ca alys s
A icle
Low-Tempe a u e Mine alisa ion o Ti ania-Siloxane
Composi e Laye s
Tomáš S oboda 1,*, Michal Veselý1, Radim Ba oš 1, Tomáš Homola 2and Pe Dzik 1
Ci a ion: S oboda, T.; Veselý, M.;
Ba oš, R.; Homola, T.; Dzik, P.
Low-Tempe a u e Mine alisa ion o
Ti ania-Siloxane Composi e Laye s.
Ca alys s 2021,11, 50. h ps://
doi.o g/10.3390/ca al11010050
Recei ed: 12 Oc obe 2020
Accep ed: 4 Decembe 2020
Published: 1 Janua y 2021
Publishe ’s No e: MDPI s ays neu-
al wi h ega d o ju isdic ional clai-
ms in published maps and ins i u io-
nal a ilia ions.
Copy igh : © 2021 by he au ho s. Li-
censee MDPI, Basel, Swi ze land.
This a icle is an open access a icle
dis ibu ed unde he e ms and con-
di ions o he C ea i e Commons A -
ibu ion (CC BY) license (h ps://
c ea i ecommons.o g/licenses/by/
4.0/).
1Facul y o Chemis y, B no Uni e si y o Technology, Pu kyˇno a 118, 612 00 B no, Czech Republic;
[email p o ec ed] (M.V.); [email p o ec ed].cz (R.B.); [email p o ec ed] (P.D.)
2Depa men o Physical Elec onics, Facul y o Science, Masa yk Uni e si y, Ko lᡠská267/2, 611 37 B no,
Czech Republic; [email p o ec ed]
*Co espondence: [email p o ec ed]
Abs ac :
This pape deals wi h low- empe a u e mine alisa ion o coa ings made wi h i ania-
siloxane composi ions (TSC). Me hyl ie hoxysilane has been adop ed as he p ecu so o he
siloxane, and du ing i s syn hesis, an oligome ic siloxane condensa e wi h me hyl moie ies ac ing
as TiO
2
binde has been p oduced. These me hyl moie ies, con ained in TSC, p o ide solubili y
and p e en gelling, bu educe he hyd ophilici y o he sys em, educe he ans e o elec ons
and holes gene a ed in he TiO
2
. In o de o a oid hese un a ou able e ec s, TSC mine alisa ion
can be achie ed by non he mal ea men , o example, by using UV- adia ion o plasma ea men .
Cha ac e isa ion o he siloxane was pe o med by gel pe mea ion ch oma og aphy (GPC), which
showed he size o he siloxane chain. The mog a ime ic analysis e ealed a empe a u e a
which he siloxane mine alises o SiO
2
. P in ed laye s o wo ypes o TSC wi h di e en siloxane
con en s we e s udied by a scanning elec on mic oscope (SEM), whe e a di e ence in he po osi y
o he samples was obse ed. TSC on luo ine-doped in oxide (FTO) coa ed glass and mic oscopic
glass we e ea ed wi h non- he mal UV and plasma me hods. TSC on FTO glass we e es ed
by ol amme ic measu emen s, which showed ha he non- he mally ea ed laye s ha e be e
p ope ies and he amoun o siloxane in he TSC has a g ea in luence on hei e iciency. Samples
on mic oscopic glass we e subjec ed o a pho oca aly ic decomposi ion es o he model pollu an
Acid o ange 7 (AO7). Non- he mally ea ed samples show highe pho oca aly ic ac i i y han he
aw sample.
Keywo ds: i anium oxide; me hyl ie hoxysilane; siloxane; plasma ea men ; UV ea men ; AO7
1. In oduc ion
The pho oca aly ic laye s a e nowadays o g ea in e es o wa e [
1
] and ai [
2
]
pu i ica ion. Ti anium dioxide (TiO
2
) laye s a e pa icula ly popula because o hei
ela i ely high e iciency, non- oxici y, and a o dabili y. TiO
2
ha e been widely applied o
con aminan emedia ion and mic oo ganism des uc ion [3,4].
TiO
2
is o en used as a coa ing on ha d and du able ma e ials such as glass. Recen ly,
he e has been an in e es in deposi ing i on lexible subs a es such as polyu e hanes,
polyes e s, poly inyl chlo ides, and o he s, o pho o ol aic, ex ile, and pape indus ies,
and o he s. The main p oblem is he s abilisa ion o TiO
2
on he subs a e in o de no o
elease i om he pho oca aly ic laye in o he en i onmen [
4
–
6
]. Gene ally, he mal me h-
ods o hund eds o deg ees Celsius [
7
] a e used o inc ease he adhesion and pho oca aly ic
ac i i y. Ano he p oblem lies in he TiO2pho oca aly ic ac i i y ha dis u bs he o ganic
ma e ials on o which TiO2is deposi ed. The main di ec ions o he esea ch a e o p o ec
he subs a es om UV and pho oca aly ic deg ada ion, o ensu e su icien pene a ion
o he pollu an s o he pho oca alys , o p o ide lexibili y, and o de elop he p ocess o
p epa ing such a pho oca aly ic sys em ha could be used on an indus ial scale [6,8].
Ca alys s 2021,11, 50. h ps://doi.o g/10.3390/ca al11010050 h ps://www.mdpi.com/jou nal/ca alys s
Ca alys s 2021,11, 50 2 o 13
One way o p o ec he subs a e om deg ada ion by he mal ea men s and pho o-
ca aly ic p ocesses is o use a sui able binde as a ma ix. The ma ix se es as a mechanical
suppo o TiO
2
and as a p o ec i e laye o he subs a e. Howe e , he bonding o he
binde i sel may no be a su icien condi ion o a unc ional pho oca aly ic laye . O he
ea men s a e needed ha may also be he mal ea men s, bu i would be possible o use
he pho oca aly ic p ocesses hemsel es in he laye using UV-i adia ion o sho - e m
exposu e o plasma o low- empe a u e ea men .
Many a eas ex ensi ely use mesopo ous oxide hin ilms as unc ional and s uc-
u al ma e ials. They include p o ec i e and low-dielec ic cons an laye s, selec i e gas
pe mea ion memb anes, we abili y laye s, and gas senso s [9–11].
In his wo k we de eloped and in es iga ed polysiloxane as he TiO
2
ancho ma ix
and we ob ain he i ania-siloxane composi ion (TSC), which can be used o deposi ion on
lexible subs a es wi hou he need o he mal ea men s which damage he subs a e.
Siloxane (oligome o polyme ) can be ob ained om o ganosilicon p ecu so s, which
can be u he doped wi hTiO
2
pa icles. Siloxane se es as a ma ix o ancho ing TiO
2
,
and, a he same ime, i can e ain po osi y o success ully adso b he pollu an s o he
su ace o he pho oca alys g ains deposi ed deepe in he laye [
6
,
12
]. The siloxane used
in his s udy con ains a ce ain p opo ion o me hyl moie ies ha p o ide solubili y and
p e en gelling. On he o he hand, he me hyl moie ies educe he ans e o elec ons
gene a ed by he TiO
2
and signi ican ly de e io a es he pho oca aly ic ac i i y o TiO
2
.
The e is also a ce ain p opo ion o hyd oxyl g oups in siloxanes. The hyd oxyl g oup, as a
esidual g oup, induces mois u e adso p ion h ough hyd ogen bonding when exposed o
mois u e [
13
]. Mois u e (H
2
O) is an impo an pa o he pho oca aly ic p ocesses aking
place in i anium dioxide. Mois e u e-binding hyd oxyl g oups imp o e he we abili y
o he su ace and he pho oca aly ic ac i i y i sel and is a be e supply o pollu an s in
he aqueous medium [
14
]. In o de o emo e he o ganic ma e om he siloxane and
imp o e he ans e o he elec ons, siloxane mus be comple ely mine alised owa ds
amo phous silica su ace. This s ep is adi ionally pe o med by he mal annealing a
hund eds o
◦
C. On he o he hand, his app oach is no compa ible i lexible and he mally-
sensi i e subs a es such as polye hylene e eph hala e (PET) and polye hylene naph hala e
(PEN) a e used. Low-cos subs a es a e a o able o u u e gene a ion manu ac u e o
eme ging echnologies, including lexible and p in ed elec onics. The e o e, i is impo an
o in es iga e no el low- empe a u e me hods compa ible wi h apid and low- empe a u e
pos - ea men o siloxanes as eplacemen s o adi ional he mal annealing ha is no
compa ible wi h lexible elec onics.
In his wo k we also in es iga ed wo non- he mal me hods o pos -p ocessing
o i ania-siloxane composi ion (TSC) laye s: UV-i adia ion and open-ai plasma. UV-
i adia ion was al eady success ully es ed o he ab ica ion o amo phous TiO
2
hin ilms.
UV-i adia ion a oom empe a u e leads o a highe conduc ion band minimum le el
o he ilm and a smalle amoun o hyd oxyl g oup a he ilm su ace, compa ed o he
he mal-assis ed (100–250
◦
C) UV-annealing o he he mal-only annealing (500
◦
C). [
15
].
Plasma ea men can be used o calcina ion and emo al o o ganic esidues om sol–gel
and gene a ion o mesopo ous ilms [
16
–
20
]. The plasma echnique is mo e a ac i e
because i has many ad an ages, such as low p ocessing empe a u e, sho p ocessing
ime and inexpensi e equipmen [9].
We s udied he p ope ies o TSC in syne gy wi h UV-i adia ion and plasma ea men
as he echniques o non- he mal cu ing o pho oca aly ic laye s o imp o e pho oca aly ic
ac i i y. The main pa ame e o he mine alisa ion o siloxane was he s udy o he dec ease
o me hyl g oups by he Fou ie - ans o m in a ed spec oscopy (FT-IR) me hod. The
pho oca aly ic ac i i y was moni o ed by ol amme ic measu emen s and pho oca aly ic
deg ada ion o AO7.
Ca alys s 2021,11, 50 3 o 13
2. Resul s and Discussion
2.1. Viscosi y o Siloxane Solu ions
Figu e 1shows iscosi y o siloxane solu ions es ed wi h a ious concen a ion o
me hanol anging om 10% o 50%. All solu ions es ed show linea dependence o dy-
namic iscosi y wi h shea a e. The sample wi h 50% o e hanol showed a sligh de ia ion
om New onian beha iou (a he lowes shea a e he e is a sign o iscosi y inc ease).
The iscosi y o he solu ions inc eases conside ably as he concen a ion inc eases, which
is impo an , especially o he u u e p in ing o composi ions con aining siloxane.
Ca alys s 2021, 11, x FOR PEER REVIEW 3 o 13
2. Resul s and Discussion
2.1. Viscosi y o Siloxane Solu ions
Figu e 1 shows iscosi y o siloxane solu ions es ed wi h a ious concen a ion o
me hanol anging om 10% o 50%. All solu ions es ed show linea dependence o dy-
namic iscosi y wi h shea a e. The sample wi h 50% o e hanol showed a sligh de ia ion
om New onian beha iou (a he lowes shea a e he e is a sign o iscosi y inc ease).
The iscosi y o he solu ions inc eases conside ably as he concen a ion inc eases, which
is impo an , especially o he u u e p in ing o composi ions con aining siloxane.
0200 400 600 800 1000
0.005
0.010
0.015
0.020
0.025
10 %
20 %
30 %
40 %
50 %
Dynamic iscosi y (Pas)
Shea a e (s–1)
Figu e 1. Viscosi y o siloxane solu ions in absolu e e hanol (concen a ions 10, 20, 30, 40, 50%).
2.2. Su ace Tension o Siloxane Solu ions
We also in es iga ed he e ec o applying concen a ions o e hanol on he su ace
ension o siloxane solu ions. The pu e e hanol has a su ace ension o 21.90 mN·m−1 a 25
°C [21]. The esul s in Table 1 show ha he e was no signi ican change in he su ace
ension o solu ions wi h a ious concen a ion o e hanol. Howe e , wi h he dec easing
concen a ion o pu e e hanol, a sligh inc ease in su ace ension occu s when he concen-
a ion o siloxane in olume, and hence on he su ace o he liquid, inc eases. We see a
ce ain simila i y wi h e hanol, conside ing he possible s uc u e o siloxane (Equa ions
(1) and (2)) whe e R−Si−(OH)3 is p esen in he solu ion, and because in ou case R means
me hyl, he hyd oca bon esidue will no ha e he weigh o signi ican ly a ec he su ace
ension. I can he e o e be assumed ha he su ace ension o he esul ing siloxane-
blended composi ions will no be signi ican ly a ec ed by he amoun o siloxane.
Table 1. Su ace ension o siloxane solu ions (concen a ions 10, 20, 30, 40, 50%).
Concen a ion (%)
10
20
30
40
50
Su ace ension (mN∙m−1)
22.58
22.96
23.52
24.08
24.75
2.3. Gel Pe mea ion Ch oma og aphy
Gel pe mea ion ch oma og aphy (GPC) esul s a e shown in Table 2. The polydis-
pe si y is close o 1, which means ha he pa icles in solu ion ha e simila dimensions,
exp essed by he adius o gy a ion and he weigh a e age mola mass.
Figu e 1. Viscosi y o siloxane solu ions in absolu e e hanol (concen a ions 10, 20, 30, 40, 50%).
2.2. Su ace Tension o Siloxane Solu ions
We also in es iga ed he e ec o applying concen a ions o e hanol on he su ace
ension o siloxane solu ions. The pu e e hanol has a su ace ension o 21.90 mN
·
m
−1
a 25
◦
C [
21
]. The esul s in Table 1show ha he e was no signi ican change in he
su ace ension o solu ions wi h a ious concen a ion o e hanol. Howe e , wi h he
dec easing concen a ion o pu e e hanol, a sligh inc ease in su ace ension occu s when
he concen a ion o siloxane in olume, and hence on he su ace o he liquid, inc eases.
We see a ce ain simila i y wi h e hanol, conside ing he possible s uc u e o siloxane
(Equa ions (1) and (2)) whe e R
−
Si
−
(OH)
3
is p esen in he solu ion, and because in ou
case R means me hyl, he hyd oca bon esidue will no ha e he weigh o signi ican ly
a ec he su ace ension. I can he e o e be assumed ha he su ace ension o he
esul ing siloxane-blended composi ions will no be signi ican ly a ec ed by he amoun
o siloxane.
Table 1. Su ace ension o siloxane solu ions (concen a ions 10, 20, 30, 40, 50%).
Concen a ion (%) 10 20 30 40 50
Su ace ension (mN·m−1)22.58 22.96 23.52 24.08 24.75
2.3. Gel Pe mea ion Ch oma og aphy
Gel pe mea ion ch oma og aphy (GPC) esul s a e shown in Table 2. The polydis-
pe si y is close o 1, which means ha he pa icles in solu ion ha e simila dimensions,
exp essed by he adius o gy a ion and he weigh a e age mola mass.
Ca alys s 2021,11, 50 4 o 13
Table 2.
The esul s o GPC. The polydispe si y is close o 1, which means ha he pa icles in
solu ion ha e simila sizes.
Dilu ion o
10% Solu ion
Injec ion Volume
(µL) MW(kDa) Polydispe si y
(MW/MN)
Radius o
Gy a ion (nm)
1:1 100 1.497 1.019 12.8
1:1 100 1.534 1.036 11.9
1:1 100 1.475 1.033 12.0
1:1 100 1.506 1.046 12.5
Diame e MW(kDa) 1.52 1.032 13
Selec i e s anda d
de ia ion 0.04 0.009 2
2.4. Speci ic Su ace A ea (SSA) and SEM o Siloxane/TiO2
Two siloxane/TiO
2
composi ions o a ios 1:1 and 1:3 we e c ea ed and compa ed.
SSA was measu ed by by ni ogen adso p ion using he BET iso he m (Figu e 2). The wo
di e en alues o siloxane/TiO
2
a ios ha e a di ec impac o he laye ex u al p ope ies
as is e iden om Figu e 3. The mo e i ania ich o mula ion exhibi ed a lu y ex u e
wi h many accessible oids while he mo e binde ich o mula ion is appa en ly dense
and mo e compac . SSA o siloxane/TiO
2
composi ions o a ios 1:1 and 1:3 was calcula ed
as 12.3 m
2
/g and 34.9 m
2
/g, espec i ely. The ques ion o phase composi ion o bo h
coa ings was add essed sepa a ely in ou p e ious communica ion [
22
] and we ound ou
ha binde mine alisa ion p ocess has no impac on he c ys allini y o i ania.
Ca alys s 2021, 11, x FOR PEER REVIEW 4 o 13
Table 2. The esul s o GPC. The polydispe si y is close o 1, which means ha he pa icles in so-
lu ion ha e simila sizes.
Dilu ion o
10% Solu ion
Injec ion Volume
(μL)
MW (kDa)
Polydispe si y
(MW/MN)
Radius o Gy-
a ion (nm)
1:1
100
1.497
1.019
12.8
1:1
100
1.534
1.036
11.9
1:1
100
1.475
1.033
12.0
1:1
100
1.506
1.046
12.5
Diame e MW (kDa)
1.52
1.032
13
Selec i e s anda d de-
ia ion
0.04
0.009
2
2.4. Speci ic Su ace A ea (SSA) and SEM o Siloxane/TiO2
Two siloxane/TiO2 composi ions o a ios 1:1 and 1:3 we e c ea ed and compa ed.
SSA was measu ed by by ni ogen adso p ion using he BET iso he m (Figu e 2). The wo
di e en alues o siloxane/TiO2 a ios ha e a di ec impac o he laye ex u al p ope ies
as is e iden om Figu e 3. The mo e i ania ich o mula ion exhibi ed a lu y ex u e
wi h many accessible oids while he mo e binde ich o mula ion is appa en ly dense
and mo e compac . SSA o siloxane/TiO2 composi ions o a ios 1:1 and 1:3 was calcula ed
as 12.3 m2/g and 34.9 m2/g, espec i ely. The ques ion o phase composi ion o bo h coa -
ings was add essed sepa a ely in ou p e ious communica ion [22] and we ound ou ha
binde mine alisa ion p ocess has no impac on he c ys allini y o i ania.
0.0 0.1 0.2 0.3
0
20
40
60
80
100
1:1
1:3
1/[W((P0/P)–1)] (g–1)
Rela i e p essu e, P/P0
Figu e 2. Mul i-poin BET plo o siloxane/TiO2 wi h a ious a io 1:1 and 1:3.
Figu e 2. Mul i-poin BET plo o siloxane/TiO2wi h a ious a io 1:1 and 1:3.
Ca alys s 2021, 11, x FOR PEER REVIEW 5 o 13
Figu e 3. SEM images. (A) Shows a less po ous laye wi h a siloxane/TiO2 a io o 1:1, and (B) shows a much mo e po ous
laye o a 1:3 a io.
2.5. The mal T ea men o TSC and TGA, DTG
The coa ings we e u he sin e ed in u nace o en a empe a u e 450 °C o 30 min
o mine alise he binde , i.e., emo e he o ganic me hyl moie ies and suppo he
anspo o elec ons om TiO2. FT-IR esul s showed in Figu e 4 e ealed ha siloxane
was no mine alised comple ely. Peaks loca ed a 2995−2950 cm−1 and 2895−2840 cm−1 co -
esponding o asymme ical and symme ical s e ching o −CH3 and indica ing he p es-
ence o me hyl g oups in polysiloxane binde .
3600 3400 3200 3000 2800
0.00
0.01
0.02
0.03
0.04
0.05
Abso bance
Wa enumbe (cm–1)
Un ea ed
450 °C
–CH3
Figu e 4. FT-IR measu emen o TSC on soda-lime glass. The p ocessing empe a u e o 450 °C
was no su icien o mine alise he siloxane because he me hyl g oups a e s ill p esen a e he -
mal ea men .
The coa ings we e u he analysed by DSC-TGA and hea ed o 1300 °C. Du ing he
he mal p ocess, he e we e se e al changes as he empe a u e ose (Figu e 5). The i s
ange was om 20 °C o 477 °C. The e was p obably a loss o so bed ai mois u e o e-
sidual sol en . Weigh dec eases s eeply. The second majo change occu s a 478 °C, when
he e was a u he apid d op in weigh . Siloxane should be calcined a his empe a u e
and hus deg ade he p ima ily me hyl g oups bonded o silicon oxide. Ano he weigh
change was signi ican ly slowed down om a empe a u e o 600 °C un il i s abilised and
no changes occu ed. I is clea om he measu emen s ha , o comple e calcina ion, i is
necessa y o achie e empe a u es highe han 478 °C, and he e o e, he he mal me hod
o mine alisa ion is only sui able o esis an ma e ials.
Figu e 3.
SEM images. (
A
) Shows a less po ous laye wi h a siloxane/TiO
2
a io o 1:1, and (
B
) shows a much mo e po ous
laye o a 1:3 a io.
Ca alys s 2021,11, 50 5 o 13
2.5. The mal T ea men o TSC and TGA, DTG
The coa ings we e u he sin e ed in u nace o en a empe a u e 450
◦
C o 30 min
o mine alise he binde , i.e., emo e he o ganic me hyl moie ies and suppo he anspo
o elec ons om TiO
2
. FT-IR esul s showed in Figu e 4 e ealed ha siloxane was
no mine alised comple ely. Peaks loca ed a 2995
−
2950 cm
−1
and 2895
−
2840 cm
−1
co esponding o asymme ical and symme ical s e ching o
−
CH
3
and indica ing he
p esence o me hyl g oups in polysiloxane binde .
Ca alys s 2021, 11, x FOR PEER REVIEW 5 o 13
Figu e 3. SEM images. (A) Shows a less po ous laye wi h a siloxane/TiO2 a io o 1:1, and (B) shows a much mo e po ous
laye o a 1:3 a io.
2.5. The mal T ea men o TSC and TGA, DTG
The coa ings we e u he sin e ed in u nace o en a empe a u e 450 °C o 30 min
o mine alise he binde , i.e., emo e he o ganic me hyl moie ies and suppo he
anspo o elec ons om TiO2. FT-IR esul s showed in Figu e 4 e ealed ha siloxane
was no mine alised comple ely. Peaks loca ed a 2995−2950 cm−1 and 2895−2840 cm−1 co -
esponding o asymme ical and symme ical s e ching o −CH3 and indica ing he p es-
ence o me hyl g oups in polysiloxane binde .
3600 3400 3200 3000 2800
0.00
0.01
0.02
0.03
0.04
0.05
Abso bance
Wa enumbe (cm–1)
Un ea ed
450 °C
–CH3
Figu e 4. FT-IR measu emen o TSC on soda-lime glass. The p ocessing empe a u e o 450 °C
was no su icien o mine alise he siloxane because he me hyl g oups a e s ill p esen a e he -
mal ea men .
The coa ings we e u he analysed by DSC-TGA and hea ed o 1300 °C. Du ing he
he mal p ocess, he e we e se e al changes as he empe a u e ose (Figu e 5). The i s
ange was om 20 °C o 477 °C. The e was p obably a loss o so bed ai mois u e o e-
sidual sol en . Weigh dec eases s eeply. The second majo change occu s a 478 °C, when
he e was a u he apid d op in weigh . Siloxane should be calcined a his empe a u e
and hus deg ade he p ima ily me hyl g oups bonded o silicon oxide. Ano he weigh
change was signi ican ly slowed down om a empe a u e o 600 °C un il i s abilised and
no changes occu ed. I is clea om he measu emen s ha , o comple e calcina ion, i is
necessa y o achie e empe a u es highe han 478 °C, and he e o e, he he mal me hod
o mine alisa ion is only sui able o esis an ma e ials.
Figu e 4.
FT-IR measu emen o TSC on soda-lime glass. The p ocessing empe a u e o 450
◦
C was no
su icien o mine alise he siloxane because he me hyl g oups a e s ill p esen a e he mal ea men .
The coa ings we e u he analysed by DSC-TGA and hea ed o 1300
◦
C. Du ing he
he mal p ocess, he e we e se e al changes as he empe a u e ose (Figu e 5). The i s
ange was om 20
◦
C o 477
◦
C. The e was p obably a loss o so bed ai mois u e o esidual
sol en . Weigh dec eases s eeply. The second majo change occu s a 478
◦
C, when he e
was a u he apid d op in weigh . Siloxane should be calcined a his empe a u e and
hus deg ade he p ima ily me hyl g oups bonded o silicon oxide. Ano he weigh change
was signi ican ly slowed down om a empe a u e o 600
◦
C un il i s abilised and no
changes occu ed. I is clea om he measu emen s ha , o comple e calcina ion, i is
necessa y o achie e empe a u es highe han 478 ◦C, and he e o e, he he mal me hod
o mine alisa ion is only sui able o esis an ma e ials.
Ca alys s 2021, 11, x FOR PEER REVIEW 6 o 13
Figu e 5. The esul s o he TGA, DTG. The mine alisa ion and emo al o he me hyl g oups om
he siloxane should occu a 478 °C.
2.6. FT-IR o Non he mal Cu ing TSC
UV-i adia ion and non- he mal a mosphe ic-p essu e plasma ea men we e u -
he in es iga ed in o de o eplace ime-consuming he mal sin e ing, which is p oblem-
a ic i polysiloxane/TiO2 is deposi ed on he mally-sensi i e ma e ials. Bo h siloxane/TiO2
composi ions o a ios 1:1 and 1:3 we e in es iga ed by FT-IR be o e and a e UV-i adi-
a ion and plasma ea men .
Figu e 6A,B shows FT-IR spec a o siloxane/TiO2 laye s p epa ed by bo h composi-
ions o a ios 1:1 and 1:3, cu ed by UV-i adia ion o 0–210 min. The dec ease o peaks
ela ed o asymme ical and symme ical s e ching o −CH3 (2995−2950 cm−1 and
2895−2840 cm−1) indica e he emo al o −CH3 g oups om siloxane su ace and appa -
en ly he ans o ma ion o siloxane owa ds amo phous SiO2. Fo bo h coa ings (1:1 and
1:3 a io), UV-i adia ion o 150 min was enough o emo e all de ec able −CH3 g oups.
This dec ease can be u he explained by pho oca aly ical eac ion be ween ana ase TiO2
wi h bandgap 3.2 eV [23] ha can p o ide addi ional ca aly ical eac ion on polysiloxane
su ace and enhance he deg ada ion o me hyl g oups.
Figu e 6C,D shows FT-IR spec a o siloxane/TiO2 laye s p epa ed by bo h composi-
ions o a ios 1:1 and 1:3, cu ed by plasma o 0–32 s. In con as wi h he esul s p esen ed
o UV-i adia ed laye s, he plasma ea ed laye s showed signi ican ly lowe e iciency
o me hyl g oups emo al. The mos p o ound di e ence was ound o he ilm o lowe
po osi y (1:1) and highe e iciency was ound o he laye o highe po osi y. This dis-
c epancy is clea ly ela ed o he di e ence in mechanism o UV-i adia ion and plasma
ea men . Whe eas he dominan ene gy- ans e mechanism in UV-i adia ion is he
anspo o pho ons o ce ain wa eleng h in o he po ous ilm, he plasma ea men
wo ks di e en ly. Apa om he UV-i adia ion, he plasma gene a ed a a mosphe ic
p essu e in ambien ai con ains a ious ene ge ic species ha can ca y ene gy owa ds
siloxane su ace: High- empe a u e elec ons, low- empe a u e ions, exci ed species and
me as ables. These species, howe e , ecombine and ex inc in con ac wi h ma e ials su -
ace and hus, he po osi y o ma e ial plays an impo an ole in he limi a ion o such
me hod. Since plasma ea men was less e icien o me hyl emo al a less po ous ma-
e ial, i can be concluded ha plasma canno e icien ly pene a e in o he coa ing bulk
and mine alise i . On he o he hand, he coa ing wi h high po osi y showed be e e i-
ciency o me hyl emo al, and he e o e, i is impo an o ind an op imal combina ion o
laye po osi y i a mosphe ic p essu e plasma is used o mine alisa ion o coa ings. Al -
hough no e icien as UV-i adia ion, a clea bene i o plasma ea men is in signi ican ly
as e ea men imes, in o de o en seconds, which allow o use his me hod on as oll-
o- oll p oduc ion lines and employ he coa ings in lexible and p in ed elec onics con-
cep .
Figu e 5.
The esul s o he TGA, DTG. The mine alisa ion and emo al o he me hyl g oups om
he siloxane should occu a 478 ◦C.
Ca alys s 2021,11, 50 6 o 13
2.6. FT-IR o Non he mal Cu ing TSC
UV-i adia ion and non- he mal a mosphe ic-p essu e plasma ea men we e u he
in es iga ed in o de o eplace ime-consuming he mal sin e ing, which is p oblema ic
i polysiloxane/TiO
2
is deposi ed on he mally-sensi i e ma e ials. Bo h siloxane/TiO
2
composi ions o a ios 1:1 and 1:3 we e in es iga ed by FT-IR be o e and a e UV-i adia ion
and plasma ea men .
Figu e 6A,B shows FT-IR spec a o siloxane/TiO
2
laye s p epa ed by bo h com-
posi ions o a ios 1:1 and 1:3, cu ed by UV-i adia ion o 0–210 min. The dec ease o
peaks ela ed o asymme ical and symme ical s e ching o
−
CH
3
(2995
−
2950 cm
−1
and
2895
−
2840 cm
−1
) indica e he emo al o
−
CH
3
g oups om siloxane su ace and appa -
en ly he ans o ma ion o siloxane owa ds amo phous SiO
2
. Fo bo h coa ings (1:1 and
1:3 a io), UV-i adia ion o 150 min was enough o emo e all de ec able
−
CH
3
g oups.
This dec ease can be u he explained by pho oca aly ical eac ion be ween ana ase TiO
2
wi h bandgap 3.2 eV [
23
] ha can p o ide addi ional ca aly ical eac ion on polysiloxane
su ace and enhance he deg ada ion o me hyl g oups.
Ca alys s 2021, 11, x FOR PEER REVIEW 7 o 13
3600 3400 3200 3000 2800
0.02
0.04
0.06
FT-IR (UV 0–210 min)
siloxane/TiO2 (1:1)
0 min
15 min
30 min
60 min
90 min
150 min
210 min
Abso bance
Wa enumbe (cm–1)
–CH3
3600 3400 3200 3000 2800
0.00
0.02
0.04
0.06
FT-IR (UV 0–210 min)
siloxane/TiO2 (1:3)
0 min
15 min
30 min
60 min
90 min
150 min
210 min
Abso bance
Wa enumbe (cm–1)
–CH3
(A)
(B)
3600 3400 3200 3000 2800
0.02
0.04
0.06 0 s
2 s
4 s
8 s
16 s
32 s
Abso bance
Wa enumbe (cm–1)
FT-IR (plasma 0–32 s)
siloxane/TiO2 (1:1)
–CH3
3600 3400 3200 3000 2800
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
FT-IR (plasma 0–32 s)
siloxane/TiO2 (1:3)
0 s
2 s
4 s
8 s
16 s
32s
Abso bance
Wa enumbe (cm–1)
–CH3
(C)
(D)
Figu e 6. FT-IR measu emen s o TSC laye s. (A,B) a e images o samples ea ed wi h UV-i adia ion; (C,D) a e images
o plasma- ea ed samples. UV-i adia ion has be e laye pene a ion and is he e o e e ec i e o laye s wi h a highe
concen a ion o siloxane.
2.7. Vol amme ic Measu emen s o Non he mal Cu ing TSC
Vol amme ic measu emen s can be aken o he pho oca aly ic ac i i y es . I he
ea ed laye is exposed o UV-i adia ion, i gene a es an elec on/hole pai . By applying
he ex e nal ol age, he elec ons can be abs ac ed om he exci on and he pho ocu en
is de ec ed in he ex e nal ci cui . The g ow h o he pho ocu en is ela ed o highe pho-
oca aly ic ac i i y [24].
The siloxane/TiO2 ilms o a ios 1:1 and 1:3 we e exposed o UV-i adia ion o 0–
120 min and plasma ea men o 0–32 s. Figu e 7 shows he ol ame ic measu emen o
pho ocu en gene a ed by he coa ing upon exposu e o UV. The UV-i adia ion p e-
ea men o 0–120 min led o a g adual inc ease o he pho ocu en o bo h coa ings
(1:1, 1:3) in es iga ed. Highe alues o pho ocu en we e measu ed o coa ing wi h a
highe concen a ion o TiO2 in siloxane/TiO2 ( a io 1:3). This is appa en ly hanks o
highe concen a ion o pho o-ca aly ically ac i e TiO2 ha can no only imp o e he min-
e alisa ion eac ion, bu also supply mo e elec ons upon UV exposu e. Fu he mo e, he
po osi y o he coa ing wi h siloxane/TiO2 a io 1:3 is highe , so he highe pho ocu en is
mos likely an in e play be ween hese ac o s.
Figu e 7C,D shows ol amme ic measu emen s o siloxane/TiO2 ilms o a ios 1:1
and 1:3 we e exposed o ai plasma o 0–32 s. The coa ing wi h small po osi y yields e y
Figu e 6.
FT-IR measu emen s o TSC laye s. (
A
,
B
) a e images o samples ea ed wi h UV-i adia ion; (
C
,
D
) a e images
o plasma- ea ed samples. UV-i adia ion has be e laye pene a ion and is he e o e e ec i e o laye s wi h a highe
concen a ion o siloxane.
Figu e 6C,D shows FT-IR spec a o siloxane/TiO
2
laye s p epa ed by bo h composi-
ions o a ios 1:1 and 1:3, cu ed by plasma o 0–32 s. In con as wi h he esul s p esen ed
o UV-i adia ed laye s, he plasma ea ed laye s showed signi ican ly lowe e iciency
Ca alys s 2021,11, 50 7 o 13
o me hyl g oups emo al. The mos p o ound di e ence was ound o he ilm o lowe
po osi y (1:1) and highe e iciency was ound o he laye o highe po osi y. This dis-
c epancy is clea ly ela ed o he di e ence in mechanism o UV-i adia ion and plasma
ea men . Whe eas he dominan ene gy- ans e mechanism in UV-i adia ion is he
anspo o pho ons o ce ain wa eleng h in o he po ous ilm, he plasma ea men
wo ks di e en ly. Apa om he UV-i adia ion, he plasma gene a ed a a mosphe ic
p essu e in ambien ai con ains a ious ene ge ic species ha can ca y ene gy owa ds
siloxane su ace: High- empe a u e elec ons, low- empe a u e ions, exci ed species and
me as ables. These species, howe e , ecombine and ex inc in con ac wi h ma e ials
su ace and hus, he po osi y o ma e ial plays an impo an ole in he limi a ion o such
me hod. Since plasma ea men was less e icien o me hyl emo al a less po ous ma e-
ial, i can be concluded ha plasma canno e icien ly pene a e in o he coa ing bulk and
mine alise i . On he o he hand, he coa ing wi h high po osi y showed be e e iciency
o me hyl emo al, and he e o e, i is impo an o ind an op imal combina ion o laye
po osi y i a mosphe ic p essu e plasma is used o mine alisa ion o coa ings. Al hough
no e icien as UV-i adia ion, a clea bene i o plasma ea men is in signi ican ly as e
ea men imes, in o de o en seconds, which allow o use his me hod on as oll- o- oll
p oduc ion lines and employ he coa ings in lexible and p in ed elec onics concep .
2.7. Vol amme ic Measu emen s o Non he mal Cu ing TSC
Vol amme ic measu emen s can be aken o he pho oca aly ic ac i i y es . I he
ea ed laye is exposed o UV-i adia ion, i gene a es an elec on/hole pai . By applying
he ex e nal ol age, he elec ons can be abs ac ed om he exci on and he pho ocu en
is de ec ed in he ex e nal ci cui . The g ow h o he pho ocu en is ela ed o highe
pho oca aly ic ac i i y [24].
The siloxane/TiO
2
ilms o a ios 1:1 and 1:3 we e exposed o UV-i adia ion o
0–120 min
and plasma ea men o 0–32 s. Figu e 7shows he ol ame ic measu emen
o pho ocu en gene a ed by he coa ing upon exposu e o UV. The UV-i adia ion p e-
ea men o 0–120 min led o a g adual inc ease o he pho ocu en o bo h coa ings
(1:1, 1:3) in es iga ed. Highe alues o pho ocu en we e measu ed o coa ing wi h
a highe concen a ion o TiO
2
in siloxane/TiO
2
( a io 1:3). This is appa en ly hanks
o highe concen a ion o pho o-ca aly ically ac i e TiO
2
ha can no only imp o e he
mine alisa ion eac ion, bu also supply mo e elec ons upon UV exposu e. Fu he mo e,
he po osi y o he coa ing wi h siloxane/TiO
2
a io 1:3 is highe , so he highe pho ocu en
is mos likely an in e play be ween hese ac o s.
Figu e 7C,D shows ol amme ic measu emen s o siloxane/TiO
2
ilms o a ios 1:1
and 1:3 we e exposed o ai plasma o 0–32 s. The coa ing wi h small po osi y yields
e y small pho ocu en s, and i seems ha plasma ea men had a nega i e e ec on he
maximal pho ocu en alues. On he o he hand, he plasma ea men o he coa ing wi h
highe po osi y (1:3) esul ed in a g adual inc ease in pho ocu en wi h plasma-exposu e
ime. The be e e iciency in pho ocu en gene a ion in he coa ing o highe po osi y is
ela ed o highe mine alisa ion e iciency, as shown in FT-IR esul s p esen ed in
Figu e 6D.
Ca alys s 2021,11, 50 8 o 13
Ca alys s 2021, 11, x FOR PEER REVIEW 8 o 13
small pho ocu en s, and i seems ha plasma ea men had a nega i e e ec on he max-
imal pho ocu en alues. On he o he hand, he plasma ea men o he coa ing wi h
highe po osi y (1:3) esul ed in a g adual inc ease in pho ocu en wi h plasma-exposu e
ime. The be e e iciency in pho ocu en gene a ion in he coa ing o highe po osi y is
ela ed o highe mine alisa ion e iciency, as shown in FT-IR esul s p esen ed in Figu e
6D.
-0.5 0.0 0.5 1.0 1.5 2.0
0
10
20
30
40
50
0 min
15 min
30 min
60 min
90 min
120 min
Pho ocu en (mA)
Po en ial s. FTO (V)
UV 0–120 min, siloxane/TiO2 (1:1)
-0.5 0.0 0.5 1.0 1.5 2.0
0
10
20
30
40
50
60
70
80
Pho ocu en (mA)
0 min
15 min
30 min
60 min
90 min
120 min
Po en ial s. FTO (V)
UV 0–120 min, siloxane/TiO2 (1:3)
(A)
(B)
-0.5 0.0 0.5 1.0 1.5 2.0
0
10
20
30
40
50
0 s
2 s
4 s
8 s
16 s
32 s
Pho ocu en (mA)
Po en ial s. FTO (V)
Plasma 0–32 s, siloxane/TiO2 (1:1)
0.0 0.5 1.0 1.5 2.0
0
5
10
15
20
25
30
35
40
45
50
0 s
2 s
4 s
8 s
16 s
32 s
Pho ocu en (mA)
Po en ial s. FTO (V)
Plasma 0–32 s, siloxane/TiO2 (1:3)
(C)
(D)
Figu e 7. Vol amme ic cha ac e isa ion o TSC p in ed on FTO-coa ed glass. (A,B) a e images o samples ea ed wi h
UV-i adia ion; (C,D) a e images o plasma- ea ed samples. Plasma can no pene a e he laye su icien ly i i con ains
oo much siloxane. In he case o (C), he pho ocu en is e y low o ze o.
2.8. Pho oca aly ic Deg ada ion o Acid O ange 7
Acid O ange 7 (AO7) se es as a model pollu an o he pho odeg ada ion es . AO7
u ns he aqueous solu ion o o ange and i is a esis an subs ance ha slow deg ades.
Figu e 8 shows he dec ease in abso bance signal e lec ing he concen a ion o AO7 upon
i adia ion o UVA o 1 h. UVA was chosen because adia ion wi h lowe wa eleng hs is
less ad an ageous in e ms o ene gy e iciency. We compa ed h ee samples wi h a io
1:3: un ea ed siloxane/TiO2, UV-i adia ed siloxane/TiO2 o 210 min and plasma- ea ed
siloxane/TiO2 o 32 s. The slowes deg ada ion o AO7 was obse ed o un ea ed silox-
ane/TiO2, whe eas he as es deg ada ion was obse ed o UV-i adia ed siloxane/TiO2
o 210 min. The plasma- ea ed siloxane/TiO2 showed signi ican imp o emen in AO7
Figu e 7.
Vol amme ic cha ac e isa ion o TSC p in ed on FTO-coa ed glass. (
A
,
B
) a e images o samples ea ed wi h
UV-i adia ion; (
C
,
D
) a e images o plasma- ea ed samples. Plasma can no pene a e he laye su icien ly i i con ains
oo much siloxane. In he case o (C), he pho ocu en is e y low o ze o.
2.8. Pho oca aly ic Deg ada ion o Acid O ange 7
Acid O ange 7 (AO7) se es as a model pollu an o he pho odeg ada ion es . AO7
u ns he aqueous solu ion o o ange and i is a esis an subs ance ha slow deg ades.
Figu e 8shows he dec ease in abso bance signal e lec ing he concen a ion o AO7
upon i adia ion o UVA o 1 h. UVA was chosen because adia ion wi h lowe wa e-
leng hs is less ad an ageous in e ms o ene gy e iciency. We compa ed h ee samples
wi h a io 1:3: un ea ed siloxane/TiO
2
, UV-i adia ed siloxane/TiO
2
o 210 min and
plasma- ea ed siloxane/TiO
2
o 32 s. The slowes deg ada ion o AO7 was obse ed o
un ea ed siloxane/TiO
2
, whe eas he as es deg ada ion was obse ed o UV-i adia ed
siloxane/TiO
2
o 210 min. The plasma- ea ed siloxane/TiO
2
showed signi ican imp o e-
men in AO7 deg ada ion in compa ison o he un ea ed sample, al hough he e iciency
o siloxane/TiO
2
i adia ed by UV was be e . I should be no ed ha UVA in ol ed in
AO7 deg ada ion es may u he con ibu e o mine alisa ion o TSC [25].
Ca alys s 2021,11, 50 9 o 13
Ca alys s 2021, 11, x FOR PEER REVIEW 9 o 13
deg ada ion in compa ison o he un ea ed sample, al hough he e iciency o silox-
ane/TiO2 i adia ed by UV was be e . I should be no ed ha UVA in ol ed in AO7 deg-
ada ion es may u he con ibu e o mine alisa ion o TSC [25].
0500 1000 1500 2000 2500 3000 3500
0.35
0.40
0.45
0.50
Blank
Raw
Plasma 32 s
UV 210 min
Abso bance
Time (s)
Figu e 8. Deg ada ion o 2 mg/L aqueous solu ion o AO7 wi h immobilized TSC laye s ea ed by
plasma and UV-i adia ion.
3. Ma e ials and Me hods
3.1. Syn hesis o Siloxane
The s a ing subs ance o he syn hesis o siloxane was me hyl ie hoxysilane
(MTEOS) (Al a Aesa , 98%, Ha e hill, MA, USA). MTEOS was hyd olysed wi h acidic
wa e . E hanol was o med du ing he hyd olysis and subsequen ly dis illed. Siloxane
was ex ac ed wi h die hyl e he (Pen a, 99.7%), and i was dissol ed in absolu e e hanol
(Pen a, 99.8%, P ague, Czech Republic) a e e apo a ion o he ex ac ing agen . The si-
loxane solu ion in e hanol was s o ed a a empe a u e below 0 °C [6]. The expec ed chem-
ical eac ion can be summa ised by he ollowing equa ion [26]:
OHH3COHSiRO3HHOCSiR52
3
2
3
52
(1)
OHROHSiOOHSiROHSi2R 2
223 -----
(2)
3.2. P epa a ion o TSC
Two se ies o samples we e p epa ed o expe imen s, as shown in Table 3. TiO2 (P25,
Sigma Ald ich, 99.7%, S . Louis, MO, USA, pa icle size ≤ 25 nm, SSA 45–55 m2/g [27]) was
dispe sed in dowanol and a siloxane 20% solu ion was added. The esul ing suspension
was u he dilu ed wi h hexanol. The TSC we e p in ed wi h a Dima ix (DMP-2800) ma-
e ial p in e on soda-lime glass and FTO-coa ed glass.
Table 3. Composi ions designed o ma e ial p in ing.
Composi ion
Siloxane (20% in E hanol)
TiO2 (20% P25 in Dowanol)
Hexanol
E38-9AD (1:1)
4 mL
4 mL
20 mL
E38-10AD (1:3)
2 mL
6 mL
20 mL
3.3. Mine alisa ion o he P in ed TSC on he Subs a e
The siloxane/TiO2 coa ings we e mine alised by wo me hods: UV-i adia ion and
plasma ea men . UV-i adia ion was gene a ed by a Syl ania UV lamp Figu e 9 (me -
cu y, 125 W, Budapes , Hunga y). The adia ion in ensi y was se and held a 9 mW∙cm−2.
Figu e 8.
Deg ada ion o 2 mg/L aqueous solu ion o AO7 wi h immobilized TSC laye s ea ed by
plasma and UV-i adia ion.
3. Ma e ials and Me hods
3.1. Syn hesis o Siloxane
The s a ing subs ance o he syn hesis o siloxane was me hyl ie hoxysilane (MTEOS)
(Al a Aesa , 98%, Ha e hill, MA, USA). MTEOS was hyd olysed wi h acidic wa e . E hanol
was o med du ing he hyd olysis and subsequen ly dis illed. Siloxane was ex ac ed
wi h die hyl e he (Pen a, 99.7%), and i was dissol ed in absolu e e hanol (Pen a, 99.8%,
P ague, Czech Republic) a e e apo a ion o he ex ac ing agen . The siloxane solu ion in
e hanol was s o ed a a empe a u e below 0
◦
C [
6
]. The expec ed chemical eac ion can be
summa ised by he ollowing equa ion [26]:
R−Si(OC2H5)3+3H2O→R−Si(OH)3+3C2H5OH (1)
2R −Si(OH)3↔R−Si(OH)2−O−Si(OH)2−R+H2O (2)
3.2. P epa a ion o TSC
Two se ies o samples we e p epa ed o expe imen s, as shown in Table 3. TiO
2
(P25,
Sigma Ald ich, 99.7%, S . Louis, MO, USA, pa icle size
≤
25 nm, SSA 45–55 m
2
/g [
27
]) was
dispe sed in dowanol and a siloxane 20% solu ion was added. The esul ing suspension
was u he dilu ed wi h hexanol. The TSC we e p in ed wi h a Dima ix (DMP-2800)
ma e ial p in e on soda-lime glass and FTO-coa ed glass.
Table 3. Composi ions designed o ma e ial p in ing.
Composi ion Siloxane (20% in E hanol) TiO2(20% P25 in Dowanol) Hexanol
E38-9AD (1:1) 4 mL 4 mL 20 mL
E38-10AD (1:3) 2 mL 6 mL 20 mL
3.3. Mine alisa ion o he P in ed TSC on he Subs a e
The siloxane/TiO
2
coa ings we e mine alised by wo me hods: UV-i adia ion and
plasma ea men . UV-i adia ion was gene a ed by a Syl ania UV lamp Figu e 9(me cu y,
125 W, Budapes , Hunga y). The adia ion in ensi y was se and held a 9 mW
·
cm
−2
.
Samples we e placed unde 5 mm o dis illed wa e and i adia ed o 0, 15, 30, 60, 90, 150,
and 210 min.
