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J. Photochem Phot obiol . A: Chem, 79 (1994)

189-195

Photochemistry of semiconductor

I. Optical properties of ZnS sols

189

particles

Ling Zang, Chun-Yan Liu and Xin-Min Rent

Insti tef Phot ogmphic Chemi sby, Academi a Si nicu, Be g 100101 (Peoples Republi c of

China)

(Received October 15, 1993; accepted

November 30, 1993)

Abstract

A ZnS sol stabilized by polyphosphate was prepared. The particle size is smaller than 3 nm. During the aging

and illuminating processes, the change in particle size was followed by the absorption and fluorescence measurement.

The mean size of ZnS particles in the solution has a limited value which did not decrease further owing to the

large surface tension. A more reasonable description of the optical processes in ZnS sols has been given. This

explains well the disappearance of discernibility of the exciton peak, as well as the band shift of

fluorescence

which was caused by the surface modification. The effects of such parameters as aging, temperature, illumination

and surface condition on the fluorescence intensity of ZnS sols were studied. The luminescence efficiency could

be increased by a factor of 5 under the most favorable conditions.

1 Introduction

In recent years, numerous investigations have

been conducted on the photophysical and pho-

tochemical properties of semiconductor sols [l-5],

such as TiOz, CdS or ZnO, while only a few studies

have been done on the ZnS sol. This may be due

to its high electrical resistance and large band gap

which prevents electrochemical and photochemical

studies on it.

The luminescence of ZnS has been extensively

studied because ZnS crystals have a high emission

efficiency [6, 71. Since the first report on the

luminescence of colloidal ZnS was made by Becker

and Bard [8], a few significant studies have been

done by some workers [g-11]. The luminescence

properties can serve as a probe of not only the

interfacial electron-hole process but also the size

change in colloidal particles. Such a change was

observed by Henglein and coworkers in their earlier

work [9, lo], in which a continuous decrease in

size was found for ZnS colloids under illumination.

It is in contrast with the results obtained in our

and some other workers experiments [ll, 121. A

more reasonable explanation of the size change

of ZnS colloids under various experimental con-

ditions (such as aging and illumination) was pro-

posed in this study.

Author to whom correspondence should be addressed.

lOlO-6030/94/ 07.00 Q 1994 Elsevier Sequoia. All rights reserved

SSDI 1010-6030(93)03760-E

A generally accepted view of the absorption

process in semiconductor colloids is that the initial

step involves the excitation of an electron into the

conduction band or the exciton level (slightly below

the conduction band) of the semiconductor by

ultraband gap radiation. If so, the absorption

spectra of semiconductor sols should be such that

the exciton peak falls slightly below the absorption

edge. However, the absorption edge often lies far

above the exciton peak. A more plausible de-

scription of the absorption process is indispensable

although a description of such process, with specific

reference to that in CdS, using a quantum-me-

chanical model has been developed by Chestnoy

et al. [13]. In this study, we tried to give a more

reasonable depiction of the optical processes in

ZnS sols, which has not, to our knowledge, been

reported in previous work.

2. Experimental details

2 1 Materials

Zn(NO,),, Na,S, NaOH and polyphosphate (PP)

were of analytical grades. They were all used

without further purification. Doubly distilled water

was used in all experiments.

2.2. Preparat ion of ZnS sols

The ZnS sol was prepared by a simple rapid

injection technique: 0.8 ml of 0.1 M Na,S solution

was added ragidly under vigorous stirring to 199.2

ml of 4

X 10-

M Zn(NO& aqueous solution with

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190

L. Zmg et al. I Photochemistry of semiconductor particles I

2 10m4 M PP, the concentration of which is given

by referring to the formula Na,P,O10 (formula

weight, 367.86). ZnS sols were aged in the re-

frigerator at about 10 C. The absorption and

fluorescence measurement were followed during

aging.

2.3. Ilumination

Two kinds of ZnS sol (fresh and aged for 3

days) were used in the illumination experiments.

The illumination was carried out with a 500 W

high pressure Hg lamp (distance, about 15 cm;

without a fllter), and the solution was saturated

with air during the process. Absorption and fluor-

escence spectra were measured at given time in-

tervals during the illumination experiments.

2.4. Apparatus

Absorption spectra were recorded on a Hew-

lett-Packard diode array 8451A spectrophotometer

and the fluorescence spectra were recorded on a

Perkin-Elmer IS-05 fluorometer equipped with a

computer for data acquisition, storage and ma-

nipulation. Fluorescence quantum yields were de-

termined by comparison with rhodamine B in

ethanol, which is known to fluoresce with a quan-

tum yield of 0.69 [14].

3.

Results

3.1. Effect of aging on the optical propetiies

The absorption spectra of ZnS sols aged for

different lengths of time are shown in Fig. 1. The

particle size of these sols cannot be larger than

3 nm since the absorption edge (about 315 nm)

is lower than 335 nm, which corresponds to the

size range of 2-3 nm [9]. Some changes in the

absorption spectra of ZnSsol occurred upon aging.

Wovelength d

Wavelength

hml

Pi.

1. Absorption spectra of ZnS sol at various aging times

(ZnS (diatomic) concentration, 4x lo- M, stabilizer, 2~10~

M PP). The sol was used in all the subsequent experiments.

Fig. 2. Fluorescence spectra of ZnS sol at various aging times.

The inset shows the dependence of fluorescence intensity cm the

aging time.

The absorbance reduced by about 10 in 9 days

(Fig. 1). This was believed to be a result of the

slow air oxidation

ZnS+202- Zn2++S0,2-

(1)

The Zn+ ions in the sol thus were in excess. The

exciton peak became pronounced and shifted

slightly to a longer wavelength, whereas hardly

any shift in the absorption edge was found.

Figure 2 shows the fluorescence spectra of ZnS

sols aged for different lengths of time. The quantum

efficiency for the fresh sol is about 1.7 and

reaches a highest level of about 5.5 for the sol

aged for 6 days. This efficiency is much higher

than that reported in the work of Dunstan et al.

[ll], in which the ZnS sols luminesced with a

quantum yield of 2X10-. In addition to the

increase in intensity with increasing aging time,

the maximum fluorescence shifted slightly to a

longer wavelength within the first 3 days. This

implies that the particle size increases slightly. No

band gap emission was observed for the ZnS ~01s.

3.2. Ef fect of il lumination on the optical

properties

The ZnS sol is very sensitive to illumination

with a photon energy higher than its band gap.

The absorption spectrum changed drastically with

illumination time, as shown in Fig. 3. First, the

shoulder (the exciton peak) at about 282 nm

became slightly pronounced. On increasing the

illumination time, it became increasingly vague

and almost disappeared after about 60 min. In

the meantime, the absorption edge shifted to a

longer wavelength. This was quite different from

the results obtained by Henglein and Gutierrez

[9], in which the onset of light absorption shifted

continuously to a shorter wavelength and the ex-

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L . Zang et al. / Photochemdy of semiconductor panicles I

191

Wavelength nm)

Fig. 3. Absorption spectra of the ZnS sol aged for 3 days at

various times of illumination in the presence of air. The fresh

sol showed a simlar behavior

.V

fresh

ol

40 60

I

6

z

:

60

c

2

.g 40

5

20

0

330 400

450 500

loo

3-day-aped

ol

1

8 60

Fig. 4. Fluorescence spectra of the two ZnS sols at various times

f

ihninatiDn in the presence Of air. The flUOreXence hItenSit&

were manipulated to be at the same kvel for the

observation

Of band shift.

citon peak became more and more pronounced

during the whole illumination process. The ab-

sorbance of ZnS sol decreases very quickly upon

illumination, as can be seen in Fig. 3.

The fluorescence spectra of ZnS sols illuminated

for different times are shown in Fig. 4. It can be

seen that the fluorescence spectra of the two sols

escence maximum of the fresh sol shifted contin-

uously to a longer wavelength in the whole illu-

mination period. In fact, the shift in the initial

period is a consequence of the decrease in fluor-

escence at a shorter wavelength. The width of the

fluorescence band became narrower and narrower

at the same time. For the sol aged for 3 days,

the fluorescence maximum shifted to a shorter

wavelength in the first 4 min and then shifted to

a longer wavelength as in the case of the fresh

sol.

It was also found that the intensity of fluor-

escence of the two sols underwent different changes

during the illumination process (Fig. 5). Consid-

ering the great change in absorbance during the

illumination, the specific intensity (the ratio of the

fluorescence intensity to the absorbance at 282

nm) was used to compare the fluorescence in-

tensities of sols in different illumination times.

With the exception of the slight decrease in the

first 4 min, the fluorescence intensity of the sol

aged for 3 days increases steadily with illumination.

However, for the fresh sol, the fluorescence in-

tensity increased sharply in the first 20 min of

illumination and then decreased quickly with fur-

ther illumination.

3.3.

Ef fect of temperatur e on f luorescence

It is known that the fluorescence intensity de-

pends strongly upon temperature [lo, 151. In the

Arrhenius plot (Fig. 6), In l / I is plotted vs. the

reciprocal temperature as is usually done for fluor-

escence processes with competing thermal deac-

tivation, where

I

is the fluorescence intensity. With

increasing temperature for 0 C to about 50 C,

-B-fresh sol

--+--3-doy-oqsd sol

01

I

I I

0

20

40 60

J

Illumination time, t min.1

Fig. 5. Fluorescence intensity of the two ZnS sols as a function

changed differently upon illumination. The fluor-

of the illumination time (from the data in Fig. 4).

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L. Zang et al. / Photochemir ny of semiconductor

parricks

I.6

1.2

z

C

5

e 0.0

D

w

.

5

0.4

0

-4-day-aged sol

--+--14-day-aged sol

I/T (I lO+K-

Fig. 6. Plot of In(W) VS. the reciprocal temperature for the hvo

zas sols.

the fluorescence intensity decreased to about 25

of its value at 0 C. The fluorescence intensity of

the two ZnS sols changed similarly with temper-

ature. In other words, the degree of dependence

does not change with the surface condition, which

may be quite different for various aging times.

The activation energy for the competing process

can be calculated from the slope of line in Fig.

6, to be about 19 kJ mol-l.

3.4. Efi ct of surface condit i on on fl uorescence

In our experiments, various ions, such as Cu+,

Ag+,Cd2+,Zn2+, m+,.S-,OH-,I- andSCN-,

were used to modify the surface of ZnS colloids.

Some of the ions were found to be very effective

in changing the optical properties of the ZnS ~01s;

this will be described in the next paper of this

series. Figure 7 showed the change in the fluor-

escence spectrum of the ZnS sol aged for 9 days

with the addition of OH- and Zn+ ions. The

fluorescence intensity increased on addition of

OH- ions up to 5 X 10e4 M, and no increase was

found on further addition (inset in Fig. 7). The

fluorescence spectra became wider owing to the

addition of OH- ions. The intensity increased

were markedly at longer wavelengths than at

shorter wavelengths. If 2.5X 10e8 M Zn2+ was

added to the ZnS sol with a prior addition of

5~10~~ M OH-, the fluorescence intensity de-

creased and the shape of the spectra did not

change. The same effect was also shown in spectrum

d in Fig. 7 which was obtained by adding 2

X

low4

M Zn2+ to the ZnS sol aged for 9 days without

any prior addition. The fluorescence intensity de-

420 460

Wavelength Lnml

Fig. 7. Fluorescence spectra of the ZnS sol aged for 9 days upon

addition of OH- and Zn2+ ions: curve a, no addition; curve b,

addition of 5 X lo- M OH-; curve c, addition of 2.5

X

10ms M

Zn2* to sample for curve b; curve d, addition of 2x10- M

zn2+.

creased about 60 . A clear red shift of fluorescence

maximum was observed, as shown in Fig. 7.

4. Discussion

4.1. Sire of ZnS colloids

It is well known that the colloids are thermo-

dynamically unstable with respect to the bulk phase.

The surface tension favors a small surface area.

This means that the colloidal particles have a

natural tendency to aggregate. The smaller the

particles, the more dramatic is the increase in the

specific surface area, and hence the stronger is

the tendency to particle aggregation. Such tendency

was found in the initial aging period of ZnS sols

(Fig. 2). The smaller particles in the fresh sol

tend to dissolve and the larger particles grow

through Ostwald ripening.

Considering the size dependence of the tendency

to particle aggregation, the average size of ZnS

particles under the condition of our experiments

should have a limited value, which does not de-

crease further even under photodegradation. The

results shown in Fig. 4 can be regarded as an

indication of the change in particle size in view

of the size quantization effect, which has been

reported elsewhere [lo, 16, 171. The blue shift

and red shift of the fluorescence maximum suggest

a decrease and an increase respectively in particle

size. This is similar to the results obtained by

Dunstan et al. [ll] and Rossetti et al. [12], but

quite contrary to the results obtained by Henglein

and Gutierrez [9], in which a continuous decrease

in particle size of the ZnS sol was found during

illumination. The contraditory results may be

caused by the different stabilizers used in the

experiments.

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L. Zang t a. I Photochemishy of semi conduct0ra r t i c l e s

193

4.2. Absorption process in ZnS sols

In our studies, the exciton peak (absorption

involved between the valence band and the exciton

level) of ZnS sols is not so pronounced as that

obtained in methanol by Rossetti et

nl.

[12]. Even

in their results, the ZnS sol prepared in aqueous

solution shows a much less pronounced exciton

peak compared with that prepared in methanol

solution. A similar phenomenon was also observed

for CdS sols prepared in water and propanol-2

solution [18]. Obviously, the solvent is the most

effective factor that affects the characteristics of

the exciton peak. In the work of Rossetti er al.,

this solvent effect was referred to as a prevention

of the colloid particles from aggregation, which

was believed to be the origin of the difference

between the exciton peaks. The ZnS sol obtained

in our experiments, which was stabilized by PP,

was very stable and no obvious aggregation can

be found after a long aging time (e.g. 14 days).

This can be seen from the fact that the onset of

the absorption spectra did not shift significantly

to a longer wavelength, while the exciton peak

became a little more pronounced upon aging. This

is also in contrast with the results of Rossetti et

al.

[12].

The disappearance of the discernibility of the

exciton peak can be attributed to either a broad-

ening or a decrease in this peak. This may be due

to trapping of the excitons by an impurity center

at the surface. This may happen as follows: the

impurity center first traps an electron (or hole)

and, once this has happened, the impurity center

(now charged) can attract a hole (or electron)

through the coulombic force. The level of such a

trapped exciton is lower than that of an untrapped

exciton by the interaction energy of the exciton

with the impurity center. Thus the exciton peak

is broadened to a longer wavelength where the

absorption associated with the trapped exciton

level occurs. The intensity of the original exciton

peak weakens owing to the decrease in the number

of untrapped excitons. Roth Zn and S2- ions

on the surface can trap photogenerated excitons

in ZnS colloids. The increase in the discernibility

of the exciton peak during aging (in the process

the Zn* ions were in excess) indicates that S2-

(hole trap) is more efficient than Zn*+ (electron

trap) in trapping excitons. This is understandable

considering the different electrode

potentials of

the

two onpairs,E(S-S), -0.51 V;E(Zn+-Zn),

-0.76 V.

The trapping of excitons is affected

by the

dielectric properties of the environment of the

ZnS particle, which in turn is determined to a

great extent by the properties of the solvent. A

solvent with a higher polarity is always favorable

for the creation of a gradient of electrostatic

potential around the ZnS particle and hence is

conducive to the movement of photogenerated

charge carriers (hole and electron) from the in-

terior to the surface. Tbe difference in the polarities

of the solvents used in the experiments of Rossetti

et al. [12] and in our experiments can therefore

be reasonably regarded as an origin of the dif-

ference in the discemibilities of the exciton peaks.

In addition to the absorption processes asso-

ciated with band gap and exciton excitation, the

absorption processes involving impurities (acceptor

and donor) in semiconductor colloids may also

occur. This is shown in Fig. 8, where a and b

represent the processes in which an electron is

excited from the valence band to a donor or from

an acceptor to the conduction band, and c illus-

trates an absorption process involving transition

from an acceptor to a donor. Such processes lead

to absorption with a photon energy lower than

the band gap if the donor or acceptor exists in

a relatively deep site.

For small particles, the number of molecules

on the surface is comparable with that in the

interior. Even a minute amount of impurities ad-

sorbed on the surface lattice can cause a striking

increase in the concentration of defects in one

particle. Zn2+ or S*- ions strongly adsorbed at

the surface lattice can be regarded as the deep

traps contributing to absorption lower than the

band gap.

It can be seen from the above discussion that

the absorption edge of the semiconductor sol is

not equivalent exactly to the band gap but is lower

than the latter in energy. In fact, the exciton peak

(if it is sufficiently pronounced) is more precise

than the absorption edge as an indicator for the

change in particle size.

CB

A

VE

Fig. 8. Various absorption processes invoting impurities (see

text for details): CB, conduction band; VB, valence band; A,

acceptor; D, donor.

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L. Zang et al. I Photochemirhy of semiconductor particles I

4.3. Fluorescence

4.3.1. I ntensity

It is commonly accepted that the S2- vacancy

(in the surface layer) is the fluorescence center

for the sols of CdS [I 213 or ZnS [lo] in the

cubic zinc-blende structure. That is to say, the

S2- ion can quench the fluorescence more effi-

ciently than the cations Cd+ or ZP. Removal

of S2- ions on the surface is always favorable to

the enhancement of fluorescence. The removal

can be achieved either by illumination (photo-

oxidation) or by aging (air oxidation). Owing to

oxidation the Zn ions on the surface were in

excess. A sufficiently high concentration of ZrP

ions can also effectively quench the fluorescence.

This is because the high concentration of Zn+

favors the electron-trapping reaction

Zn2+ +2e---, Zn

(2)

which hardly occurs in the condition of a low

concentration of Zn+ ions.

Of particular interest is .the sharp decrease in

the fluorescence intensity of the ZnS sol. This can

be attributed to the high concentration of defects

generated in the colloidal particles. The size dis-

tribution of the fresh ZnS particles is not narrow.

Under illumination, larger particles in a poor crystal

structure were formed through Ostwald ripening,

which proceeded much faster under illumination

than under aging [X3]. In contrast, few of these

poor crystalline particles can be formed in the

ZnS sol aged for 3 days owing to its narrow size

distribution.

As reported by the previous workers [19, 221,

the fluorescence intensity also depends to some

extent on the pH of the solution. The OH- ion

is effective for blocking the defect on the surface

of colloid. Such a defect is believed to be the

center for the radiationless recombination of the

photogenerated charge carriers. The increase in

the fluorescence intensity caused by OH- has not

been clearly explained to date, although Henglein

et al. [17] postulated a possible explanation for

the CdS ~01s: he removal of SH- groups and the

accumulation of Cd* on the surface in the form

of S2-. . .Cd2*.. .OH- seem to destroy the sites

where radiationless recombination of the charge

carriers occurs. This is hard to accept because the

excess Cd+ ions in the case of a high pH should

precipitate to form Cd(OH)2 and they could not

be in the postulated form. The binding of OH-

with the excess cations Cd2 or Zn2+ at the surface

lattice seems to be the main reason for the fluor-

escence enhancement because the binding can

weaken the ability of the cation+ for trapping the

photogenerated electron (eqn. (2)). This was ver-

ified by the results shown in Fig. 7. The addition

of OH- to the ZnS sol aged for 9 days (with

excess Zn2 + ions) resulted in an enhancement in

fluorescence. The subsequent addition of Zn2+ (in

the form of Zn(OH),, n 22) [24], however, de-

creased the intensity. This led us to conclude that

the excess Zn(OH), which covered the surface

acted as new centers for the radiationless recom-

bination of the photogenerated charge carriers.

Temperature has another significant effect on

the intensity of fluorescence. The photogenerated

charge carriers decay in two competitive ways,

namely radiation and radiationless recombination.

radiationles

ZnSh ZnS (e--h*)

rJ

heat + ZnS

photodecomposition products

\

(Zn+ S, SOJz-etc.)

radiation

hv + ZnS

The radiationless process is temperature depen-

dent with an exp(

-E/RT)

law, where

E

is the

activation energy. The higher the temperature, the

faster is the radiationless process, and hence the

lower is the fluorescence intensity. The dependence

of the radiationless process on temperature was

little affected by the surface condition (Fig. 6).

Thii implies that the radiationless recombination

of the charge carriers possibly happens in the inner

part of ZnS colloids.

4.3.2.

Wavelength

In addition to the size change, the shift in the

fluorescence maximum can also be caused by sur-

face modification with adsorbed ions. The adsorbed

ions often act as shallow traps of photogenerated

charge carriers in the luminescence process. As

a result, the fluorescence maximum shifts to a

longer wavelength. The excess OH- ions (possibly

ppose that the binding form of M(OH)2, the standard

potential of the reaction M(OH),+2e- =M+20H-(aq) can be

obtained as

&(M(OH),OH-, M)==E(ti+-M)+ g In rz,

where M represents the element Cd or Zn and K,_ is the solubility

product of &OH) Using the standard potent& and solubili6

oroducts EKd2-Cd~ = - 0.40 V, EfZn2+-ZnI= -0.76 V.

jC,(Cd(OH)& 2.5 X lo-l4

and Kz(kn(Ok)z)= 1.2~ lo-l7 froi

ref. 23, one can ohtain the standard potential EO(M(OH)+H-,

M) as follows: E(OH--Cd(OH)2, Cd)= -0.80 V,

E(OH--ZII(OH)~, Zn)= -1.26 V. So the negative potential

makes it hard for Cd(OH)2 and Zn(OH), to trap the photo-

generated electrons.

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L. Zang et al. I Photochemi ptry

of

emi conductor parti cles I

195

in the form of Zn(OH),, n B 2) at the surface of

It should be emphasized here that the optical

ZnS particles can be regarded as shallow traps

process in semiconductor sols is very complicated

which caused a red shift in the fluorescence spectra and cannot be thoroughly discussed in this article.

(Fig. 7).

Some further studies are necessary in this regard.

The absence of band gap emission (Fig. 2)

indicates that electrons in the conduction band

or exciton level cannot recombine directly with

holes in the valence band. As postulated by Weller

[25], at least one of the charge carriers is trapped

in the luminescence process. In this paper, the

trapped charge carrier is further described in the

form of trapped exciton as described above. Prior

to luminescence, the trapped exciton relaxes

through interaction with other charge carriers or

defects in the surface layer where the trapping is

believed to take place. This relaxation causes a

decrease in energy of the trapped exciton which

now luminesces with a longer wavelength. One

can see that the decrease in energy of the trapped

exciton depends upon the concentration of defects

in the surface layer of the particle. For zinc sulfide,

it is hard to obtain colloidal particles with a perfect

crystal structure by&situ generation. Some defects

always exist in the generated particles (especially

in the surface layers) owing to the peculiar prop-

erties of the Zn+ ion in alkaline solutions [24].

During the rapid precipitation, such ions as

Zn(OH),- and Zn(OH)42- can be formed as

interstitials in the particle. On the contrary, band

Acknowledgments

Thanks are due to the National Natural Science

Foundation of China and Eastman Kodak Com-

pany for their financial support for this work.

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J.A. Dean (ed.).

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13th edn.,

McGraw-Hill, New York, 198.5.

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Ckemiwy: A Guide to

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Angew. Chem, Int. Edn Engl,

32 (1993) 41.

5. Conclusions

For a stable semiconductor sol, a limiting low

value of the particle size exists because the large

surface tension of small particles favors aggre-

gation. The decrease in absorbance of ZnS sols

under illumination is caused by the reduction in

the number of particles and not by the decrease

in particle size.

Owing to the small size of the colloidal particles

and thus the large surface-to-bulk ratio, the nature

of the surface is responsible for the physi-

cal-chemical properties of the particle. Therefore

surface effects should be taken into account when

studying optical processes in semiconductor ~01s.

As the particle size decreases, not only quantization

effects but also surface effects become significant.

The changes in optical properties are caused si-

multaneously by the two kinds of effect instead

of by just one of them.

17

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