in the MoS2 are readily transferred to Ag3PO4 crystals, and the photoinduced holes on the surface of the Ag3PO4 migrate to MoS2 facilitating effective separation of charge carriers thereby reducing their recombination probability. The photo excited electrons accumulated in CB of Ag3PO4 react with dissolved oxygen, to form superoxide radical anions ($O2–) which in turn is responsible for oxidation of organic dye. However, holes that remained in VB of MoS2 react with water to form hydroxyl radical ($OH), which serves as a powerful oxidizing agent for the degradation of dye. Moreover, the holes can also directly oxidize MB dye molecules adsorbed on the catalyst surface. Thus, incorporation of Ag3PO4 nanoparticles on MoS2 sheets improves separation of the electron–hole pairs, thereby enhancing photocatalytic activity of the nanocomposite. The effective charge separation on Ag3PO4 and MoS2 not only improves photocatalytic activity but also inhibits the decomposition of Ag3PO4 into metallic Ag particles [39]. The large surface area of 2D MoS2 sheets further facilitates interfacial charge transfer between the photocatalyst and dissolved oxygen molecules in water [40]. Also, the presence of monodispersed and smaller sized Ag3PO4 nanoparticles in the composite may enhance the photocatalytic activity due to the availability of larger surface sites for interaction with dye molecules. Thus, these synergistic properties separate electron hole pairs effectively, resulting in higher degradation rate. The above analysis can further be established, by studying the PL spectra of the bare and composite sample. PL is an effective tool to understand the dynamics of separation and recombination of photo-generated charge carriers. The weaker the luminescence intensity higher is the separation of charge carriers and better is the degradation or photocatalytic efficiency. The graph shown in Fig. 8 provides a comparative PL study for both Ag3PO4 and composite sample. It can be seen that both the samples exhibit broad emission spectra ranging from 420 to 700 nm. The obtained broad PL curve consists of multiple peaks at 470, 530 and 560 nm. The peak centered at 470 nm corresponds to the electronic recombination at surface oxygen vacancies and defects [41–42]. The PL emission at 530 nm is close to fundamental absorption edge thus related to the band edge recombination [41,43–44]. The green emission at 560 nm originates due the electron hole radiative recombination

Fig. 8 PL spectra of Ag3PO4 and Ag3PO4/MoS2 nanocomposite using an excitation wavelength of 370 nm.

at surface Ag vacancies [44]. The PL spectra shows that the emission intensity of Ag3PO4/MoS2 composite weakened reasonably compared with that of bare Ag3PO4 indicating efficient separation of photogenerated carriers resulting improved photocatalytic performance [5,43]. Therefore, it can be inferred that implanting MoS2 nanosheets with Ag3PO4 nanoparticles is beneficial for photoinduced charge carrier separation.

4

Conclusions

In summary, Ag3PO4/MoS2 photocatalyst have been successfully synthesized using a wet chemical route. It has been demonstrated that Ag3PO4/MoS2 composite is a potential photocatalyst for photodegradation of dye. The Ag3PO4/MoS2 composite catalyst displays enhanced photocatalytic activity for dye degradation compared to the bare Ag3PO4 nanoparticles under visible light irradiation. The interfacial energy band alignment between MoS2 sheets and Ag3PO4 nanoparticles increases the charge separation, thereby improving degradation efficiency. The efficient interfacial charge separation has also been established from PL results. Also, the large surface area of well exfoliated MoS2 sheets and the presence of uniformly distributed smaller sized Ag3PO4 nanoparticles further contribute towards higher photocatalytic activity. Thus it can be concluded that the Ag 3 PO 4 /MoS 2 nanocomposite is supposed to be an efficient visible-light active photocatalyst material for degradation of organic pollutants present in water. Acknowledgements The authors gratefully acknowledge nanomission DST, WOS-A DST, Government of India for financial support and CRNTS, IIT Bombay for providing us HRTEM, HRSEM and Raman facilities.


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