Development of porous α-Fe2O3 microstructure by forced hydrolysis of FeCl3 solutions in the presence of AOT

Journal of Alloys and Compounds 532 (2012) 41–48

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Development of porous ␣-Fe2 O3 microstructure by forced hydrolysis of FeCl3 solutions in the presence of AOT ˇ Jasenka Stajdohar, Mira Ristic´ ∗ , Svetozar Music´ Rud¯jer Boˇskovi´c Institute, Bijeniˇcka 54, P.O. Box 180, HR-10002 Zagreb, Croatia

a r t i c l e

i n f o

Article history: Received 29 February 2012 Received in revised form 30 March 2012 Accepted 4 April 2012 Available online 16 April 2012 Keywords: ␤-FeOOH ␣-Fe2 O3 AOT XRD Spectroscopy FE-SEM

a b s t r a c t The effect of AOT (sodium di-2-ethylhexyl sulfosuccinate) on the formation and microstructure of ␣-Fe2 O3 by the forced hydrolysis of FeCl3 solutions was investigated. The precipitates formed were characterized by XRD, 57 Fe Mössbauer, FT-IR and optical spectroscopies, and also with FE-SEM. The presence of AOT influenced the shape of ␤-FeOOH particles and led to the formation of microporous ␣-Fe2 O3 . The preferential adsorption of sulfonate/sulfate groups played an important role in these effects. Generally, the 57 Fe Mössbauer and optical spectra were sensitive to low crystallinity, as well as to the particle size and shape. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Hematite (␣-Fe2 O3 ) is an important material owing to its colloid and surface properties. It is important in the fundamental studies of colloid stability and adsorption phenomena, because ␣-Fe2 O3 shows excellent acid/base surface properties and can also be prepared in different shapes and dimensions from nano to micro size. It has various applications, for example as pigment, catalyst, sensor, anticorrosion paint or as starting material in the production of ferrites and other ceramics. Different precipitation methods were used to produce ␣-Fe2 O3 , such as forced hydrolysis, solvothermal or microwave hydrolysis of Fe3+ ions, microemulsion precipitation, recrystallization from ferrihydrite or goethite suspensions at higher pH or sol–gel precipitation. Forced hydrolysis of FeCl3 solutions via recrystallization of ␤-FeOOH at low pH values is a simple way to produce ␣-Fe2 O3 particles. Music´ et al. [1] investigated the forced hydrolysis of 0.1 M Fe(NO3 )3 , FeCl3 or Fe2 (SO4 )3 solutions. It was suggested that the hydroxy polymers formed in the Fe(NO3 )3 solutions did not include nitrate ions in the polymer chain, whereas the polymers formed in the chloride solution contained some chloride ions in the place of hydroxyl ions. The next step in the forced hydrolysis of FeCl3 solutions is the formation of oxy bridges and the development of ␤-FeOOH structure. Bottero et al. [2] experimentally confirmed the

∗ Corresponding author. Tel.: +385 1 4680 107; fax: +385 1 4680 098. ´ E-mail address: [email protected] (M. Ristic). 0925-8388/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.04.011

incorporation of Cl− ions into the ␤-FeOOH structure. Matijevic´ and Scheiner [3] also investigated the forced hydrolysis of Fe3+ ions in chloride, nitrate or perchlorate solutions. The ␣-Fe2 O3 cubes of narrow size distribution were prepared [4] by FeCl3 hydrolysis in water/ethanol solutions. The effect of FeCl3 and HCl concentrations, as well as that of the specific adsorption of sulfate anion on the forced hydrolysis of FeCl3 solutions was investigated by Music´ et al. [5,6]. The forced hydrolysis of mixed Fe(NO3 )3 + FeCl3 solutions showed [7] that the phase composition of hydrolytical products was determined by the concentration of the dominant Fe(III)-salt. The size of ␤-FeOOH particles decreased to nano dimensions upon the forced hydrolysis of 0.1 M FeCl3 solutions for 5 h at 90 ◦ C with the starting HMTA (hexamethylenetetramine) concentrations varying from 0.005 to 0.100 M, whereas for 0.250 M HMTA amorphous particles for XRD were produced [8]. The effect of metal ions (Cu2+ , Ni2+ , Co2+ or Cr3+ ) on the microstructural properties of hydrolytical products produced by the forced hydrolysis of FeCl3 -HCl solutions was investigated [9]. Spherical and a small number of double-sphere ␣-Fe2 O3 particles were altered to diamond-shaped particles with increased Me2+ cation concentration. The spheres and double-spheres of ␣-Fe2 O3 were also obtained in the presence of Cr3+ ions; however, at higher concentrations of Cr3+ ions the ␤-FeOOH particles additionally precipitated. Gotic´ et al. [10] synthesized ␣-Fe2 O3 nanorings by thermal treatment of FeCl3 + NH4 H2 PO4 + Me (Mn2+ , Cu2+ , Zn2+ or Ni2+ ) in aqueous media at 231 ◦ C. The effect of some inorganic anions on the formation and shape of ␣-Fe2 O3 particles in hydrolyzing solutions was reported by Matijevic´ et al. [11,12], Morales et al.

J. Sˇ tajdohar et al. / Journal of Alloys and Compounds 532 (2012) 41–48

42 Table 1 Experimental conditions for sample preparation. Sample

2 M FeCl3 /ml

H2 O/ml

S1 S2 S3 S4 S5 S6 S7 S8 S9 S10

4 4 4 4 4 4 4 4 4 4

36 36 36 28 28 28 28 16 16 16

AOT/ml 1% solution

T/◦ C

Crystallization time/h

8 8 8 8 20 20 20

160 160 160 160 160 160 160 160 160 160

2h 6h 3d 2h 6h 24 h 3d 2h 6h 24 h

AOT = sodium di-2-ethylhexyl sulfosuccinate; h = hour; d = day.

[13] and Sugimoto and Muramatsu [14]. Zˇ ic et al. found a strong effect of ammonium amidosulfonate [15] or Na2 HPO4 addition [16] on the phase composition, morphology and size of ␣-Fe2 O3 particles crystallized from dense ␤-FeOOH suspensions. Kandori et al. [17] prepared mesoporous ␣-Fe2 O3 particles by the forced hydrolysis of FeCl3 -HCl solutions containing l-phenylalanine and N-(3-dimethylaminoethylaminopropil)-N-ethylcarbodiimide hydrochloride. All these works [9–17] show a very strong effect of various addings on the size and shape of ␣-Fe2 O3 particles formed from hydrolyzing FeCl3 solutions via ␤-FeOOH phase. In a follow-up to the cited works we are investigating the effect of AOT (sodium di2-ethylhexyl sulfosuccinate) on the formation and microstructure of ␣-Fe2 O3 by the forced hydrolysis of FeCl3 solutions. AOT surfactant has found important applications in many investigations of microemulsions from a different standpoint.

XRD measurements were performed at 20 ◦ C with an APD 2000 powder diffractometer manufactured by Italstructures (GNR-Analytical Instruments Group, Italy). 57 Fe Mössbauer spectra were recorded at 20 ◦ C in the transmission mode using a standard configuration of Mössbauer spectrometer by WissEl GmbH (Starnberg, Germany). The 57 Co in the rhodium matrix was used as a Mössbauer source. The velocity scale and Mössbauer parameters refer to the metallic ␣-Fe absorber at 20 ◦ C. Deconvolution analysis of Mössbauer spectra was made using the MossWin program. FT-IR spectra were recorded at RT with a Perkin Elmer spectrometer (model 2000). Powders were mixed with spectroscopically pure KBr, then pressed into tablets using the Carver press. Optical spectra were recorded at 20 ◦ C with Shimadzu UV-Vis-NIR spectrometer (model UV-3600) using the integrated sphere. The precipitated powders were inspected with a thermal field emission scanning electron microscope (FE-SEM, model JSM-7000F) manufactured by Jeol Ltd. The inspected powders were not coated with an electrically conductive layer.

2. Experimental

3.1. XRD and Mössbauer analyses

2.1. Sample preparation

The characteristic XRD and Mössbauer spectroscopic results are summarized in Figs. 1–4. The calculated 57 Fe Mössbauer parameters at 20 ◦ C for samples S1–S10 are given in Table 2. Mössbauer spectroscopy has found important applications in the investigation of iron oxides (group name) [18]. XRD analysis of the precipitates obtained by the forced hydrolysis of 0.2 M FeCl3 solutions at 160 ◦ C between 2 h and 3 d showed the presence of single phase ␤-FeOOH (for 2 h) or ␣-Fe2 O3 (for 6 h and 3 d), as shown in Fig. 1. In the same figure the corresponding Mössbauer spectra are also shown. The Mössbauer spectrum of sample S1 is characterized as a superposition of two quadrupole doublets. The deconvolution of this spectrum gave the quadrupole parameters ı = 0.38 mm s−1 and  = 0.55 mm s−1 for doublet 1, and ı = 0.37 mm s−1 and

FeCl3 ·6H2 O p.a. was supplied by Kemika and AOT by Merck. Twice distilled water was prepared in own laboratory and used in all experiments. The starting solutions were prepared by mixing proper volumes of FeCl3 + H2 O or FeCl3 + H2 O + AOT solutions in accordance with experimental conditions required for sample preparation, as given in Table 1. A teflon-lined, non-stirred pressure vessel (autoclave) manufactured by Parr Instruments (model 4744) was used. The autoclaves were heated at 160 ◦ C for different times from 2 h up to 3 d in DX 300 gravity oven (Yamoto; temperature uniformity ±1.9 ◦ C at 100 ◦ C or ±3 ◦ C at 200 ◦ C, as supplied by Cole Parmer). The autoclaving times were corrected for the time the autoclave needs to reach the predetermined temperature. After a proper autoclaving time the autoclaves were abruptly cooled with cold water. The mother liquor was separated from the precipitate using the ultra-speed centrifuge (Sorvall, model Super T21), then the precipitates were washed with twice distilled water. The isolated precipitates were dried at 60 ◦ C.

2.2. Instrumentation

3. Results and discussion

Table 2 57 Fe Mössbauer parameters for samples S1–S10 at 20 ◦ C. Sample

Line

ı/mm s−1

Eq/mm s−1

HMF/T

 /mm s−1

A/%

0.30; 0.33

57; 43

0.35 0.34

100 100

S1

D1; D2

0.38; 0.37

0.55; 0.99

S2 S3

M M

0.37 0.37

−0.20 −0.21

S4

D1; D2

0.38; 0.38

0.50; 0.98

0.26; 0.40

40; 60

S5

M1; M2

0.37; 0.37

−0.21; −0.21

50.9; 48.4

0.34; 0.54

100

S6 S7

M M

0.37 0.37

−0.21 −0.21

51.3 51.4

0.31 0.26

100 100

S8

D1; D2

0.38; 0.37

0.97; 0.55

0.33; 0.37

60; 40

S9

M1; M2

0.37; 0.37

−0.21; −0.21

50.9; 49.0

0.31; 0.58

100

S10

M

0.37

−0.21

51.3

0.32

100

51.2 51.5

All data are given relative to the ␣-Fe standard. Key: ı = isomer shift;  or Eq = quadrupole splitting; HMF = hyperfine magnetic field;  = line-width; A = surface under the peaks. Errors: ı = 0.01 mm s−1 ;  or Eq = ±0.01 mm s−1 , HMF = ±0.2 T.

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43

310

β-FeOOH

S1

S1

211 330 420 400 301

521 411

510 321

220

600 440 431

710 611 541 002 312

-2

-1

0

1

2

α-Fe2O3

110

Count rate / a.u.

Relative intensity

104

S2

116 012

024

113

214 300 018

S2

104

S3

α-Fe2O3

110

024 116

012

214 300

113 018

20

30

40

50

S3

60

70

-10

-5

0

o

2 θ / (CuKα)

5

10

-1

Velocity / mm s

Fig. 1. Characteristic XRD patterns and 57 Fe Mössbauer spectra of samples S1, S2 and S3. Measurements taken at 20 ◦ C.

 = 0.99 mm s−1 for doublet 2. In accordance with XRD analysis of sample S1 these quadrupole doublets can be assigned to ␤-FeOOH phase. The Mössbauer spectrum of ␤-FeOOH is discussed in reference literature [19–22]. Barrero et al. [23] also investigated the Mössbauer spectrum of ␤-FeOOH at different temperatures. The RT Mössbauer spectrum was fitted as a superposition of two quadrupole doublets, while at low temperature the spectra were fitted as a superposition of four sextets. Two sextets were assigned to two nonequivalent Fe3+ sites located close to the Cl− ions, while the other two sextets were assigned to Fe3+ sites located at vacant chloride sites. A small amount of chloride ions enter the ␤-FeOOH structure and for that reason chloride ions cannot be removed 310

211

from ␤-FeOOH particles by simply washing the precipitate. The Mössbauer spectra of samples S2 and S3 are characterized by one sextet with parameters that can be assigned to ␣-Fe2 O3 phase. The increase in hyperfine magnetic field (HMF) from 51.2 to 51.5 T for samples S2 and S3 can be generally assigned to the crystal ordering of ␣-Fe2 O3 in the particles. The measured value HMF = 51.5 T is close to that recorded for well-crystallized ␣-Fe2 O3 [24]. Fig. 2 shows characteristic XRD of samples S4, S5, S8 and S9 obtained by precipitation in the presence of AOT. Samples S4 and S8 were identified as ␤-FeOOH phase, whereas samples S5 and S9 were identified as Fe2 O3 phase. The corresponding Mössbauer spectra are shown in Figs. 3 and 4. Due to broadening of spectral lines of

S8

S4 211

310

400

Relative intensity

220

521 411 710 510 600 611 541 420 321 312 431 002 440 301

330

104

330

400

S5

521

301

710 600 611 541 431 312 440 002

411

420 321 510

S9

104

110 110 116

012 113

116

024 214

Al

024

012

300

018

20

30

40

50 o

2 θ / (CuKα)

60

Al

70 20

300 214

113

30

40

018

50

60

o

2 θ / (CuKα)

Fig. 2. Characteristic XRD patterns of samples S4, S5, S8 and S9, recorded at 20 ◦ C.

70

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44

S6

Count rate / a.u.

S4 -2

-1

0

1

2 -10

-5

-5

0

5

10 -10

Velocity / mm s Fig. 3.

57

5

10

5

10

S7

S5 -10

0

-5

0

-1

Fe Mössbauer spectra of samples S4, S5, S6 and S7, recorded at 20 ◦ C.

samples S5 and S9 (␣-phase) the corresponding Mössbauer spectra were fitted as superposition of two sextets. With prolonged time of hydrolysis the broadening of spectral lines decreases indicating better ordering of ␣-Fe2 O3 phase. 3.2. FT-IR and optical spectroscopy FT-IR spectra of all precipitated samples confirmed the presence of ␤-FeOOH or Fe2 O3 phase in accordance with XRD and Mössbauer spectroscopy. For illustration, FT-IR spectra of samples S2 and S9 are shown in Fig. 5. Both FT-IR spectra can be assigned to ␣-Fe2 O3 . Iglesias and Serna [25] investigated the impact of the shape of ␣Fe2 O3 particles on their IR spectrum. The authors recorded IR bands at 575, 483, 385 and 360 cm−1 for ␣-Fe2 O3 spheres, whereas the IR bands at 650, 525, 440 and 300 cm−1 were recorded for ␣-Fe2 O3 laths. In the FT-IR spectrum of sample S9 additional IR bands of very weak intensities at 1210, 1128, 1033 and 980 cm−1 were recorded. The IR bands at 1210, 1128 and 1033 cm−1 can be related to sulfate groups. The ␯3 (SO4 ) fundamental vibration is split due to the formation of a surface bidentate bridging complex between the sulfate group and iron. The IR band at 980 cm−1 can be associated with ␯1 (SO4 ) vibration. The sulfonate group shows a tendency to transform into sulfates in aqueous media at elevated temperature. The IR band of small intensity at 904 cm−1 could be related with residual OH groups and lower degree of crystallinity of ␣-Fe2 O3 particles in sample S9. Fig. 6 shows the optical spectra of samples S1–S10 in the absorption mode. In accordance with previous measurements the spectra of samples S1, S4 and S8 can be assigned to ␤-FeOOH phase. A very weak and broad IR band centered at ∼910 nm for these samples is due to the transition 6 A1 → 4 T1 in ␤-FeOOH [26]. Very strong and broad shoulders at 477 and 380 nm can also be assigned to ␤-FeOOH, showing a tendency to decrease the shoulder’s relative intensity at 380 nm in relation to the shoulder at 477 nm in series S1, S4 and S8. This effect noticed in the optical spectra of samples S4 and S8 is a direct consequence of the presence of AOT during

the forced hydrolysis of FeCl3 solutions. The shoulder at ∼240 nm is typical of charge transfer. Changes in the shape of the optical spectra S2, S3, S5–S7, S9 and S10 can also be related to precipitation conditions and the properties of the precipitates. Phase characterization of all these samples shows only the presence of ␣-Fe2 O3 as a single phase. The optical spectra of samples S2 and S3 show two very strong and broad bands centered at 897 and 893 nm, respectively. The bands can be assigned to the 6 A1 → 4 T1 transition in ␣-Fe2 O3 . For the same samples two very strong shoulders were noticed at 661 and 586 nm. The optical spectra of ␣-Fe2 O3 in samples precipitated in the presence of AOT showed distinct changes in the shape and positions of spectral bands. For samples S5, S6 and S7 the transition 6 A1 → 4 T1 is shifted to 867 nm, whereas the transition 6 A1 → 4 T2 is centered at 645 nm. The spectral band at 555 nm noticed for sample S5 is shifted to 521 nm for sample S6 and 528 nm for sample S7 (2(6 A1 ) → 2(4 T1 ) transition). Sample S9 showed the optical transition at 880 nm and 550 nm, with shoulders at 640 and 400 nm. The shoulder at 400 nm is due to the 6 A1 → 4 T2 transition. Sherman and Waite [27] investigated the spectra of Fe(III)oxides and -oxyhydroxides in the near-IR and near-UV regions. The origin of spectral bands was discussed in terms of the ligand field and ligand-to-metal charge transfer transitions, which energies are similar to those found in other Fe(III) oxygen compounds. The Fe3+ ligand field transitions are strongly influenced by the magnetic coupling of adjacent Fe3+ cations in the crystal structure of these iron oxide compounds. Sherman et al. [28] also investigated the UV/vis reflectance spectra of Fe(III)-oxide and -oxyhydroxides and the potential of this method to differentiate Fe(III)-oxide from -oxyhydroxide polymorphs. The measurement temperature is an important factor when the identification of these phases is based on the position of the 6 A1 → 4 T1 absorption transition. This investigation is useful in the interpretation of optical spectra that can be recorded on the Martian surface. Various factors that may influence the optical spectra of iron oxides are discussed in reference literature [29–31].

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45

S2

-1

0

1

2

Count rate / a.u.

-2

Relative transmittance

S8

515 475 564

S9 980 904 1210

1128 1033 481 980

S9

1210

1128

1033

1200

1500

578

1000

1250

1000

750

500

-1

Wave number / cm

Fig. 5. FT-IR spectra of samples S2 and S9, recorded at RT.

240

S10 -10

-5

0

Velocity / mm s Fig. 4.

57

5

242

10

-1

242

380

477

3.3. FE-SEM

Absorbance / a.u.

910

640

910

630

910

586 661

897

586 661

893

360 490

Fe Mössbauer spectra of samples S8, S9 and S10, recorded at 20 ◦ C.

Fig. 7 shows the FE-SEM images of samples (a) S1, (b and c) S2 and (d) S3. Fig. 7a shows ␤-FeOOH particles in the form of rods and spindles and it can be seen that these particles possess their own substructure. This substructure consists of smaller elongated ␤-FeOOH particles. The autoclaving of 0.2 M FeCl3 solution for 6 h yielded ␣-Fe2 O3 double-spheres with ring (Fig. 7b). These ␣-Fe2 O3 particles also showed the substructure. The smaller elongated ␣-Fe2 O3 particles are directed from the center to the surface of the double-spheres, whereas the particles in the ring are oriented parallel to their surface. Fig. 7c shows the image of one broken double-sphere particle (sample S2) at higher optical magnification. It can be clearly seen that the bulk of this particle consists of pseudospherical nanoparticles of ␣-Fe2 O3 (average diameter 14 nm). With a prolonged autoclaving (3 d) these nanoparticles dissolve, providing material for crystal growth of much larger elongated ␣-Fe2 O3 subparticles, as shown in the FE-SEM image in Fig. 7d. Fig. 8 shows the FE-SEM images of samples S4, S5, S6 and S7 prepared in the presence of AOT (Table 1). The forced hydrolysis of 0.2 M FeCl3 after 2 h and in the presence of AOT yielded ␤FeOOH particles of varying shape (sample S4) compared with those

645

360 482

400

555

400 521

S1 S4 S8 S2 S3

867

645

S5

645

867

645

867

640

880

640

880

528 400 400

550

S6 S7

550

S9 S10

200

400

600

800

1000

1200

Wave length / nm Fig. 6. Optical spectra of samples S1–S10, recorded at 20 ◦ C.

46

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Fig. 7. FE-SEM images of samples (a) S1, (b and c) S2 and (d) S3.

Fig. 8. FE-SEM images of samples (a) S4, (b) S5, (c) S6 and (d) S7.

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47

formed a porous microstructure (Fig. 9c). Some ␣-Fe2 O3 particles showed sizeable holes inside them. The presented results show that the addition of AOT plays a crucial role in the development of porous ␣-Fe2 O3 . The preferential adsorption of sulfonate/sulfate groups controls the shape of ␤-FeOOH and that of the ␣-Fe2 O3 particles, as well as the development of a porous microstructure. It can be suggested that the organic part of AOT also plays a certain role in the formation of the ␣-Fe2 O3 microstructure. The influence of AOT starts at a very early stage of Fe3+ hydrolysis and continues with the precipitation of ␤-FeOOH, its dissolution, and the crystal growth of ␣-Fe2 O3 which with the prolonged time of autoclaving transforms into a porous microstructure. 4. Conclusions • The forced hydrolysis of 0.2 M FeCl3 solutions at 160 ◦ C yields ␤-FeOOH particles for short times, whereas for longer times ␤-FeOOH transforms to ␣-Fe2 O3 through the dissolution/recrystallization process. • ␣-Fe2 O3 particles produced upon 6 h of autoclaving in the absence of AOT are formed as double spheres with ring. The bulk of these particles is filled with nanosize ␣-Fe2 O3 particles generating Fe3+ ions for further much larger ␣-Fe2 O3 subcrystals, as observed upon 3 d of autoclaving. • The forced hydrolysis of 0.2 M FeCl3 solutions at 160 ◦ C is influenced by AOT. The morphologies of ␤-FeOOH and ␣-Fe2 O3 particles are changed in the presence of AOT. A microporous structure of ␣-Fe2 O3 particles develops. • The action of AOT can be primarily assigned to the preferential adsorption of sulfonate/sulfate groups onto the particles and less so to the effect of the organic part of AOT. Acknowledgments This study was supported by Ministry of Science, Education and Sport, Croatia, project: 098-0982904-2952 and project “Investigations of factors influencing the properties of metallic and metal oxide nanoparticles”, Croatian-Austrian bilateral scientific cooperation. References

Fig. 9. FE-SEM images of samples (a) S8, (b) S9 and (c) S10.

obtained without AOT (sample S1) in the same experimental conditions. The particles in sample S4 (Fig. 8a) were predominantly in the form of square rods. The FE-SEM image of sample S5 did not show the formation of double spheres with ring like sample S2 (Fig. 7b). Upon 1 d of autoclaving porous ␣-Fe2 O3 particles are formed, showing a substructure consisting of ␣-Fe2 O3 rods. This porous ␣-Fe2 O3 microstructure is more pronounced upon 3 d of autoclaving (sample S7), as shown in Fig. 8d. Furthermore, for a higher amount of AOT added to the precipitation system (Table 1) the development of porous ␣-Fe2 O3 microstructure is visible (Fig. 9). Fig. 9a shows a large section of laterally arrayed ␤-FeOOH particles. After 6 h of autoclaving the ␣Fe2 O3 particles also showed a substructure (Fig. 9b) which after 1 d

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