Electrical conductivity of thermally hydrogenated nanodiamond powdersTakeshi Kondo, Ioannis Neitzel, Vadym N. Mochalin, Junichi Urai, Makoto Yuasa et al. Citation: J. Appl. Phys. 113, 214307 (2013); doi: 10.1063/1.4809549 View online: http://dx.doi.org/10.1063/1.4809549 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v113/i21 Published by the American Institute of Physics. Additional information on J. Appl. Phys.Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors

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Electrical conductivity of thermally hydrogenated nanodiamond powders

Takeshi Kondo,1,2 Ioannis Neitzel,3 Vadym N. Mochalin,3 Junichi Urai,1 Makoto Yuasa,1,2

and Yury Gogotsi3,a)

1Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science,2641 Yamazaki, Noda, Chiba 278-8510, Japan2Research Institute for Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda,Chiba 278-8510, Japan3Department of Materials Science and Engineering and A.J. Drexel Nanotechnology Institute,Drexel University, 3141 Chestnut Street, Philadelphia, Pennsylvania 19104, USA

(Received 7 March 2013; accepted 20 May 2013; published online 7 June 2013)

Electrical properties of detonation diamond nanoparticles (NDs) with individual diameters of �5 nm

are important for many applications. Although diamond is an insulator, it is known that hydrogen-

terminated bulk diamond becomes conductive when exposed to water. We show that heating ND in

hydrogen gas at 600–900 �C resulted in a remarkable decrease in resistivity from 107 to 105 X cm,

while the resistivity was essentially unchanged after treatment at 400 �C and lower temperatures.

Fourier Transform Infrared Spectroscopy and X-ray photoelectron spectroscopy (XPS) studies

revealed that hydrogenation of ND occurs at 600–900 �C, suggesting that the decrease in resistivity is

based on transfer doping at the hydrogenated ND surface. Oxidation of the hydrogenated sample at

300 �C recovers resistivity to its original value. The resistivity of treated ND as a function of the O/C

atomic ratio showed a transition from resistive (O/C ratio > 0.033) to conductive (O/C ratio < 0.033)

state. This is consistent with the idea that the change in the resistivity is caused by the shift of the

valence band maximum to above the Fermi level due to the dipole of the C-H bonds leading to

transfer doping. VC 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4809549]

I. INTRODUCTION

Diamond nanoparticles (NDs) fabricated by detonation

of explosives have superior chemical stability, hardness, and

biocompatibility.1 Based on these excellent properties, many

applications of NDs are being developed in the fields of

nanocomposites,2–5 electrochemical energy storage,6 biologi-

cal fluorescent probes,7–10 and drug delivery carriers.11–13

Nanoparticles have a very high surface-to-volume (surface-

to-weight) ratio; therefore, they exhibit surface-dependent

properties more prominently than their bulk counterparts.

Diamond surface can be modified by various treatments,

including dry (thermal and plasma) and wet (chemical, elec-

trochemical, and photochemical) techniques.14 Hydrogenated

diamond surfaces are known to exhibit unique properties

including hydrophobicity15,16 and, in the case of powder, a

high positive zeta potential.17 Negative electron affinity18,19

and surface conductivity20 were also reported for hydrogen-

ated diamond. Surface conductivity of hydrogenated diamond

has been explained via the transfer doping mechanism: elec-

tron transfer from the valence band of the hydrogenated dia-

mond to the redox species in the electrolyte (adsorbed water)

results in accumulation of holes on the diamond surface. This

allows two-dimensional electric conduction.20–22 Since ND

has a much higher surface-to-volume ratio compared to bulk

diamond, the effect of surface hydrogenation of ND on its

conductivity should be even more pronounced than for bulk

diamond. Recently, Su et al. reported that hydrogenation of

detonation ND by hydrogen plasma treatment increased the

electrical conductivity, as estimated by impedance spectros-

copy, by four orders of magnitude.23 In the present study, we

investigated the surface hydrogenation of NDs by high tem-

perature treatment in hydrogen gas and the resistivity of the

NDs as a function of treatment temperature. Based upon the

estimation of the surface oxygen content and the sp2/sp3 car-

bon ratio of the treated NDs, the decrease in the resistivity

was found to be consistent with the transfer doping caused by

the surface hydrogenation of ND.

II. EXPERIMENTAL

The ND used in this study (UD90, nominal particle di-

ameter 5 nm) was provided by Nanoblox, Inc., USA. The

UD90 was first oxidized in air in a muffle furnace at 430 �Cfor 5 h to remove the non-diamond carbon from the particle

surface.24,25 The oxidized ND was then refluxed in a mixture

of concentrated hydrochloric and nitric acids at 100 �C for

24 h to remove metallic impurities. The purified ND was

obtained by pipetting out the supernatant of the acidic aque-

ous dispersion and adding fresh deionized water in steps

until the supernatant became neutral, and drying the powder.

The purified ND (denoted here as UD90p) was heated in

hydrogen gas atmosphere using a quartz tube furnace at a

fixed temperature (200–900 �C) for 1 h. After the heat was

turned off, the sample was kept under hydrogen atmosphere

until the furnace temperature dropped to below 100 �C. The

UD90p after the heat treatment (denoted as H-ND) was char-

acterized by Fourier Transform Infrared Spectroscopy (FTIR

in KBr disks; Excalibur FTS-3000, Varian) and X-ray photo-

electron spectroscopy (XPS; Axis-nova, Kratos). The XP

spectra were calibrated by assigning the C 1 s peak position

a)Author to whom correspondence should be addressed. Electronic mail:

gogotsi@drexel.edu.

0021-8979/2013/113(21)/214307/5/$30.00 VC 2013 AIP Publishing LLC113, 214307-1

JOURNAL OF APPLIED PHYSICS 113, 214307 (2013)

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to be 284.8 eV. The O/C atomic ratio of the ND samples was

estimated by quantitative XPS analysis. The sp2/sp3 carbon

ratio of the ND sample was estimated by peak deconvolution

of the XPS C 1 s spectra with Gaussian band profiles. The re-

sistivity of ND was calculated from the I-V curves of ND

packed in a glass tube (inner diameter 1 mm). Both ends of

the ND cylinder inside the tube were pressed with copper

wires which were connected to the power source. The

applied voltage oscillated between �5 and þ5 V at a rate of

5 V s�1, and the electric resistance was calculated from the

slope of the I-V curve in the range of �1 to þ1 V.

III. RESULTS AND DISCUSSION

A. Characterization of hydrogenated ND

The effect of hydrogenation temperature on the surface

chemical structure of H-NDs was investigated by FTIR and

XPS. Figure 1(a) shows FTIR spectra of the H-NDs treated

at 400–900 �C. The spectrum for the sample after hydrogena-

tion at 400 �C showed a band of the C¼O stretching mode at

1750 cm�1.26 The spectrum for the sample treated at 200 �Clooked similar. The presence of the C¼O peak and the ab-

sence of clear CH stretching modes at around

2800–3000 cm�1 indicate that heat treatment in hydrogen

gas at 400 �C is not sufficient for hydrogenation of the

UD90p surface. On the other hand, the samples treated in a

hydrogen atmosphere at 600–900 �C showed pronounced

C-H stretch vibrations of CH3 and CH2 groups at

2800–3000 cm�1. In the spectrum of the H-ND treated at

800 �C, bands of CH3 antisymmetric (2957 cm�1), CH2 anti-

symmetric (2932 cm�1), and CH2 symmetric (2876 cm�1)

vibrations are clearly observed (Fig. 1(b)).27 The relative

intensity of the C¼O band at 1750 cm�1 was found to

decrease as the treatment temperature increased. The band at

1630 cm�1, which remains in H-ND samples produced in the

temperature range of 400–600 �C, originates from O-H bend

vibrations and is likely due to covalently bonded surface

O-H groups or water adsorbed on the surface of H-ND.28

These changes in the spectra indicate that the oxidized ND

surface is reduced by heating in a hydrogen gas atmosphere

at 600–900 �C. The extent of reduction at fixed treatment

time correlates well with the temperature of hydrogenation.

The hydrogenation of UD90p was also investigated with

XPS by monitoring the content of oxygen, which is substi-

tuted with hydrogen as hydrogenation progresses. Figure

2(a) shows XP spectra of H-NDs treated at 400 and 900 �C.

The spectra for the H-ND sample treated at 400 �C showed

an O 1 s peak at 530 eV. The O 1 s peak was less prominent

for the H-ND (900 �C) than for the H-ND (400 �C). The O/C

atomic ratio estimated from the XPS quantitative analysis

plotted as a function of hydrogenation temperature in Fig.

2(b). The O/C ratio of 0.38 for H-ND (400 �C) indicates high

oxygen content on the surface. In comparison, the O/C ratio

of an oxygen plasma-treated diamond surface was reported

to be 0.18.15 The O/C ratio decreased as the hydrogenation

temperature increased from 400 �C to 600 �C, consistent

FIG. 1. FTIR spectra of NDs after treat-

ment in hydrogen. (a) The spectra for

various treatment temperatures: (i) 400,

(ii) 500, (iii) 600, (vi) 800, and (v)

900 �C. (b) (800 �C).

FIG. 2. (a) XPS of H-NDs. Hydrogenation temperatures are (i) 400 and (ii)

900 �C. (b) O/C atomic ratio of H-NDs as a function of hydrogenation

temperature.

214307-2 Kondo et al. J. Appl. Phys. 113, 214307 (2013)

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with Williams et al.,17 and then remained constant (<0.02)

at 600–900 �C. Thus, the range of temperatures correspond-

ing to hydrogenation of ND is 600–900 �C, which is consist-

ent with the FTIR results (Fig. 1).

Figures 3(a) and 3(b) show XPS C 1 s spectra of H-ND

(400 �C) and H-ND (900 �C), respectively. The C1s peaks

were fit by four Gaussian profiles corresponding to C¼C

(283.5 eV), C-C and C-H (284.8 eV), C-O (286.5 V), and

C¼O (288.5 eV). The intensity of the C¼O peak (288.5 eV)

in H-ND (900 �C) is slightly lower compared to H-ND

(400 �C), while the intensity of C-O (286.5 eV) is much

higher in H-ND (900 �C) compared to H-ND (400 �C) due to

reduction of C¼O into C-OH, in agreement with FTIR data

(Fig. 1). The sp2/sp3 carbon ratio, estimated from the area of

the peaks at 283.5 eV (C¼C) and 284.8 eV (C-C and C-H),

decreased as the hydrogenation temperature increased (Fig.

3(c)). The trend was more pronounced for samples treated at

higher temperatures (800–900 �C). Heating at 700 �C or

above can cause partial graphitization of ND in both vacuum

and argon atmospheres.29,30 According to Fig. 3, hydrogen

suppresses the ND graphitization at temperatures up to

900 �C. At these temperatures, hydrogen can react with sp2

carbon atoms, converting them into sp3 hybridized atoms,

and eventually eliminating surface carbon in the form of vol-

atile hydrocarbons.31 Hence, we conclude that the heating of

ND in hydrogen at temperatures up to 900 �C does not

induce graphitization of ND.

B. Resistivity of hydrogenated ND

The resistivities of the H-NDs (Fig. 4) were calculated

from the I-V curves. The H-ND samples treated at or below

500 �C exhibited high resistivity on the order of 107 X cm,

similar to that of UD90 before purification (1.0� 107 X cm)

and UD90p (1.9� 107 X cm). On the other hand, the H-ND

samples treated at 600 �C or above showed almost two orders

of magnitude lower resistivity than those treated at tempera-

tures below 500 �C or untreated NDs. Further increase in hy-

drogenation temperature did not result in significant changes

of resistivity (Fig. 4). The relationship between the hydro-

genation temperature and the sp2/sp3 carbon ratio of ND

(Fig. 3(b)) indicates that the treatment at high temperature

did not cause graphitization of ND. Thus, it is obvious that

the decrease of resistivity is not related to surface graphitiza-

tion of ND. On the other hand, ND surface hydrogenation

was evidenced by FTIR and XP spectra at 600 �C and above.

Thus, the decrease of resistivity should be related to the ND

surface hydrogenation. It is well known that hydrogen-

terminated diamond films can exhibit significant electric

conductivity when in contact with water or ambient humid

air due to transfer doping.32–34 This mechanism was used to

explain significant two-dimensional surface conductivity of

hydrogen-terminated diamond.22,35,36 The sharp drop in the

resistivity of our ND samples after hydrogenation at 600 �Cand above can also be related to the transfer doping of the

hydrogenated ND surface exposed to ambient air.

Hydrogenated ND can be considered as a surface-conductive

ND, a material with many potential applications in electro-

chemistry, biomedical sensing, and nanocomposites.

C. Loss of conductivity by oxidation of H-ND

In order to further confirm that the increased electric

conductivity of the H-ND is related to hydrogen termination,

we measured the resistivity of H-NDs after oxidation. Figure 5

shows resistivity of the H-ND (600–900 �C) samples after

FIG. 5. Resistivity of H-NDs (600–900 �C) after oxidation in air at 200 and

300 �C.

FIG. 3. XPS C 1 s spectra of (a) H-ND (400 �C) and (b) H-ND (900 �C). (c)

Estimated sp2/sp3 carbon ratio as a function of hydrogenation temperature.

FIG. 4. Resistivity of ND as a function of hydrogenation temperature.

214307-3 Kondo et al. J. Appl. Phys. 113, 214307 (2013)

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heating in air at 200 and 300 �C for 1 h. While the resistivity

of all H-ND samples showed almost no change after heating

in air at 200 �C, �50 times increased resistivities were meas-

ured for the samples after heating in air at 300 �C. Figure 6

shows FTIR spectra of H-ND (600 �C) before and after heat-

ing in air at 200 and 300 �C. Although the spectrum of a

sample treated at 200 �C in air is similar to the spectrum

before the treatment, the increase of C¼O stretching band in-

tensity at ca. 1720 cm�1 and the decrease of CH2 symmetric

and antisymmetric stretching band intensity at around

2900 cm�1 was found after the treatment at 300 �C. This

result indicates that the H-ND surface can be oxidized by

heating in air at 300 �C or above, while 200 �C is not suffi-

cient for oxygen chemisorption, which is consistent with our

previous results on ND oxidation.24 The O/C atomic ratio of

H-ND samples after heating in air was quantified by XPS

(Fig. 7(a)). For all the H-ND samples prepared at 600–900 �C,

the O/C ratio increased only slightly after the treatment at

200 �C. After the treatment at 300 �C, the increase in the O/C

ratio was greater than that after the treatment at 200 �C. This

result also confirms that the loss of electric conductivity of

H-ND after heating in air is due to surface oxidation. In con-

trast, the sp2/sp3 ratio was almost unchanged for all the H-ND

samples even after the treatment in air at 300 �C (Fig. 7(b))

likely because 300 �C is too low for oxidation of sp2 carbon in

air, which starts at temperatures above 375 �C.24 The O/C ra-

tio of the H-ND samples after heating in air at 300 �C was

around 0.04, much lower than before surface hydrogenation

(0.38). Thus, the heating in air at 300 �C for 1 h results only in

a mild oxidation of ND surface. However, even this mild oxi-

dation is sufficient to reduce the conductivity of ND by almost

two orders of magnitude (Fig. 5).

Figure 8(a) summarizes resistivity of the ND samples

treated under various conditions as a function of the O/C

FIG. 6. (a) FTIR spectra of H-ND

(600 �C) after oxidation in air. Before

oxidation (i) and oxidized at 200 (ii) and

300 �C (iii). (b) CH2/CH3 stretching

mode region of the spectra.

FIG. 7. (a) O/C atomic ratio and (b) sp2/sp3 carbon ratio estimated by XPS

analysis of H-NDs (600–900 �C) after oxidation in air at 200 and 300 �C.

FIG. 8. (a) Resistivity of H-NDs treated under various conditions as a func-

tion of O/C atomic ratio measured by XPS. (b) Schematic illustration of

band diagrams for H-NDs with various O/C ratios.

214307-4 Kondo et al. J. Appl. Phys. 113, 214307 (2013)

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ratio. The resistivity shows a steep transition between low

(�105 X cm, region I) and high (�107 X cm, region II)

regions at a critical O/C ratio of ca. 0.033. This behavior is

consistent with the concept of transfer doping. The position

of the valence band maximum, which is below the Fermi

level in oxidized ND, shifts gradually to higher energy with

increasing density of the surface C-H groups due to the

dipoles with dþ charge on the H atom.37,38 However, in

order for the transfer doping to occur, it is critical that the va-

lence band maximum is above the Fermi level (redox poten-

tial derived from surface adsorbates, possibly oxygen and

water in this case21,36) (Fig. 8(b)), which can only be realized

at very low O/C ratios. The O/C atomic ratio of 0.033 is less

than 1/10 of that for the oxidized surface (0.38). Thus, the

highly hydrogenated surface is necessary for surface conduc-

tivity, which, again, is consistent with the transfer doping

mechanism.

IV. CONCLUSION

The electric conductivity of ND increased by almost 2

orders of magnitude after heat treatment in hydrogen gas at

600–900 �C. Characterization with FTIR and XPS revealed

that the surface hydrogenation of ND occurred at 600 �C and

higher temperatures. On the other hand, analysis of the XPS

C 1 s spectra showed no evidence of surface graphitization

after heating ND in hydrogen gas at temperatures up to

900 �C. Based on these results, we conclude that the increase

of the conductivity is due to the surface hydrogenation rather

than graphitization. From the relationship between the O/C

ratio and the resistivity of the NDs, we found a sharp transi-

tion of ND powder from electrically insulating to conductive

state at a critical O/C ratio of ca. 0.033. This stepwise transi-

tion is consistent with the transfer doping mediated by sur-

face hydrogenation of ND.

ACKNOWLEDGMENTS

This work was partly supported by the Overseas

Research Program of Tokyo University of Science. The

authors thank Ms. Amanda Pentecost, Drexel University, for

proofreading the manuscript.

1V. N. Mochalin, O. Shenderova, D. Ho, and Y. Gogotsi, Nat. Nanotechnol.

7, 11 (2012).2K. Hanada, K. Yamamoto, T. Taguchi, E. �Osawa, M. Inakuma, V.

Livramento, J. B. Correia, and N. Shohoji, Diamond Relat. Mater. 16,

2054 (2007).3K. D. Behler, A. Stravato, V. Mochalin, G. Korneva, G. Yushin, and Y.

Gogotsi, ACS Nano 3, 363 (2009).4O. Shenderova, C. Jones, V. Borjanovic, S. Hens, G. Cunningham, S.

Moseenkov, V. Kuznetsov, and G. McGuire, Phys. Status Solidi A 205,

2245 (2008).

5V. N. Mochalin, I. Neitzel, B. J. M. Etzold, A. Peterson, G. Palmese, and

Y. Gogotsi, ACS Nano 5, 7494 (2011).6I. Kovalenko, D. G. Bucknall, and G. Yushin, Adv. Funct. Mater. 20, 3979

(2010).7C.-C. Fu, H.-Y. Lee, K. Chen, T.-S. Lim, H.-Y. Wu, P.-K. Lin, P.-K. Wei,

P.-H. Tsao, H.-C. Chang, and W. Fann, Proc. Natl. Acad. Sci. U.S.A. 104,

727 (2007).8Y.-R. Chang, H.-Y. Lee, K. Chen, C.-C. Chang, D.-S. Tsai, C.-C. Fu, T.-S.

Lim, Y.-K. Tzeng, C.-Y. Fang, C.-C. Han, H.-C. Chang, and W. Fann,

Nat. Nanotechnol. 3, 284 (2008).9V. N. Mochalin and Y. Gogotsi, J. Am. Chem. Soc. 131, 4594 (2009).

10L. P. McGuinness, Y. Yan, A. Stacey, D. A. Simpson, L. T. Hall, D.

Maclaurin, S. Prawer, P. Mulvaney, J. Wrachtrup, F. Caruso, R. E.

Scholten, and L. C. L. Hollenberg, Nat. Nanotechnol. 6, 358 (2011).11H. Huang, E. Pierstorff, E. Osawa, and D. Ho, Nano Lett. 7, 3305 (2007).12X.-Q. Zhang, R. Lam, X. Xu, E. K. Chow, H.-J. Kim, and D. Ho, Adv.

Mater. 23, 4770 (2011).13M. Chen, E. D. Pierstorff, R. Lam, S.-Y. Li, H. Huang, E. Osawa, and D.

Ho, ACS Nano 3, 2016 (2009).14A. Krueger and D. Lang, Adv. Funct. Mater. 22, 890 (2012).15I. Yagi, H. Notsu, T. Kondo, D. A. Tryk, and A. Fujishima, J. Electroanal.

Chem. 473, 173 (1999).16L. Ostrovskaya, V. Perevertailo, V. Ralchenko, A. Dementjev, and O.

Loginova, Diamond Relat. Mater. 11, 845 (2002).17O. A. Williams, J. Hees, C. Dieker, W. J€ager, L. Kirste, and C. E. Nebel,

ACS Nano 4, 4824 (2010).18J. van der Weide, Z. Zhang, P. K. Baumann, M. G. Wensell, J. Bernholc,

and R. J. Nemanich, Phys. Rev. B 50, 5803 (1994).19D. Takeuchi, H. Kato, G. S. Ri, T. Yamada, P. R. Vinod, D. Hwang, C.

E. Nebel, H. Okushi, and S. Yamasaki, Appl. Phys. Lett. 86, 152103

(2005).20F. Maier, M. Riedel, B. Mantel, J. Ristein, and L. Ley, Phys. Rev. Lett. 85,

3472 (2000).21V. Chakrapani, J. C. Angus, A. B. Anderson, S. D. Wolter, B. R. Stoner,

and G. U. Sumanasekera, Science 318, 1424 (2007).22A. Bolker, C. Saguy, M. Tordjman, L. Gan, and R. Kalish, Phys. Rev. B

83, 155434 (2011).23S. Su, J. Li, V. Kundrat, A. M. Abbot, and H. Ye, J. Appl. Phys. 113,

023707 (2013).24S. Osswald, G. Yushin, V. Mochalin, S. O. Kucheyev, and Y. Gogotsi,

J. Am. Chem. Soc. 128, 11635 (2006).25S. Osswald, V. N. Mochalin, M. Havel, G. Yushin, and Y. Gogotsi, Phys.

Rev. B 80, 075419 (2009).26J. Mona, J. S. Tu, T. Y. Kang, C.-Y. Tsai, E. Perevedentseva, and C. L.

Cheng, Diamond Relat. Mater. 24, 134 (2012).27T. Jiang, K. Xu, and S. Ji, J. Chem. Soc., Faraday Trans. 92, 3401

(1996).28V. Mochalin, S. Osswald, and Y. Gogotsi, Chem. Mater. 21, 273 (2009).29T. Petit, J.-C. Arnault, H. A. Girard, M. Sennour, and P. Bergonzo, Phys.

Rev. B 84, 233407 (2011).30J. Chen, S. Z. Deng, J. Chen, Z. X. Yu, and N. S. Xu, Appl. Phys. Lett. 74,

3651 (1999).31M. Yeganeh, P. R. Coxon, A. C. Brieva, V. R. Dhanak, L. �Siller, and Y. V.

Butenko, Phys. Rev. B 75, 155404 (2007).32P. Strobel, M. Riedel, J. Ristein, and L. Ley, Nature 430, 439 (2004).33J. Ristein, Science 313, 1057 (2006).34C. E. Nebel, Science 318, 1391 (2007).35L. Ley, J. Ristein, F. Meier, M. Riedel, and P. Strobel, Physica B 376–377,

262 (2006).36C. E. Nebel, B. Rezek, D. Shin, and H. Watanabe, Phys. Status Solidi A

203, 3273 (2006).37F. Maier, J. Ristein, and L. Ley, Phys. Rev. B 64, 165411 (2001).38M. Riedel, J. Ristein, and L. Ley, Diamond Relat. Mater. 13, 746

(2004).

214307-5 Kondo et al. J. Appl. Phys. 113, 214307 (2013)

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