Graphene Oxide Coordination Composites for High

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Brianyoungite/Graphene Oxide Coordination Composites for HighPerformance Cu2+ Adsorption and Tunable Deep-Red Photoluminescence Xiaobin Zhu, Yun Shan, Shijie Xiong, Jiancang Shen, and Xinglong Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05464 • Publication Date (Web): 03 Jun 2016 Downloaded from http://pubs.acs.org on June 7, 2016

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ACS Applied Materials & Interfaces

hydrothermal reaction that produces BY-GO composites with hollow spherical and flake-like morphologies that are easy to remove.

By producing abundant

oxygen-containing groups on GO, the Cu2+ adsorption capacity increases to 1724.1 mg/g, which is the highest value in graphene-related materials.

The experimental and

theoretical analysis clearly shows that the infrared spectral shifts towards the low-frequency side of C-O-H and O=C-O bending vibrations in the BY-GO composites stem from the Zn2+ (or Cu2+) coordination with O atoms in GO.

The BY-GO also

exhibits tunable deep-red photoluminescence up to 750 nm with a quantum yield of about 1%, which may be useful in infrared optoelectronic devices and solar energy exploitation.

The photoluminescence which is different from that previously reported

from chemically-derived GO can be attributed to the optical transition in the disorder-induced localized states of the carbon-oxygen functional groups.

2

ACS Paragon Plus Environment


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1. INTRODUCTION Heavy metals in discharged industrial waste cause pollution and release of copper to soil is becoming more serious.

Although copper is an essential element in the

human body, divalent copper (Cu2+) is toxic and carcinogenic when ingested in excessive

amounts.

Techniques

such

as

adsorption,1-10

ion

exchange,11,12

precipitation,13 and membrane processes14,15 have been used to remove heavy metal ions. In particular, adsorption is attractive due to the simple and rapid operation1-10 but it requires an effective, economical, and environmentally friendly adsorbent to be practical. Graphene oxide (GO) is the soluble form of graphene with functional groups containing oxygen atoms with lone electron pairs that can combine with metal ions to form coordination bonds.

As a result, GO can act as a good adsorbent of heavy metal

ions with the oxygen functional groups acting as adsorption active sites.3-10 Since the adsorption efficiency hinges on the active sites, it is desirable to increase the amount of the oxygen-containing groups on GO and chemically derived GO nanoflakes may be a good candidate.16

However, small-size GO with dense oxygen-containing groups

increase the water solubility making separation difficult and a possible solution is to produce a cluster which can coordinate with small GO as shown in Figure 1a. Zn2+ can coordinate with oxygen atoms with lone electron pairs.

A well known

example is the Zn-based metal-organic framework (MOF), MOF-5, synthesized by Li and co-workers.17 Zn2+ and O can bond by Zn-O coordination in Zn-based minerals including

hydrozincite

[Zn5(OH)6(CO3)2]

and

brianyoungite

(BY)

[Zn3(CO3,SO4)(OH)4].18-20 These Zn-based MOFs and minerals may thus be good 3


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alternatives to bond with GO via Zn-O coordination.

In fact, the process can be

designed to take place during growth of the initial central cluster and then form large coordinated composites that can be removed. In this work, we design and prepare BY-GO coordinated composites with a highly efficient adsorption capacity of Cu2+ at 1724.1 mg/g, which is the highest value in graphene-related materials.

The composites

also show tunable photoluminescence (PL) in the deep-red range, which is different from previously reported PL from chemically derived GO.16,21-26 Our designs and results provide insights into the materials and process that possibly have important applications in environmental control and graphene-related optoelectronic devices.

2. EXPERIMENTAL SECTION The materials, synthesis of GO, and characterization can be found in Experimental Section of Supporting Information. Synthesis of BY-GO composites: 0.6 g ZnSO4⋅7H2O were added to 250 mL of the GO solution.

The pH of the solution was adjusted to 4-5 with anhydrous Na2CO3 and then

5.5 with Na2CO3.

25 mL of the solution were introduced into a 50 mL autoclave

which was put in oven at 200 °C for 3.5 h and then cooled to room temperature.

The

hollow spheres were obtained by centrifugation, washed with absolute ethanol and distilled water several times, and dried at 60 °C.

The flakes were prepared by the same

process except that 35 mL of the solution were put in the autoclave.

The reference

sample was synthesized by substituting the GO solution with a mixture of H2SO4 and HNO3. 4



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Adsorption experiments: The Cu2+ adsorption isotherms were acquired by adding 3 mg of the sample into 9 mL of CuCl2 with different concentrations (pH is adjusted to 4.95). The influence of pH was acquired by adding 3 mg of the sample into 9 mL of CuCl2 (the concentration of Cu2+ is 493.9 mg/L) with different pH. The solution was magnetically stirred in the absence of light for 10 h.

The adsorbent settled without

stirring and thus centrifugation was not necessary for separation.

To eliminate the

impact of the residual adsorbent on subsequent measurement, the solution was centrifuged at 6,000 rpm for 5 min and the supernatant was collected for inductance coupled plasma-atomic emission spectroscopy (ICP-AES) analysis.

PE-5300DV was

used for ICP-AES analysis.

3. RESULTS AND DISCUSSION The synthesis procedures for the coordinated composite are illustrated in Figure 1b. Chemically derived GO nanoflakes with a planar size of 2 to 5 nm, most probable size of 3.2 nm, and thickness of 1 to 3 nm determined by atomic force microscopy (Figure S1) are prepared by using previous method.16

The BY clusters are formed

simultaneously during the reaction with GO due to supersaturation in the solution and the GO nanoflakes coordinate with the BY rapidly.

Usually, GO and BY are

sheets16,19,20 and so the BY/GO assemble into plates initially.

As the reaction

proceeds, more plates are formed to interlace with each other producing a hierarchical structure.

Finally, flakes and hollow spheres emerge at different vapor pressures.

If

the hydrothermal process continues, the Zn-O coordination breaks and BY decomposes. 5


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As a result, a mixture containing ZnO and BY-GO composites is produced after the hydrothermal treatment for 8 h.

However, the BY-GO composite disappear when the

reaction time is increased to 16 h as indicated by scanning electron microscopy (SEM, Figure S2a) and X-ray diffraction (XRD) (Figure S2b). Figure 2 depicts the SEM images of two kinds of BY-GO composites.

The flake

sample has the planar size between tens of µm and 1-2 cm (Figure 2a) and the thickness is several µm (the inset of Figure 2b).

Figure 2b exhibits the magnified surface which

consists of many interlaced small plates with a thickness of about 0.1 µm and planar size of several µm2.

The spherical sample has a diameter of 15-25 µm (Figure 2c) and

shell thickness of 2.1 µm which is similar to the thickness of the flake sample (Figure 2d).

Similar to the flake sample, the spherical shells are assembled by interlacing

small plates.

Since the two kinds of samples have the same structural unit and the

hollow spheres are formed by decreasing the volume of the reactant solution and vapor pressure, it can be inferred that the flake samples are formed at the beginning of the reaction and the flakes curl up to form hollow spheres subsequently.

BY without the

addition of GO can be synthesized by Schelonka’s method20 but the BY resembles a lump.

Although the morphology is rather irregular compared to the coordinated

composites, many small plates can be observed from the surface (Figure S3) indicating that BY grows preferentially into sheets as consistent with our observation in the BY-GO composites. The XRD patterns of the two coordinated composites and synthesized BY are presented in Figure S4 and all the peaks can be indexed to the standard card (JCPDS 6



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card No. 46-1431) of Zn12(CO3)3(SO4)(OH)16.19,20 The XRD patterns of the two composites are similar.

Energy dispersive X-ray (EDX) spectrum reveals that BY

contains C, O, S, and Zn (Figure S5), but the BY-GO composites have no S element. The BY in the composites appears to be Zn3(CO3)(OH)4, an isomorphic state of Zn12(CO3)3(SO4)(OH)16, after sulfate is replaced by carbonate at the high temperature and pressure during synthesis.

As the sulfate occupies a small proportion in the unit

cell, replacement of sulfate by carbonate will only cause small change of the crystal structure.

So the XRD of the BY in the coordinated composites and synthesized BY

are almost the same. If GO is not added or the reaction proceeds for a long time, hexagonal prisms of typical ZnO are produced (Figures S2c and S2d) and XRD (Figure S2b) also confirms the formation of ZnO instead of BY. No obvious XRD peak arising from graphene is observed as the GO nanoflakes are too small similar to the observation from other graphene composites.27-29

The

materials are further characterized by Raman scattering and Fourier transform infrared (FTIR) spectroscopy. As shown in Figure S6, the typical D band at 1370 cm-1 and G band at 1586 cm-1 indicate the presence of GO in the composites.

The Raman spectra

of the composites and pristine GO are alike suggesting that the layered structure of GO is not alternated by the hydrothermal process.

The relative intensity of the disorder D

band to crystalline G-band changes from 91.0% to 95.3% after the coordination, indicating that the GO is more oxidized. As shown in the FTIR spectrum of the pristine GO (Figure 3), the peaks at 1612, 1435, 1340 and 1131 cm-1 can be assigned to C=C ring stretching, C-O-H, O=C-O and C-C bending vibrations,30-33 respectively. 7


The

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FTIR spectra of the hollow sphere and flake samples are almost the same.

Compared

to the FTIR spectrum of GO, the relative intensity of the C=C ring stretching at 1612 cm-1 is smaller and C-C bending at 1131 cm-1 almost disappears.

This indicates that

there are more oxygen-containing functional groups after coordination of GO with BY, as confirmed by the enhanced C-O-H (1435 cm-1) and O=C-O (1340 cm-1) bending vibrations.

Hence, the present GO is highly oxidized and easier to assemble on BY to

form the composites.

In addition, the C-O-H and O=C-O bending peaks obviously

shift to the low-frequency side from 1435 and 1340 cm-1 to 1420 and 1328 cm-1, respectively, indicative of the interactions between O and coordinated Zn2+.34,35

The

as-made BY shows the same FTIR spectrum as natural BY,19 but all the vibration modes are not observed from the BY-GO coordinated composites with the exception of the strong peak at 472 cm–1 stemming from Zn-O coordination bonds. Hence, it can be inferred that the 472 cm-1 peak observed from the composite arises from new Zn-O bonds formed during coordination between BY and GO.

Furthermore, as shown in the

XPS spectra of the BY and BY-GO composites (Figure S7), the peak area ratios of Zn-O to CO3 binding in both BY-GO composites are larger than those in the as-made BY, which also approves the coordination of GO and BY. The adsorption isotherms of Cu2+ acquired from the BY-GO composites and BY are displayed in Figure 4 showing that the amount of adsorbed Cu2+ increases with Cu2+ concentration for the coordinated samples. The Langmuir and Freundlich adsorption isothermal models are adopted to fit the experimental data and as shown in Table S1, the Langmuir model is more suitable for each sample (Figure 4). 8


According to the


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Langmuir model, the adsorption capacities of the hollow spheres, flakes, and BY are obtained to be 1724.1, 1603.7, and 349.5 mg/g, respectively.

The BY-GO composites

deliver the best performance pertaining to adsorption of heavy metals.

It is noted that

the adsorption capacity of BY is not as good as that of the composites even though the specific surface area of the former is larger (Figure S8).

Because the solubility product

constant KSP of Cu(OH)2 at room temperature is 2.2 x 10-20 mol3/L3, Cu(OH)2 will not precipitate before the concentration of Cu2+ reaches 1773 mg/L at pH 4.95. precipitation of Cu(OH)2 can be eliminated in the adsorption process.

So the

The high

adsorption capacities of the BY-GO composites can be attributed to the Cu-O coordination. As described above, the C-O-H bending vibration peak in the FTIR spectrum shifts from 1435 to 1420 cm-1 after the coordination of BY with GO (Figure 3). With further Cu2+ adsorption, the C-O-H bending peak shifts to 1429 cm-1, which indicate the interactions of C-O-H bonds with Cu2+.

In addition, the broadening of the

Zn-O vibration peak at 472 cm-2 also suggests the combination of Cu2+ with O atoms (partial Zn-O bonds are replaced by Cu-O bonds).

The huge amount of the

oxygen-containing groups in the GO nanoflakes makes the BY-GO composites better than common GO and its derivatives reported previously (Table S2).3-10 The effect of solution pH is illustrated in Figure S9.

It can be observed that the Cu2+ adsorption

increases with pH value for all the adsorbents.

At a low pH, protonation of the

oxygen-containing groups reduces the number of the active sites for Cu2+ adsorption.3-10 With increasing pH value, the positive charge density on the surface sites of adsorbent decreases and as a result, the adsorption of Cu2+ rises.

It is worthy to mention that

9


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when the pH value is greater than 5.23, the formation of Cu(OH)2 also causes the decrease of Cu2+, which leads to the unreliable adsorption amount.

Furthermore, we

also test the adsorption capacity of Pb2+ and the values are determined to be 910.7 mg/g for the hollow spheres and 837.0 mg/g for the flakes when the concentration of Pb2+ is 621 mg/L (3 mM). out as well.

The absorption performances of methylene blue (MB) are carried

The adsorption capacities of the hollow spheres and flakes are

respectively obtained to be 540.47 and 529.95 mg/g (Figure S10 and Table S3) by Langmuir model, which are also satisfactory. To clarify the coordination of Zn2+ and Cu2+ in BY with GO and spontaneously show the mechanism of Cu2+ adsorption, we calculate the infrared vibration properties of different coordinated composites with special emphasis on the two vibration modes at 1435 and 1340 cm-1.

The calculation is performed with Gaussian 03 code.36

Perdew-Burke-Ernzerhof potential functional37 is adopted. is carried

out by

approximation.38,39

using

The

The geometry optimization

density functional method

with

the frozen-core

Calculations of phonon frequencies and infrared spectrum are

based on the numerical differentiation of energies from the DFT results.40,41

As we

focus on the two modes at 1435 and 1340 cm-1, we only take units containing the C-O-H and O=C-O bending vibrations to produce these peaks in the infrared spectrum. The calculated infrared spectra for composites with and without Zn2+ and Cu2+ adsorptions are shown in Figure 5. We can see that in the unit without Zn2+ and Cu2+ adsorptions, the two peaks appear near 1435 and 1340 cm-1, respectively.

With the

adsorption of Zn2+ near the C-O-H and O=C-O bonds, the two peaks are red-shifted and 10



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strengthened.

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This can be easily understood from the fact that the attachment of Zn2+

increases the effective mass of the constituents relevant to these vibrations, and enhances the relative infrared activity due to the participation of more charged particles in the vibrations. By replacing Zn2+ with Cu2+, the two peaks are still red-shifted and strengthened compared to those in the case without Zn2+ and Cu2+ adsorption, but both the effects become weaker because both the size and mass of Cu2+ are smaller than those of Zn2+.

As can be seen from Figure 5, Zn2+ and Cu2+ are adsorbed towards the

oxygen atoms in the units, because oxygen atoms usually have local negative charges, while Zn2+ and Cu2+ are positively charged.

This calculation clearly indicates that this

is Zn2+ or Cu2+ coordination on BY-GO that leads to the red-shift of C-O-H and O=C-O infrared vibration bands. In addition to enhanced heavy metal adsorption, the coordinated composites also exhibit tunable PL in the deep-red range to 750 nm (Figure 6a). photos when excited by different wavelengths.

Figure 6c shows the

The absolute PL quantum yields

excited by different wavelengths vary slightly with the most value being 1%. Deep-red emission from GO is desirable because of infrared optoelectronic devices and solar cells.42 The PL spectra reported here are different from those acquired from chemically derived GO which shows the strongest emission in the blue or green region,16,21-26 as shown in Figures S11a and S11c.

The blue or green emission is

attributed to the size effect of the sp2 clusters,21 whereas red emission usually derives from disorder-induced localized states consisting of distorted carbon atoms and linked oxygen-containing functional groups (Figure S11d).23,25 Since the small GO flakes 11


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have many localized defect levels, the red PL peak is broad.

BY does not luminesce

and has no PL excitation (PLE) spectrum (Figure S12).19 The new Zn-O bonds formed during coordination between BY and GO would not luminesce as well since Zn-O bonds also exist in BY.

Hence, the deep-red emission in our BY-GO composites is

related to the modified GO nanoflakes implying that the disorder-induced localized states are dominant in the luminescence.

As shown by the FTIR spectra, the GO

nanoflakes with a large degree of oxidation are easier to assemble on BY into the composites and the BY-GO composites have a large density of disorder-induced localized states related to oxygen-containing groups. PL when excited by different wavelengths.

This leads to the observed broad

This is consistent with the fact that our

composites have the excellent Cu2+ adsorption capacities. The PLE spectra of the BY-GO composites in Figure 6b are different from those acquired from chemically derived GO (Figure S11b). consist of two parts.

They are asymmetrical and

The first one is the short-wavelength excitation in the blue region

which depends on the monitored emission wavelength and according to previous results,21,23 it stems from the size effect of the sp2 clusters.

The second one is the

long-wavelength excitation in the red region and its position does not depend on the monitored emission wavelength, indicating that the excitation is associated with the disorder-induced localized states of the carbon-oxygen functional groups.

Based on

the PLE spectra, the deep-red PL mechanism is illustrated in Figure 6d. The excitation process of the BY-GO coordinated composites takes place in both the sp2 clusters and carbon-oxygen-related defects, whereas radiative recombination only occurs in the 12



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defect levels.

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The enhanced deep-red radiative transition partially originates from the

resonance energy transfer from the excitation states of the sp2 clusters to the localized defect levels due to overlapping of the energy levels between the two excitation states.43

4. CONCLUSION BY-GO composites resembling hollow spheres and flakes are synthesized by a hydrothermal method exploiting GO coordination with BY.

The composites possess

an excellent Cu2+ adsorption capacity at 1724.1 mg/g which is higher than those observed from other graphene-related materials. These composites are easy to remove from the solution due to the large size of the coordinated BY.

The excellent

adsorption capacity can be attributed to the abundant oxygen-containing groups on the GO nanoflakes. The BY-GO composites also exhibit tunable deep-red PL arising from the optical transition in the disorder-induced localized states associated with the carbon-oxygen functional groups due to selective assembly of the highly oxidized GO nanoflakes on BY during the synthesis.

ASSOCIATED CONTENT Supporting Information available: Materials, synthesis of GO, and sample characterization. Table S1: Parameters of the used adsorbents in Langmuir and Freundlich models for Cu2+. Table S2: Adsorption capacities of different adsorbents for Cu2+. Table S3: Parameters for the Langmuir and Freundlich models of MB adsorption on different adsorbents. Figure S1: TEM images 13


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and size distribution of the GO nanoflakes. Figures S2 and S3: SEM images and XRD patterns of the reference samples. Figures S4 and S5: XRD patterns and EDX spectra of the hollow sphere and flake composites and synthesized BY. Figure S6: Raman spectra of GO and the flake BY-GO composites. Figure S7: XPS spectra of the GO nanoflakes, BY and BY-GO composites. Figure S8: N2 adsorption−desorption isothermals of the hollow sphere and flake samples and BY. Figure S9: The influence of pH on adsorption capacity. Figure S10: The adsorption performance of MB on different adsorbents. Figure S11: PL and PLE spectra from the GO nanoflakes and optical photos of the GO nanoflakes taken under excitation with different wavelengths. Figure S12: PL spectrum from BY. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected] (XL Wu) Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by National Basic Research Programs of China under Grant Nos. 2014CB339800 and 2013CB932901 and National Natural Science Foundation of China (Nos. 11374141, 21203098, 61521001, and 21375067). Partial support was provided by Qing Lan Project of Jiangsu Province. 14



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Figure captions

Figure 1. (a) Design of chemically derived nanoflake GO-based composites. (b) Schematic illustration of the growth procedures for the BY-GO coordinated composites.

Figure 2. (a) and (b) Low- and high-magnification SEM images of the flake-like BY-GO composite. The inset in (b) shows the cross-sectional SEM image of the flakes; (c) and (d) Low- and high-magnification SEM images of the hollow spherical BY-GO composites.

Figure 3. FTIR spectra of the hollow sphere and flake composites before and after Cu2+ adsorption. The corresponding spectra of GO nanoflakes and BY are also given for comparison.

Figure 4. Adsorption performance of Cu2+ for the BY, hollow sphere and flake samples with pH = 4.95 and T = 293 K.

The Langmuir adsorption isothermal model is used to

fit the experimental data (line).

Figure 5. Calculated infrared spectra and corresponding structures of GO units responsible for the enhanced C-O-H (1435 cm-1) and O=C-O (1340 cm-1) bending vibrations. (a) The spectrum of unit without Zn2+ or Cu2+ adsorption. (b) The spectrum of unit with Zn2+ adsorption. (c) The spectrum of unit with Cu2+ adsorption. In the insets, 22



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gray, red, white, blue, and pink balls denote C, O, H, Zn, and Cu elements, respectively.

Figure 6. (a) PL and (b) PLE spectra of the flake BY-GO composite. (c) Photos of the BY-GO flakes taken under excitation by different wavelengths from 360 to 640 nm (from left to right) with an increment of 40 nm. (d) Schematic illustration of the deep-red PL mechanism of the BY-GO composite.

23


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Figure 1, Zhu et al. 24



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Figure 2, Zhu et al.

25


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Nanoflake-like GO 1612

1435 1340 1131

Hollow sphere 472 Hollow sphere (Cu uptake)

Transmittance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1420

1328

Flake

1331

1429

Flake (Cu uptake)

1329

1420

Brianyoungite

1330

472

1430

2000

1500

1000 -1

Wavenumbers (cm )



500


1000

Hollow sphere sample Flake sample Brianyoungite

800

qe(mg/g)

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600

400

200

0 0

100

200

300

400

Ce (mg/L)



500

600

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Fig. 5, Zhu et al.,

28



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Figure 6, Zhu et al.

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Table of Contents

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Graphene Oxide Coordination Composites for High

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