1. In oduc ion
In gene al, colloidal sys ems con ain a la ge numbe o
indi idual nanopa icles dispe sed in some con inuous
medium mo ing by B ownian mo ion and colliding wi h
one ano he so ha i) hey emain sepa a ed as indi idual
nanopa icles o ii) o m agglome a es by a p ocesses
called coagula ion o loccula ion [1]. Colloidal sys ems
a e mos ly s abilized by a ious capping (su ace coa ing)
addi i es p o iding hei s e ic and/o elec os a ic
s abiliza ion o by deposi ion on solid suppo s [2-4].
Nanopa icles ha e many unique p ope ies, such as
high su ace a ea o olume a io, la ge su ace a ea and
ene gy e c. [5]. ZnS nanopa icles ha e some impo an
p ope ies ypical o o he semiconduc o nanopa icles,
including a quan um size e ec [6], which p ede e mine
hem o luminescence and pho oca aly ic applica ions
[7]. Syn hesis, cha ac e iza ion and applica ions o
semiconduc o nanopa icles we e e iewed in de ail in,
e.g. a book edi ed by Smi h [2]. In pa icula , p ecipi a ion
o ZnS nanopa icles in he p esence o ca ionic
su ac an s was e iewed in ou ecen pape s [8,9].
The aim o his s udy was o in es iga e he
agglome a ion o ZnS nanopa icles wi hou capping
addi i es a low empe a u es educing hei kine ic
ene gy o B own’s mo emen . These colloid dispe sions
we e s abilized only by epulsi e elec os a ic in e ac ions
among ZnS nanopa icles. Acco ding o he on Weima n
law, colloidal dispe sions can be ob ained om e y
dilu ed o e y concen a ed solu ions, he e o e, ZnS
nanopa icles we e p ecipi a ed in dilu ed solu ions o
zinc and sulphide ions. Expe imen al echniques we e
Cen al Eu opean Jou nal o Chemis y
Agglome a ion o ZnS nanopa icles wi hou
capping addi i es a di e en empe a u es
* E-mail: pe . [email p o ec ed]
Recei ed 12 May 2013; Accep ed 12 No embe 2013
Abs ac :
© Ve si a Sp. z o.o.
Keywo ds: ZnS nanopa icles • Agglome a ion • Low empe a u es • Molecula modelling
1Depa men o Analy ical Chemis y and Ma e ial Tes ing,
VŠB-Technical Uni e si y o Os a a,
708 33 Os a a-Po uba, Czech Republic
2Ins i u e o Physics, VŠB-Technical Uni e si y o Os a a,
708 33 Os a a-Po uba, Czech Republic
3Depa men o Chemical Physics and Op ics,
Cha les Uni e si y in P ague, Facul y o Ma hema ics and Physics,
121 16 P ague 2, Czech Republic
Pe P aus1*, Richa d D o ský2,
Pe Ko ář3, Ladisla S oboda1
Sho Communica ion
ZnS nanopa icles we e p ecipi a ed in dilu ed aqueous solu ions o zinc and sulphide ions wi hou capping addi i es a a empe a u e
in e al o 0.5–20oC. ZnS nanopa icles we e a anged in la ge locs ha we e disagg ega ed in o smalle agglome a es wi h
hyd odynamic sizes o 70–150 nm depending on empe a u e. A linea ela ionship be ween hyd odynamic adius (Ra) and empe a u e
(T) was heo e ically de i ed as Ra =652 – 2.11 T.
The adii o 1.9–2.2 nm o indi idual ZnS nanopa icles we e calcula ed on he basis o gap ene gies es ima ed om hei UV
abso p ion spec a. Low ze a po en ials o hese dispe sions o -5.0 mV o -6.3 mV did no depend on empe a u e. In e ac ions
be ween indi idual ZnS nanopa icles we e modelled in he Ma e ial S udio en i onmen . Wa e molecules we e ound o s abilize ZnS
nanopa icles ia elec os a ic in e ac ions.
312
Cen . Eu . J. Chem. • 12(3) • 2014 • 312-317
DOI: 10.2478/s11532-013-0385-2
P. P aus e al.
comple ed by molecula modelling o unde s and
in e ac ions be ween indi idual nanopa icles in
aqueous en i onmen s.
2. Expe imen al p ocedu e
2.1. Ma e ial and chemicals
The chemicals used we e o analy ical eagen g ade:
zinc ace a e and sodium sulphide (bo h om Lachema,
Czech Republic). Wa e deionized by e e se osmosis
(Aqua Osmo ic, Czech Republic) was used o
p epa a ion o all solu ions.
2.2. P ecipi a ion o ZnS nanopa icles
In a ypical p ocedu e, 50 mL o he aqueous solu ion o
Na2S (2.5 mmol L-1) was added d op-wise o 250 mL o
aqueous solu ion o Zn(CH3COO)2 (0.3 mmol L-1) unde
igo ous s i ing and cooling. The mola a io be ween
zinc and sulphu p ecu so s was kep a 1:1.7.
2.3. UV abso p ion spec a measu emen s
UV abso p ion spec a o ZnS colloidal dispe sions we e
measu ed by a double-beam spec ome e Lambda 25
(Pe kin Elme , USA). All spec a we e eco ded using
1 cm qua cu e es wi hin a wa eleng h ange om
200 nm o 400 nm.
2.4. Ze a po en ial and hyd odynamic size
measu emen s
Ze a po en ials and hyd odynamic sizes we e measu ed
by dynamic ligh sca e ing (DLS) a l = 633 nm using
a Ze asize Nano ZS ins umen (Mal e n Ins umen s,
UK). This ins umen is able o measu e pa icle
sizes om 0.6 nm o 8.9 µm and ze a-po en ials om
-500 mV o + 500 mV. A sample o he ZnS dispe sion
was il e ed h ough a memb ane il e wi h a po e size
< 100 nm and injec ed in o he disposable capilla y cell
(DTS1061). The measu emen s o he ze a po en ials
and hyd odynamic sizes we e pe o med immedia ely
a e p ecipi a ion using he SOP Playe measu emen
p og amme sequence. In his manne , he ac ions
o nanopa icles wi h low ze a po en ials could be
egis e ed be o e hei loccula ion.
2.5. T ansmission elec on mic oscopy
T ansmission elec on mic oscopy (TEM) was pe o med
on a JEM 1230 (Jeol, Japan) mic oscope ope a ed a
80 kV. F eshly p epa ed samples o ZnS nanopa icles
we e placed on a coppe g id (400 mesh) coa ed by a
Fo m a ilm (1.5-3 w .% o poly inyl o maldehyde in
chlo o o m), d ied by blo ing pape and analyzed a e
2 days. Con as o mic og aphs was imp o ed by a
1 w .% solu ion o ammonium molybda e added o he
samples.
2.6. Molecula simula ions
Molecula modelling simula ions we e pe o med in
he Fo ci e module o he Ma e ials S udio modelling
en i onmen [10]. The wu zi e s uc u e da a we e
used o building ZnS nanopa icles. C ys allog aphic
da a o wu zi e a e he ollowing: space g oup P63mc,
a = b = 0.382 nm, c = 0.626 nm [11]. The a omic
posi ions we e a iable du ing geome y op imiza ion.
Molecula dynamics simula ions we e pe o med in an
NVT s a is ical ensemble a a empe a u e o 298 K;
one dynamics s ep was 0.5 ps and 500–1000 ps we e
pe o med. A omic posi ions o ZnS nanopa icles we e
ixed du ing he simula ions. A e dynamics simula ions
he models we e op imized wi h a iable a omic
posi ions.
ZnS nanopa icles we e su ounded by 5860 wa e
molecules. Cha ges o a oms in ZnS nanopa icles we e
calcula ed by he QEq me hod [12] and o al cha ge o
each ZnS nanopa icles was se o ze o. Cha ges o
wa e molecules we e assigned by he Compass o ce
ield [13]. The models we e op imized in he Uni e sal
o ce ield [14]. Elec os a ic and an de Waals
in e ac ions we e calcula ed by a cubic spline wi h a cu -
o dis ance o 1.55 nm.
3. Resul s and discussion
3.1. Agglome a ion o ZnS nanopa icles
ZnS dispe sions we e examined by TEM, ze a
po en ials and hyd odynamic size measu emen s.
TEM mic og aphs in Fig. 1 show la ge locs o di e en
sizes composed o indi idual ZnS nanopa icles, which
seem o no c ea e bulky pa icles bu emain mu ually
sepa a ed. The locs can be di ided in o smalle
agglome a es indica ed by ci cles.
Ze a po en ials and hyd odynamic sizes (diame e ),
which depend on he empe a u e o ZnS dispe sions,
we e measu ed using DLS as shown in Fig. 2. The ze a
po en ials wi hin he empe a u e in e al o 0.5–20oC
we e nea ly cons an , andomly changing be ween
-5.0 mV and -6.0 mV. The low ze a po en ials alues
we e likely as a esul o low amoun s o adso bed
sulphide ions in he dilu ed solu ions. In gene al, ze a
po en ials equal o o highe han ca ±30 mV a e
necessa y o s abilize colloid dispe sions. The e o e,
he nex expe imen s we e pe o med immedia ely a e
p ecipi a ion o a oid coagula ion a longe imes. In
313
Agglome a ion o ZnS nanopa icles wi hou
capping addi i es a di e en empe a u es
p ac ice he expe imen s we e pe o med a abou 30 s
a e p ecipi a ion because ou equipmen was no able
o analyse ZnS dispe sions in e y sho ime in e als
a e hei p ecipi a ion like e.g. [15].
As men ioned abo e, we assumed ha he la ge
locs depic ed in Fig. 1 we e composed o smalle
agglome a es, which can be cha ac e ized by a mean
adius Ra. Fig. 2 shows ha he hyd odynamic size
o ZnS agglome a es signi ican ly dec eased wi h
inc easing empe a u e. Hyd odynamic sizes we e
logno maly dis ibu ed and only hei mode alues
we e e alua ed. Each poin in Fig. 2 was calcula ed
as an a e age om 5–6 measu emen s; ela i e
s anda d de ia ion a ied om 4 o 20%. A mean mu ual
equilib ium dis ance h0 be ween hese agglome a es
co esponds o he seconda y shallow minimum on a
po en ial unc ion desc ibed by he DLVO heo y [1].
Acco ding o his unc ion he o al in e ac ion po en ial u
(sum o a ac ion and epulsion po en ial) depends on a
dis ance h be ween he agglome a es as ollows
(1)
whe e e is he pe mi i i y, z is he ze a po en ial, l is
he e ec i e hickness o an elec ic bilaye o sulphide
and zinc ions and H is he Hamake cons an . I is
possible o emphasise he e ha bo h he locs and
smalle agglome a es we e assumed o be composed
o indi idual ZnS nanopa icles as demons a ed below.
A s abili y o agglome a es o he adius Ra ha ing he
B own kine ic ene gy o
3
2b
kT can be exp essed as
(2)
whe e he o al po en ial u(Ra) co esponds o he mean
dis ance h0 among hem. Fo maximal Ra a empe a u e
T we can ob ain om Eqs. 1 and 2
(3)
Now we can de i e a dependence o he agglome a e
adius Ra on empe a u e as ollows
(4)
whe e a and b a e he cons an s; he pa ame e a can
be conside ed cons an because he ze a po en ials z
we e ound o be cons an as demons a ed in Fig. 2.
The dec ease o he measu ed hyd odynamic adii Ra
wi h inc easing empe a u e can be well cha ac e ized
by a linea ela ionship Ra =652 – 2.11 T (wi h co ela ion
coe icien c = 0.9807) in ag eemen wi h Eq. 4, which
con i ms he alidi y o ou assump ions.
Figu e 1. TEM mic og aphs o ZnS dispe sions a 0.5oC. O e all iew o ZnS locs (le ), de ail iew o one ZnS loc.
Figu e 2. Ze a-po en ials (le ) and hyd odynamic sizes o ZnS colloidal dispe sions as a unc ion o he modynamic empe a u e.
314
P. P aus e al.
3.2. Size o ZnS nanopa icles
The ZnS colloidal dispe sions we e s udied by UV
spec ome y supposing ha he agglome a es we e
composed o indi idual nanopa icles exhibi ing he
quan um size e ec [6]. The UV abso p ion spec a we e
eco ded 30 s a e p ecipi a ion wi hin he empe a u e
in e al o 0.5–20oC o es ima e gap ene gies o
ZnS nanopa icles acco ding o Tauc’s me hod and
consequen ly o calcula e hei adii simila ly as in ou
ea lie s udies [8,9]. Bulk ZnS pa icles ha e a gap
ene gy o abou 3.7 eV, howe e , ZnS nanopa icles
wi h sizes lowe han 10 nm ha e la ge gap ene gies
as a esul o he quan um size e ec as
(5)
whe e Eg(nano) and Eg(bulk) a e he gap ene gies o
ZnS nanopa icles and bulk pa icles, espec i ely, h is
Planck’s cons an , is he adius o nanopa icles, me
and mh a e he e ec i e masses o elec on and hole,
espec i ely, and ε is he ela i e pe mi i i y dielec ic
cons an o he ma e ial, εo is he pe mi i i y o acuum.
Fo ZnS, me = 0.42m0 and mh = 0.61m0, whe e m0 is he
ee elec on mass and ε = 8.76 [16].
The nanopa icles adii calcula ed acco ding o
Eq. 5 changed e y li le om 1.9 nm o 2.2 nm wi hin
he es ed empe a u e in e al. Such small adii
con i med ou assump ion ha he agglome a es we e
composed o indi idual and mu ually sepa a ed ZnS
nanopa icles.
3.3. Molecula simula ions o ZnS nanopa icles
In e ac ions be ween wo ZnS nanopa icles su ounded
by wa e molecules we e simula ed in he Fo ci e module
o he Ma e ials S udio modelling en i onmen . The o al
cha ge o ZnS nanopa icles was se o ze o, which
was close o he de e mined low ze a po en ials. The
wu zi e ZnS s uc u e [8] da a we e used o building
he ZnS nanopa icles. Acco ding o he expe imen al
da a ZnS nanoc ys als in he shape o a sphe e wi h he
adii o 2.0 nm we e c ea ed and used o simula ions.
Two ypes o sys ems we e c ea ed: i) sys em 1 wi h
a igh a angemen o ZnS nanopa icles, whe e he
dis ance be ween he cen es o ZnS nanopa icles was
4.0 nm and ii) sys em 2 wi h ZnS nanopa icles isola ed
by wa e molecules whe e he mu ual dis ance be ween
he nanopa icle su aces was abou 2.0 nm and he
dis ance be ween he nanopa icles cen es was abou
6.0 nm (Fig. 3).
Bo h sys ems we e cha ac e ized by o al sublima ion
ene gy E o al calcula ed as a sum o non-bonded ene gy -
an de Waals (E dw) and elec os a ic (Eels ) in e ac ions
(E o al = E dw + Eels ) be ween he igid bodies in he
sys ems. The igid body is a pa o a sys em, whe e
all he in e ac ions (bonded and non-bonded) be ween
he a oms a e no calcula ed. The o al sublima ion
ene gy can gi e us in o ma ion on s abili y, p obabili y o
exis ence and mu ual in e ac ions in he sys ems. In ou
case, igid bodies we e ep esen ed by wa e molecules
and ZnS nanopa icles. The simula ion esul s a e
summa ized in Table 1.
Each non-bonded in e ac ion E (Eels , E dw and hei
sum E o al) was calcula ed as E = EZnS+H2O - EH2O, whe e
EZnS+H2O is he in e ac ion ene gy be ween all pa s o
he sys ems and EH2O ep esen s he in e ac ion ene gy
wi hin a bulk o wa e su ounding ZnS nanopa icles.
To al sublima ion ene gy E o al in ol es he in e ac ions
be ween ZnS nanopa icles and wa e molecules and
he in e ac ions be ween ZnS nanopa icles hemsel es.
One can see ha elec os a ic in e ac ions a e dominan
in he E o al ene gy e ms and he o al ene gy o sys em
Figu e 3. Simula ions o wo ZnS nanopa icles su ounded by wa e molecules in igh a angemen (le ) and wi h mu ual dis ance o 2.0 nm.
Table 1. Calcula ed sublima ion ene gies o sys ems o ZnS
nanopa icles and wa e .
Sys em No. E o al
(kcal mol-1)
Eels
(kcal mol-1)
E dw
(kcal mol-1)
1-33475 -33217 -258
2-36453 -36159 -294
315
Agglome a ion o ZnS nanopa icles wi hou
capping addi i es a di e en empe a u es
2 is abou 8% lowe han in sys em 1. This shows ha
sys em 2 o he isola ed ZnS nanopa icles is mo e
s able han sys em 1 wi h hei igh a angemen .
Excep ZnS and H2O in e ac ions i is use ul o
conside he in e ac ions be ween ZnS nanopa icles.
The in e ac ion ene gies E o al be ween he ZnS
nanopa icles we e nega i e and o sys em 1 he
alue was o hund eds o kcal mol-1. Wi h inc easing
dis ance be ween ZnS nanopa icles he in e ac ion
ene gy inc eased. Fo example, a he dis ance o
1.2 nm he in e ac ion ene gy was a hund ed imes lowe
(-2 kcal mol-1) han o he sys em wi h hei igh
a angemen (-150 kcal mol-1). This means ha mu ual
in e ac ions be ween ZnS nanopa icles can be in luenced
by hei dis ance and he o al sublima ion ene gy o bo h
sys ems is signi ican ly in luenced by wa e molecules.
I we conside he low concen a ion o ZnS
nanopa icles in aqueous dispe sions we can conclude
ha ZnS nanopa icles p obably will be isola ed by
wa e molecules in hei agglome a es. This ag ees
wi h he low ZnS adii o 1.9-2.2 nm es ima ed om
he UV abso p ion spec a. In addi ion, he molecula
simula ions also indica ed ha wa e molecules can
media e elec os a ic in e ac ions s abilizing ZnS
nanopa icles in hei agglome a es.
4. Conclusions
ZnS nanopa icles we e p ecipi a ed in dilu ed aqueous
solu ions o zinc and sulphide ions wi hou any capping
addi i es a empe a u es o 0.5–20oC. ZnS dispe sions
consis ed o locks which we e obse ed by TEM. Wi h
inc easing empe a u e hese locs we e b oken down
o smalle agglome a es wi h hyd odynamic sizes
o 70–150 nm. The linea dependence be ween he
hyd odynamic adius and he modynamic empe a u e
was heo e ically de i ed and con i med by
expe imen s. The ze a po en ials o hese dispe sions
we e cons an a he empe a u e in e al and eached
he low alues o -5 mV o -6.3 mV. The adii o
1.9–2.2 nm o indi idual ZnS nanopa icles we e
calcula ed on he basis o gap ene gies es ima ed om
hei UV abso p ion spec a.
In o de o unde s and in e ac ions be ween ZnS
nanopa icles hey we e modelled in he Ma e ial S udio
en i onmen . The sys em wi h ZnS nanopa icles
sepa a ed by wa e molecules was mo e s able han
one wi h hei igh a angemen . This indica es ha
wa e molecules in e media e s abilizing elec os a ic
in e ac ions be ween he nanopa icles and i con i ms
he assump ion ha he nanopa icles we e mu ually
sepa a ed in hei agglome a es.
Bo h expe imen al and molecula simula ions
e ealed ha ZnS nanopa icles o med agglome a es
bu hey exis ed as indi idual objec s in hei dilu ed
dispe sions. This s udy allowed us o in es iga e he
beha iou o single ZnS nanopa icles wi hou any
capping addi i es like ca ionic su ac an s. They a e
able o s abilize nanopa icles bu s ongly in luence
ela ionships among hem du ing hei g ow h and/o in
agglome a ion [8,9]. Nex , expe imen s will be pe o med
o in es iga e g ow h mechanisms o uncapped ZnS
nanopa icles a di e en empe a u es.
Acknowledgemen
This wo k was suppo ed by he Czech Science
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