Morphologies of zinc oxide particles and their effects on photocatalysis

Chemosphere 51 (2003) 129–137 www.elsevier.com/locate/chemosphere

Morphologies of zinc oxide particles and their effects on photocatalysis Di Li, Hajime Haneda

*

Advanced Materials Laboratory, National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba 305-0047, Japan Received 15 January 2002; received in revised form 23 May 2002; accepted 6 November 2002

Abstract ZnO powders with different morphologies were synthesized by alkali precipitation, organo-zinc hydrolysis, and spray pyrolysis. Acetaldehyde decomposition was used as a probe reaction to evaluate the photocatalysis of these ZnO powders. We investigated the relationship between photocatalytic activity and crystallinity, surface area, or morphology. Results indicate that the photocatalytic activity of ZnO powder depends on crystallinity rather than surface area for the same original ZnO powders prepared by equal conditions other than the difference in calcination temperature. However, no direct relationship between photocatalytic activity and crystallinity or surface area was found for the differently original ZnO powders prepared by different methods, or the same method with different conditions. Instead, we find that the particle morphology significantly affects its photocatalysis. Ó 2003 Elsevier Science Ltd. All rights reserved. Keywords: Zinc oxide; Morphology; Crystallinity; Photocatalysis; Acetaldehyde

1. Introduction As a well-known photocatalyst, ZnO has received much attention in the degradation and complete mineralization of environmental pollutants (Richard et al., 1997; Driessen et al., 1998; Villase
oor et al., 1998; Yeber et al., 2000). Since ZnO has almost the same band gap energy (3.2 eV) as TiO2 , its photocatalytic capacity is anticipated to be similar to that of TiO2 . However, in the case of ZnO, photocorrosion frequently occurs with the illumination of UV lights, and this phenomenon is considered as one of the main reasons resulting in the decrease of ZnO photocatalytic activity in aqueous solutions (Dijken et al., 1998; Neppolian et al., 1999). However, for the application in the gas phase, this disadvantage should not exist (Yamaguchi et al., 1998). Additionally, some studies have confirmed that ZnO

*

Corresponding author. Tel.: +81-298-51-3354x575/3203; fax: +81-298-55-1196. E-mail address: [email protected] (H. Haneda).

exhibits a better efficiency than TiO2 in photocatalytic degradation of some dyes, even in aqueous solution (Gouvea et al., 2000; Dindar and Icli, 2001). ZnO particle morphologies are very complex and diversiform in comparison with that of TiO2 . So far, monodispersed ZnO particles with well-defined morphological characteristics, such as spherical, ellipsoidal, needle, prismatic, and rod-like shapes have been obtained. Aggregates composed of these basic shape particles have also been achieved. The methods used for synthesis of these ZnO powders include alkali precipitation (Chittofrati and Matijevic, 1990; Verges et al., 1990; Trindade et al., 1994), thermal decomposition (Auffredic et al., 1995), hydrothermal synthesis (Lu and Yeh, 2000), organo-zinc hydrolysis (Kamata et al., 1984), spray pyrolysis (Milosevic et al., 1994) and other routes. However, the effects of ZnO particle morphologies on its photocatalysis have not been discussed extensively so far. Crystallinity is regarded as one of important factors for determining photocatalysis (Villase
oor et al., 1998; Bamwenda and Arakawa, 2001; Bamwenda et al., 2001;

0045-6535/03/$ - see front matter Ó 2003 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 5 - 6 5 3 5 ( 0 2 ) 0 0 7 8 7 - 7

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D. Li, H. Haneda / Chemosphere 51 (2003) 129–137

Jing et al., 2001). However, the implication about this terminology is ambiguous. It is often estimated qualitatively by the peak intensity or FWHM in X-ray diffractograms (Bamwenda and Arakawa, 2001; Bamwenda et al., 2001). On the contrary, its quantitative evaluation is less reported. In this study, ZnO powders with different morphologies were synthesized. The photocatalysis of these ZnO powders in acetaldehyde decomposition was investigated. Crystallinity of ZnO powders was calculated from lattice strain data. The dependence of the photocatalytic activity on crystallinity, surface area, and particle morphology or acetaldehyde adsorptivity is discussed in detail.

solution (150 ml) was added dropwise to a 0.2 M Zn[OC(CH3 )3 ]2 hexane solution (100 ml) under argon atmosphere at 298 K. ZnO powder was obtained by centrifugation and drying under reduced pressure (<10
4 Pa) at 353 K for 4 h. Spray pyrolysis: A 0.05 M Zn(CH3 COO)2 (WAKO, 99%) aqueous solution was atomized by a nebulizer and then passed through a high temperature quartz tube under the suction of an aspirator. The pyrolysis proceeded quickly as droplets passed through the quartz tube at high temperature within a second. The formed ZnO powder was collected with a glass filter installed at the end of the quartz tube. 2.2. Characterization

2. Experimental 2.1. Synthesis of ZnO powders Alkali precipitation: The main procedures of this method consisted of mixing at room temperature a Zn(NO3 )2 (Wako Pure Chemical Industries, Ltd., Japan (WACO), 99%) aqueous solution with an ammonia or triethanolamine (TEA) (WAKO, 99%) or hexamethylenetetraamine (HMT) (WAKO, 98%) aqueous solution in proper volumes, followed by aging the mixed solution at a constant temperature for constant periods of time. Finally, ZnO powder was obtained after filtration and drying under reduced pressure (<10
4 Pa) at 353 K for 4 h. The other concrete preparation conditions will be shown in Table 1. Organo-zinc hydrolysis: Zinc t-butoxide was used as precursor of organo-zinc, which was prepared by mixing t-butanol (WAKO, 99%) and diethylzinc hexane solution (Aldrich Chemical Co., Inc.,) under nitrogen at 273 K. The hydrolysis proceeded as a 0.5 M H2 O ethanol

The particle morphology was observed by a JEOL S-5000 scanning electron microscopy (SEM) (Hitachi, Japan). The XRD patterns were recorded on a Philips PW1800 X-ray diffractometer (Philips Research Laboratories, The Netherlands) with monochromatized Cu Ka radiation operated at 40 kV/50 mA. The BET surface area of ZnO sample was determined by N2 adsorption and desorption at 77 K in a Model 4201 Automatic Surface Area Analyzer (Beta Scientific Corp., USA). 2.3. Photocatalytic system Photocatalytic decomposition of acetaldehyde was carried out in a closed circulation system (250 cm3 ) interfaced to a gas chromatograph (HITACHI G-35A) with a TCD and a PID detector for acetaldehyde and CO2 analysis, respectively. Sampling was performed at intervals of 15 min. Prior to catalytic experimentation, the ZnO samples (0.1000 g) were outgassed under vacuum of 10
5 Pa at 398 K for 2 h. Then a gas mixture

Table 1 Summary of ZnO synthesis Method

Precursor

Synthesized conditions

Alkali precipitation

0.01 M ZnðNO3 Þ2 þ 0:10 M TEA 0.01 M TEA 0.02 M HMT

Aging at 363 K for 3.0 h Drying at 373 K for 48 h

0.03 M NH4 OH

BETa (m2 g
1 )

Particle morphologyb (sizec )

7.3  0.3 9.3  0.3

SP (3.5  0.28 lm) EA (3.0  0.22 lm)

12.1  0.7

RP (0:8  0:07  0:24  0:05 lm)

3.2  0.2

NA (5.5  0.35 lm)

Organo-zinc hydrolysis

0.05 M Zn[OC(CH3 )3 ]2

At 298 K at Ar

40.3  1.7

SC (0.14  0.03 lm)

Spray pyrolysis

0.05 M Zn(CH3 COO)2

873–1273 K

28.7  0.9

IP (0.9  0.09 lm)

a

As-synthesized samples. SP: spherical particle; EA: ellipsoidal aggregate; RP: rod-like particle; NA: needle aggregate; SC: single crystallite particle; IP: irregular particle. c Average size of 200 particles from SEM photographs. b

D. Li, H. Haneda / Chemosphere 51 (2003) 129–137

of 93.3 kPa CH3 CHO–He (930 ppm) and 13.3 kPa O2 was introduced. The samples were irradiated from the outside of the reactor by a 200 W Hg–Xe lamp (Hayashi, LA300UV-1, kmax ¼ 365 nm) with an incident intensity of 20 mW cm
2 . 2.4. Adsorption system The adsorption experiment was performed volumetrically at 298 K in the same equipment where the photocatalytic reaction was carried out. The adsorbed amount of acetaldehyde was calculated from the change of acetaldehyde concentration in gas phase, i.e. subtracting the residual acetaldehyde in gas phase after the adsorption equilibrated from the introduced acetaldehyde.

3. Results and discussion 3.1. Powder characterization 3.1.1. Morphology Six ZnO powders were synthesized by three methods. Preparation conditions, BET surface areas and particle sizes of as-synthesized samples are listed in Table 1. The SEM images of calcined samples are shown in Fig. 1, the used calcination temperature for each sample corresponded to its optimum catalytic activity that would be discussed below. The monodispersed spherical particles with a rough surface (SP, Fig. 1a), intertwined ellipsoidal aggregates (EA, Fig. 1b), rod-like particles (RP, Fig. 1c), and intertwined needle aggregates (NA, Fig. 1d) were obtained from alkali precipitation by changing the used alkali and alkaline concentration. Here, ÔmonodispersedÕ refers only to the particle shape because the size was variable in many cases. SP and EA were prepared from the same Zn(NO3 )2 –TEA system but different TEA concentration; RP was from Zn(NO3 )2 –HMT solution, and NA was from Zn(NO3 )2 –NH4 OH system. The morphology of ZnO particles from organo-zinc hydrolysis is given in Fig. 1e, exhibiting that most of the particles are of similarly spherical shape and the average size of the particles (135 nm) is much smaller than that from alkali precipitation (Fig. 1a–d). Additionally, the particle size is almost the same with its crystallite size (127 nm) determined by XRD results, therefore, one particle is considered as one crystallite. This ZnO powder was symbolized as SC meaning single crystallite particle. ZnO powder prepared by spray pyrolysis (Fig. 1f) has irregular particles (IP). No obvious change in morphology was observed when the pyrolysis temperature was elevated from 873 to 1273 K. The internal structure of particles seems to be hollow since holes can be found on some particles.

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In addition, we must emphasize here that the ‘‘particle’’ in this study means a group of ‘‘elementary particles’’, and the ‘‘elementary particles’’ refers to ‘‘crystallites’’. The crystallite size of the samples, given in Fig. 1, is 37.5 nm for SP-1073, 35.4 nm for EA-873, 76.8 nm for RP-873, 49.9 nm for NA-873, 127 nm for SC-1073, and 18.3 nm for IP-1073 from the XRD results. There is disagreement with particle size for each sample except SC. Therefore, the synthesized ZnO samples except SC (single crystallite particle) are aggregate particles that are composed of some crystallites. 3.1.2. XRD patterns and crystallinity The XRD patterns of six ZnO powders were determined and similar results were obtained. Here, the SC was selected as an example to reveal the effect of calcination temperature on the XRD patterns, shown in Fig. 2. There are no noticeable changes in the crystallographic patterns and intensity ratios among peaks. But, a clear sharpening and attenuation of peaks can be observed with increasing the calcination temperature. In general, the peak sharpening in XRD patterns can be ascribed to the increasing of crystallite size or the decreasing of lattice strain or both. Crystallite size (D) as well as lattice strain (g) of the calcined powders were calculated by the X-ray line broadening technique performed on the (1 1 2 0) diffraction of ZnO lattice using computer software (APD 1800, Philips Research Laboratories) based on the so-called Hall Eq. (Klug and Alexander, 1954): b cos h ¼ Kk=D þ 2g sin h

ð1Þ

where h is the Bragg angle of diffraction lines; K is the shape factor of the average crystallite (K ¼ 0:90); k is the wavelength of incident X-ray (k ¼ 0:15406 nm). Corrections of instrumental and spectral broadening were made by means of data from a high purity SiO2 (99.9995%, Aldrich) with quartz crystal form after calcination at 1273 K for 24 h. Therefore, the corrected half-width was given by: b2 ¼ b2m
b2SiO2

ð2Þ

where bm is the measured half-width and bSiO2 is the half-width of the silica sample with a known crystallite size of larger than 150 nm. Accordingly, crystallite size (D) and lattice strain (g) were calculated by Eqs. (1) and (2). The results are given in Fig. 3 for calcined SC samples, indicating that the sharpening of the peaks with the calcination temperature results not only from the increasing of crystallite size but also from the decreasing of lattice strain. However, we will consider the BET surface area instead of the crystallite size in our further discussion since less crystallite surface is accessible for reactant molecules. This consideration will be proved later.

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D. Li, H. Haneda / Chemosphere 51 (2003) 129–137

Fig. 1. SEM images of calcined ZnO powders: (a) SP-1073; (b) EA-873; (c) RP-873; (d) NA-873; (e) SC-1073; (f) IP-1073. The numbers represent the calcination temperature except IP-1073, in which the number refers to pyrolysis temperature.

On the other hand, we cited ‘‘crystallinity’’ terminology instead of lattice strain since crystallinity is frequently used in photocatalytic studies as stated above. Here, the crystallinity was calculated by: Cm ¼ Cs gs =gm

ð3Þ

here, the Cm and Cs are the crystallinity of the measured and standard samples, respectively. The gm and gs are the lattice strain of the measured and standard samples, respectively. The SC, which was calcined at 1273 K for 2 h, was selected as the standard sample. Its lattice

strain (gs ) is 0.053% (Fig. 3), and its crystallinity (Cs ) is assumed to be 100%. 3.2. Photocatalysis 3.2.1. Dependence of photocatalytic activity on calcination temperatures All ZnO powders except the IP were calcined in air at different temperatures for 2 h in order to get an optimum photocatalytic performance for each sample. Special attention was also paid to the carbonaceous substance

D. Li, H. Haneda / Chemosphere 51 (2003) 129–137

Intensity / 1000 cps

10 8 6 1273K

4

1073K 873K

2

673K 473K

0 20

30

40

50 60 2 θ/ deg

70

80

500

1.0

400

0.8

300

0.6

200

0.4

100

0.2

Lattice strain / %

Crystallite size / nm

Fig. 2. XRD patterns of SC ZnO powders obtained by calcination at different temperatures.

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temperature at which no carbonaceous substance residues were detected for that sample. In this study, the initial rate R0 (mol min
1 ) of CO2 formation was used to stand for the photocatalytic activity if not otherwise describe. The results in Fig. 4 clearly show that both synthesis methods and calcination temperatures have a significant effect on the ZnO photocatalytic activity. The R0 increases with the calcination temperatures, however, max. R0 exists for all samples. The decreasing of R0 as the sample was calcinated at above 873 K is probably associated with the drastic change of particle surface, including sintering among particles and surface defects. Although we need additional evidence for the last assumption, the sintering among particles was confirmed by SEM image observation. The max. R0 Õs values for the six ZnO samples decrease in the following order: SC-1073 > NA-873 > SP-1073 > IP-1073 > EA-873 > RP-873. Here, the numbers represent the calcination temperature except IP-1073, in which the number refers to pyrolysis temperature. For the photocatalytic activity, besides the initial rate, the time-course of CO2 yields (Obuchi et al., 1999), shown in Fig. 5, and quantum yield for CO2 formation (Sopyan et al., 1996) after the reaction started for 1 h were also calculated for these samples on which the max. R0 was obtained. They are listed in Table 2. The magnitude orders of the CO2 yields and quantum yields for CO2 formation are the same with that of max. R0 s, confirming that using the initial rate R0 (mol min
1 ) of CO2 formation to evaluate the photocatalytic activity is plausible. For comparison, the photocatalytic activity of TiO2 powder (Degussa P25) was also determined as a reference sample since TiO2 is a photocatalyst that

5 0.0

0 673

873

1073 1273

Calcination temperature / K Fig. 3. Dependences of crystallite size and lattice strain on calcination temperatures for SC ZnO powders.

residues on the ZnO samples. First, we checked each ZnO sample calcined at 673 K by blank experiment (only oxygen gas was introduced to reaction system). If CO2 production was found in blank experiment, this sample calcined at 673 K was not considered for our research and the calcination temperature for this sample was then raised to 873 K. On the contrary, if CO2 production was not found in blank experiment, a lower calcination temperature (473 K) was used. Then, the samples calcined at the newly used temperature were checked again until we found the lowest calcination

R0 /10 -7 mol min-1

473

4 SC 3 NA 2 IP EA

1

SP RP 0 473

673

873

1073 1273

Calcination temperature / K Fig. 4. Dependence of photocatalytic activity on calcination temperatures for ZnO powders.

D. Li, H. Haneda / Chemosphere 51 (2003) 129–137

has been widely studied (Yamashita et al., 1996; Anpo, 2000). The photocatalytic activity of TiO2 , shown in Fig. 5 and Table 2, is higher than that of any one of the synthesized ZnO samples. The above results also show that the single crystallite particles (SC-1073) from organo-zinc hydrolysis are the best for photocatalytic decomposition of acetaldehyde among ZnO samples. For the ZnO powders from alkali precipitation, a better result was observed on NA-873 sample in which ammonia was used than on SP-1073, EA-873, or RP-873 sample in which organic base (TEA or HMT) was applied. ZnO powder prepared by spray pyrolysis has been reported to demonstrate a better photocatalytic performance for degradation of trichloroethylene even in comparison with the commercial TiO2 (Degussa P25) (Jung et al., 1997). However, similar results were not obtained for decomposition of acetaldehyde.

5

100 BET

4

80

3

60 C

2

40

1

20

0

0 473

3.2.2. Photocatalytic activity versus BET surface area, crystallinity and particle morphology In order to evaluate the factors that influence ZnO photocatalysis, the relationship between photocatalytic

R0

BET / 3 m2 g-1 or C / %

Fig. 5. Plots of the increase in CO2 yield with irradiation time on some representative ZnO powders. t ¼ 0 refers to the yield before the UV light was switched on.

activity (R0 ) and surface area (BET), crystallinity (C), or particle morphology was examined. Firstly, the investigation was carried out on the same original ZnO samples. Here, the Ôsame original samplesÕ means that the samples were synthesized by the same conditions other than the difference in calcination temperature. The effects of BET and C on R0 for SC sample are given in Fig. 6. R0 rises with the increasing of the crystallinity irrespective of the decreasing of BET, indicating that crystallinity, rather than BET surface area, has a pronounced effect on photocatalytic activity for the same original ZnO particles. Generally, BET surface area is considered as a key parameter since it can significantly influence the catalytic performance for a thermocatalyst. However, crystallinity is more important for a photocatalyst. This indicates that a high surface reactivity of ZnO sample is not necessarily related to a high surface area. An abundance of specific sites, which strongly depends on the preparation route, is the most important feature for an active ZnO. This result is in agreement with the conclusion proposed by other researchers (Bowker et al., 1983; Bolis et al., 1989). Secondly, the investigation was performed on the differently original ZnO samples. Here, the Ôdifferently original samplesÕ means that the samples were synthe-

-7

x

R0 /10 mol min-1

134

673

873

1073 1273

Calcination temperature / K Fig. 6. Relationship between photocatalytic activity and BET or crystallinity for SC ZnO powders.

Table 2 Photocatalytic activities of differently original ZnO powders Photocatalytic activity

SC-1073

NA-873

SP-1073

IP-1073

EA-873

RP-873

P25

R0 (10
7 mol min
1 ) CO2 yield (%) Quantum yield (%)

4.4 73.9 3.8

3.1 61.5 3.2

2.1 51.5 2.6

1.9 46.8 2.4

1.4 35.3 1.8

1.0 26.3 1.4

6.3 84.3 4.3

D. Li, H. Haneda / Chemosphere 51 (2003) 129–137

40

660

4

SXRD

NA

3

IP

2

SP

EA

1 RP

0 0

5

10 BET / m2 g-1

15

20

Fig. 7. Photocatalytic activity of ZnO powders as a function of BET surface area.

D

30

50

20

40

10

30

Crystallite size D /nm

SC

SBET or SXRD / m2 g-1

-7

R0 /10 mol min-1

5

135

SBET 0

20 400

600

800

1000

Calcination temperature / K

-7

R0 /10 mol min-1

5 Fig. 9. Dependences of the BET surface area, calculated surface area and crystallite size on calcination temperatures for NA ZnO powders.

SC

4

NA

3 IP

2 1

SP EA

RP

0 0

20

40 C /%

60

80

Fig. 8. Photocatalytic activity of ZnO powders as a function of crystallinity.

sized by the different methods or the same method but different conditions. R0 versus BET and R0 versus C for the six kinds of ZnO particles are shown in Figs. 7 and 8, respectively. Here, the R0 is the maximum value for each sample. No systematic trend in the variation of R0 with BET or C was noted. This result suggests that we cannot evaluate the photocatalytic activity of a ZnO sample only by its BET surface area or crystallinity if the sample originates from different preparation conditions or different methods. However, as mentioned above, the prepared six ZnO powders exhibit an obvious difference in particle morphology. This decisive difference in morphology probably influences the photocatalytic activity because the exposed crystal faces or the ratios among exposed crystal faces are noticeably different for the ZnO particles composed with different crystallite forms (Bowker et al., 1983; Bolis et al., 1989). Here, we emphasized the particle morphology rather than crystallite form since less crystallite surface is accessible for reactant molecules. Fig. 9 gives the dependences of experimental surface area (SBET ), calculated surface area (SXRD ), and crystallite size on the different calcination temperatures. The

result shows that experimental surface area is much smaller than that calculated by crystallite size from XRD, e.g. the SBET is only 2.2 m2 g
1 for NA-873, but, the SXRD is up to 20.5 m2 g
1 from its crystallite size of 49.9 nm, indicating that only 10.7% of the crystallite surface is accessible for nitrogen molecules. This feature is remarkable for the case of ZnO powder (Lou€er et al., 1983; Lou€er et al., 1984). Additionally, the experimental BET surface area is only considered as an extra surface area rather than interior surface area since no porous ZnO powders were observed in this study. Yamaguchi and co-workers have shown that photocatalytic activity of ZnO thin films increases with the peak intensity ratio of (1 0 1 0) to (0 0 0 2) and suggested that the decomposition of acetaldehyde would proceed faster on the oxygen–zinc alternating atomic layer, i.e. (1 0 1 0) face than on the oxygen–oxygen monotonous atomic layer (0 0 0 1) (Yamaguchi et al., 1998). Moreover, Bowker investigated the interaction of methyl formate and ethanol with polycrystalline ZnO by temperature-programmed reaction and confirmed that methyl formate and ethanol, which have a similar molecular structure as acetaldehyde, adsorb and decompose on the cation–anion dual site (Bowker et al., 1982). If these conclusions are plausible, the photocatalytic activity of ZnO particles should strongly depend on the particle morphologies on which the specified crystal faces expose. Acetaldehyde adsorptivity on ZnO powders was determined to confirm this deduction since it is difficult to analyze directly the exposed crystal faces by TEM observation. Accordingly, the adsorbed amounts of acetaldehyde on differently original ZnO samples are shown in Fig. 10. Here, the initial rate R0 of CO2 formation, as described

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References

1.5

NA

SC

SP 1.0

-6

R0 /10 mol min-1 m-2

2.0

0.5

EA RP

0.0

IP 2 6 8 4 -6 Adsorbed amount /10 mol m-2

10

Fig. 10. Dependence of photocatalytic activity on adsorbed amount of acetaldehyde for ZnO powders.

above, was divided by BET surface area to stand for the photocatalytic activity in order to elucidate the effectiveness caused by particle morphology. The adsorbed amount of acetaldehyde was also presented in an absolute amount per square meter BET surface area. A good linearity between the photocatalytic activity and the adsorbed amount of acetaldehyde was observed. The samples that adsorb larger amount of acetaldehyde have high photocatalytic activity. This supports indirectly the dependence of ZnO photocatalysis on particle morphology.

4. Conclusions The morphology of ZnO particles is very sensitive to the preparation conditions and preparation methods. The photocatalysis of ZnO powder depends on crystallinity for the same original ZnO samples and on the particle morphology or acetaldehyde adsorptivity for the differently original ZnO samples.

Acknowledgements This study was supported by a Millennium Project, Development of Catalysts for the Decomposition of Environmental Pollutants, from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

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