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Elec ochemis y Communica ions
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Fab ica ion o TiO
2
nano ubes on Ti sphe es using bipola elec ochemis y
Hanna Sopha
a,b,⁎
, Ludek H omadko
a,b
, Ma in Mo ola
a
, Jan M. Macak
a,b
a
Cen e o Ma e ials and Nano echnologies, Facul y o Chemical Technology, Uni e si y o Pa dubice, Nam. Cs. Legii 565, 53002 Pa dubice, Czech Republic
b
Cen al Eu opean Ins i u e o Technology, B no Uni e si y o Technology, Pu kyno a 123, 612 00 B no, Czech Republic
ARTICLE INFO
Keywo ds:
TiO
2
nano ube laye s
Ti sphe es
Bipola elec ochemis y
Pho oca alysis
ABSTRACT
In his wo k, he anodiza ion o Ti sphe es using bipola elec ochemis y is epo ed o he i s ime. TiO
2
nano ubes we e ound o e he en i e su ace a ea o he Ti sphe es when a squa e-wa e po en ial was employed.
The TiO
2
nano ubes we e ~77 nm in inne diame e and had a hickness o ~2 µm on he ex emi ies o he Ti
sphe es. Due o hei inc eased su ace a ea, he Ti sphe es co e ed wi h TiO
2
nano ubes had a a e cons an o
he pho oca aly ic deg ada ion o me hylene blue which was app oxima ely 2.15 imes highe han ha o non-
anodized Ti sphe es wi h a hin he mal oxide laye .
1. In oduc ion
Since hei in oduc ion [1,2], anodic TiO
2
nano ube (TNT) laye s
ha e ecei ed a g ea deal o scien i ic a en ion due o hei possible
applica ions in many di e en ields, as desc ibed in a ious e iews
[3–5]. In a con en ional se -up, hese TNT laye s a e p oduced in a wo-
elec ode con igu a ion using a Ti subs a e as he anode and a P
elec ode as he ca hode in a luo ide-con aining elec oly e. Bo h
elec odes a e usually plana . Howe e , in his con igu a ion i is no
possible o anodize small Ti objec s which canno be p ope ly elec-
ically connec ed o he po en ios a , such as small Ti sphe es.
Bipola elec ochemis y, a echnique known o se e al decades [6,7],
can be used o gene a e edox eac ions on he su ace o conduc i e ob-
jec s wi hou a di ec elec ical con ac [8,9]. The elec ochemical se -up
consis s o an elec onic conduc o in an elec oly e solu ion, exposed o an
ex e nal elec ical ield applied be ween wo eede elec odes. Due o he
ex e nal elec ical ield be ween he eede elec odes, he conduc o , he
so-called bipola elec ode (BPE), is pola ized wi h espec o he su -
ounding solu ion. The BPE hus beha es like an anode on he side acing
he eede ca hode and like a ca hode on he side acing he eede anode,
and i he applied po en ial is high enough edox eac ions can be d i en
on i s ex emi ies [8]. This echnique can be employed, o ins ance, o
elec odeposi ion and he p oduc ion o asymme ically modi ied objec s
on he mic o- o nanoscale (so-called Janus pa icles) [10–12], he de-
elopmen o swimme s [13,14] o he anodiza ion o me als o p oduce
nano ubes o nanopo es [15,16].
In he case o TNT laye s, only a ew publica ions using bipola
elec ochemis y can be ound, desc ibing ei he he p oduc ion o
nano ubes o di e en leng hs and diame e s on one single Ti subs a e
[15,17,18] o TNT g ow h in he ho izon al di ec ion on Ti oils using
small amoun s o elec oly e solu ion [19]. Recen ly, i was also shown
ha bipola elec ochemis y can be used o deco a e TNT laye s wi h
Ag nanopa icles o di e en sizes and densi ies a di e en places on
he TNT laye [20]. Fu he mo e, Asoh e al. showed he possibili y o
using bipola elec ochemis y o he indi ec oxida ion o Al unde an
AC elec ic ield [16,21]. In his wo k, po ous alumina ilms we e
o med homogenously on Al shee s as well as on small Al sphe es o e
he en i e mac oscopic su ace a ea. Howe e , po en ial applica ions o
he anodized Al sphe es we e no shown o discussed.
In his communica ion, he anodiza ion o small Ti sphe es (wi h a
diame e o 3 mm) using bipola elec ochemis y is epo ed o he
i s ime. Fo he con o mal anodiza ion o he en i e mac oscopic
su ace a ea, a squa e-wa e ol age was applied in a luo ide-con-
aining e hylene glycol-based elec oly e. The pho oca aly ic deg ada-
ion o an o ganic dye (me hylene blue) is shown as a po en ial appli-
ca ion o he anodized Ti sphe es.
2. Expe imen al
The anodiza ion se -up consis ed o wo P elec odes, used as eede
elec odes, sepa a ed by a dis ance o ~1 cm, connec ed o a high-
ol age po en ios a (PGU-200 V; Elek oniklabo GmbH). Ti sphe es
(Good ellow, 99.6+%, 3 mm diame e ), cleaned in isop opanol and
ace one, we e placed cen ally be ween he wo eede elec odes (see
Fig. 1a) on adhesi e ape and anodized in 200 ml o an e hylene glycol-
based elec oly e con aining 170 mM NH
4
F and 1.5 ol% H
2
O. The
h ps://doi.o g/10.1016/j.elecom.2020.106669
Recei ed 19 Decembe 2019; Recei ed in e ised o m 10 Janua y 2020; Accep ed 14 Janua y 2020
⁎
Co esponding au ho a : Cen e o Ma e ials and Nano echnologies, Facul y o Chemical Technology, Uni e si y o Pa dubice, Nam. Cs. Legii 565, 53002
Pa dubice, Czech Republic.
E-mail add ess: [email p o ec ed] (H. Sopha).
Elec ochemis y Communica ions 111 (2020) 106669
A ailable online 24 Janua y 2020
1388-2481/ © 2020 The Au ho s. Published by Else ie B.V. This is an open access a icle unde he CC BY-NC-ND license
(h p://c ea i ecommons.o g/licenses/BY-NC-ND/4.0/).
T
adhesi e ape was used o a oid andom mo emen o he Ti sphe es in
he elec oly e du ing anodiza ion. The anodiza ion was ca ied ou
using a ec angula squa e wa e po en ial wi h an ampli ude o ±
120 V and a equency o 0.0166 Hz (i.e. holding he po en ial o
1 min a ± 120 V be o e changing he di ec ion). The o al anodiza ion
ime was 200 min. The elec oly e was cooled be o e and du ing ano-
diza ion using a he mos a and a cooling coil. The cooling empe a u e
was se o 10 °C. A e anodiza ion he Ti sphe es we e sonica ed in
isop opanol o 5 min and d ied in ai . Be o e es ing hei pho o-
ca alysis p ope ies, he Ti sphe es we e annealed in a mu le o en a
400 °C o 1 h o p oduce he ana ase phase.
The s uc u al and mo phological ea u es o he anodized Ti
sphe es we e cha ac e ized by a ield-emission scanning elec on mi-
c oscope (FE-SEM, JEOL, JSM 7500F). Fo c oss-sec ional images, a Ti
sphe e was ca e ully sc a ched o ob ain some nano ube bundles sui-
able o inspec ion.
The X- ay di ac ion (XRD) pa e ns we e measu ed on a
Panaly ical Empy ean di ac ome e using B agg-B en ano geome y.
Mic o-di ac ion analysis was used. The di ac ome e was equipped
wi h a Cu X- ay ube in poin ocus mode and mono-capilla y o
135 mm leng h wi h 0.1 mm diame e size ou pu beam. The de ec o
was Pixcel
3D
, wo king in scanning mode. The pa e ns we e measu ed
in he 2θ ange o 20–65° wi h a s ep size o 0.026°.
Fo he pho oca aly ic deg ada ion o me hylene blue (MB), en
anodized and annealed o en non-anodized and annealed ( o p oduce a
he mal TiO
2
laye ) Ti sphe es we e imme sed in 4.4 ml o a 1 × 10
−5
M MB solu ion o 60 min unde cons an s i ing o achie e a dye
adso p ion/deso p ion equilib ium. A e wa ds, he samples we e i a-
dia ed by a LED-based UV lamp (10 W, λ = 365 nm ± 5 nm) and he
abso bance o he MB solu ion was pe iodically measu ed (10 o 30 min
s eps) using a UV–VIS spec ome e (S-200, Boeco) a a wa eleng h o
670 nm o moni o he deg ada ion a e. The olume o he MB solu ion
was chosen o ha e a simila a io o mac oscopic su ace a ea o o-
lume o MB as in o he publica ions by ou g oup, whe e anodized Ti
s ips wi h a mac oscopic su ace a ea o 2.25 cm
2
we e imme sed in
3.5 ml o MB solu ion [22–25].
3. Resul s and discussion
Fig. 1b shows he po en ial- ime plo du ing he i s six minu es o
anodiza ion. As one can see, +120 V o −120 V we e applied in u n
o a pe iod o 1 min each. In o al, a pola iza ion ime o 200 min was
employed. The squa e-wa e po en ial was used o anodize he en i e Ti
sphe e, i.e. on all sides. As desc ibed in he in oduc ion, he po en ial
applied be ween he eede elec odes pola ized he Ti sphe e, which
ac s as a bipola elec ode (BPE). Thus, one side o he Ti sphe e was
oxidized while a educ ion eac ion p oceeded on he o he side (i.e.
educ ion o H
+
o H
2
). Due o he squa e-wa e po en ial bo h sides o
he Ti sphe e we e oxidized al e na ely o e 100 cycles. When a single
pola iza ion cycle was used, i.e. +120 V was applied o 100 min and
hen −120 V o 100 min, TiO
2
nano ubes (TNTs) we e ound only on
he la e anodized side o he Ti sphe e. The eason o his is mos
likely ha he massi e and long- e m H
2
e olu ion o 100 min on he
ca hodic side o he Ti sphe e des oyed he p e iously p oduced TNTs.
I is sel -e iden ha se e al Ti sphe es can be placed be ween he
eede elec odes and anodized simul aneously (see Fig. 1a).
Fig. 2a shows he scanning elec on mic oscope (SEM) images o he
uppe pa o he TNT laye on a Ti sphe e (a). Nano ubes we e ound all
o e he Ti sphe es, excep on he e y small pa ixed o he ape in he
elec ochemical cell, as his pa was no in con ac wi h he elec oly e.
The a e age inne diame e o he TNTs was ~77 nm, while he hickness
was ~2 µm a he ex emi ies o he Ti sphe es acing he eede elec-
odes. I should be no ed ha he inne TNT diame e was bigge a he
ex emi ies, compa ed o hose a he middle o he Ti sphe e (~43 nm) o
loca ed a posi ions be ween he ex emi ies and he middle (~60 nm)
(Fig. 2b and c). The same phenomenon was ound o he hickness o he
TNTs. This can be explained by he mechanism o bipola elec ochemis y
and is desc ibed in de ail in he li e a u e [8,9,15]. B ie ly, he imposed
po en ial g adually d ops ac oss he elec oly e solu ion be ween he
eede elec odes. Consequen ly he pola iza ion po en ial on he Ti sphe e
also d ops ac oss he leng h o he Ti sphe e. Thus he pola iza ion po-
en ial is a a maximum a he ex emi ies o he Ti sphe e and dec eases
owa ds he middle o he Ti sphe e. As a consequence, he inne TNT
diame e and hickness a y o e he Ti sphe e, wi h he smalles inne
diame e and minimum hickness a he middle o he sphe e [15].
Taking in o accoun he ac ha he pola iza ion po en ial on he Ti
sphe e in luences he dimensions o he TNT, he e a e wo possible
me hods o a ying he dimensions o he TNTs on he sphe e: (i)
changing he ex e nally applied po en ial, and (ii) changing he dis-
ance be ween he eede elec odes. Fig. 2d and e show TNT laye s
ob ained a anodiza ion po en ials o 100 V (diame e ~ 53 m) and
140 V (diame e ~ 86 nm), espec i ely. When inc easing he ex-
e nally applied po en ial o p oduce TNTs wi h la ge dimensions, he
empe a u e o he elec oly e inc eases du ing he anodiza ion, as he
o al cu en be ween he eede elec odes inc eases. Hence, an e ec-
i e cooling me hod has o be employed o p e en o e hea ing o he
elec oly e. In his wo k, he elec oly e was cooled o 10 °C be o e use
and a cooling coil (also a 10 °C) was imme sed in he elec oly e du ing
he anodiza ion. Ne e heless, he elec oly e empe a u e inc eased
signi ican ly du ing he anodiza ion, ising o app oxima ely 30 °C.
Changing he dis ance be ween he eede elec odes, on he o he
hand, leads o a change in he pola iza ion po en ial o he Ti sphe e.
Thus, he smalle he dis ance be ween he eede elec odes, he la ge
he diame e o he TNTs. Fig. 2 shows a TNT laye ob ained a 120 V
wi h an elec ode dis ance o ~3 cm (TNT diame e ~ 20 nm). How-
e e , in his case only he ex emi ies o he Ti sphe e we e anodized as
Fig. 1. (a) Scheme showing he posi ion o he Ti sphe es be ween he eede elec odes, and (b) po en ial- ime plo o he i s 6 min o he anodiza ion.
H. Sopha, e al. Elec ochemis y Communica ions 111 (2020) 106669
2
he pola iza ion po en ial a he middle o he sphe e was oo small o
induce TNT o ma ion.
Fig. 2g shows he XRD pa e n o an anodized annealed Ti sphe e
and a Ti sphe e wi h a he mal oxide laye (i.e. no anodized bu an-
nealed). As expec ed, he TNTs consis ed o he ana ase phase wi h he
main signal a 2θ = 25.4° co esponding o he (1 0 1) o ien a ion
(ICDD: 00-021-1272). The signals o Ti (ICDD: 04-005-7594) s em om
he unde lying Ti sphe es. In he case o he Ti sphe e wi h a he mal
oxide laye , he XRD pa e n did no show any signals o TiO
2
. This
means ha he he mal oxide laye on hese Ti sphe es is e y hin and
canno be seen by XRD as he X- ay beam pene a es oo deep in o he
Ti sphe e. Un o una ely i was no possible o use g azing inciden XRD
o educe he pene a ion dep h o he X- ay beam due o he ound
shape o he Ti sphe es. Howe e , i can be expec ed ha unde he
gi en annealing condi ions, he he mal TiO
2
laye will be c ys alline.
In addi ion, i was isible o he naked eye ha he Ti sphe es changed
colou a e he annealing p ocedu e in he mu le o en, s emming om
he o ma ion o a hin TiO
2
laye .
The Ti sphe es we e hen employed o he pho oca aly ic de-
g ada ion o MB as a model dye. Fig. 3 shows he pho oca aly ic de-
composi ion a es o MB on anodized Ti sphe es and on Ti sphe es wi h
a he mal oxide laye on he su ace, p oduced by annealing in a mu le
o en unde he same condi ions as he anodized Ti sphe es. As ex-
pec ed, a highe deg ada ion a e was obse ed o he TNTs g own on
Ti sphe es wi h a a e cons an o k= 0.02 min
−1
. This was 2.15 imes
as e han he deg ada ion a e on he Ti sphe es wi h a he mal oxide
laye , wi h a a e cons an o k= 0.0093 min
−1
. The eason o his is
no only he inc eased su ace a ea on he anodized Ti sphe es com-
pa ed o he non-anodized Ti sphe es, bu also he unexpec edly high
pho oca aly ic decomposi ion a e o he he mal TiO
2
laye . In any
case, he a e cons an o MB deg ada ion ob ained o he TNTs g own
on Ti sphe es is compa able o he a e cons an s o TNT laye s p o-
duced on Ti s ips [22–25]. This shows a possible applica ion o he
anodized Ti sphe es in pho oca alysis. Fo ins ance, he anodized Ti
sphe es could be used as ca alys s in low-bed eac o s.
4. Conclusions
In summa y, i was shown in his communica ion ha he use o
bipola elec ochemis y enables he anodiza ion o Ti sphe es. TNTs
we e ound o e he en i e su ace o he Ti sphe es due o he appli-
ca ion o a squa e-wa e po en ial. The TNT dimensions a ied o e he
Ti sphe es, wi h he smalles inne diame e and hickness in he middle
o he sphe es. Fu he mo e, i was shown ha he pho oca aly ic de-
g ada ion a e o MB on anodized annealed Ti sphe es was 2.15 highe
han on non-anodized Ti sphe es wi h a hin he mal oxide laye .
CRediT au ho ship con ibu ion s a emen
Hanna Sopha: Concep ualiza ion, In es iga ion, Me hodology,
Da a cu a ion, W i ing - o iginal d a , W i ing - e iew & edi ing.
Ludek H omadko: Visualiza ion, Da a cu a ion, W i ing - e iew &
edi ing. Ma in Mo ola: Da a cu a ion, W i ing - e iew & edi ing. Jan
M. Macak: Concep ualiza ion, Funding acquisi ion, W i ing - e iew &
edi ing.
Fig. 2. (a)–( ) SEM images o he ops o he TNT laye s ob ained a (a)–(c)
120 V a di e en posi ions on he Ti sphe e: (a) ex emi y, (b) middle, and (c)
be ween ex emi y and middle; (d) 100 V, (e) 140 V (all wi h a ~1 cm elec ode
dis ance), ( ) 120 V wi h a ~3 cm elec ode dis ance, (g) XRD pa e n o an
anodized annealed Ti sphe e (TNTs on Ti sphe e) and o a non-anodized an-
nealed Ti sphe e ( he mal oxide on Ti sphe e). The inse in (b) shows a c oss-
sec ional SEM image o he TNT laye s ob ained a an ex emi y o he Ti sphe e
anodized a 120 V wi h a ~1 cm elec ode dis ance.
Fig. 3. Pho oca aly ic deg ada ion o MB on anodized annealed Ti sphe es
(TNTs on Ti sphe es) and non-anodized annealed Ti sphe es ( he mal oxide on
Ti sphe es). Pho o ading desc ibes he deg ada ion o MB in UV ligh in he
absence o a ca alys .
H. Sopha, e al. Elec ochemis y Communica ions 111 (2020) 106669
3
Decla a ion o Compe ing In e es
The au ho s decla e ha hey ha e no known compe ing inancial
in e es s o pe sonal ela ionships ha could ha e appea ed o in lu-
ence he wo k epo ed in his pape .
Acknowledgemen s
The inancial suppo om he Eu opean Resea ch Council (ERC No.
638857) and he Minis y o Educa ion, You h and Spo s o he Czech
Republic (p ojec s LM2015082, LQ1601, CZ.02.1.01/0.0/0.0/17_048/
0007421) is g a e ully acknowledged.
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