Applied Su ace Science 549 (2021) 149306
A ailable online 16 Feb ua y 2021
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Anodic TiO
2
nano ube walls econs uc ed: Inne wall eplaced by ALD
TiO
2
coa ing
Ma in Mo ola
a
,
1
, Raul Zazpe
a
,
b
, Ludek H omadko
a
,
b
, Jan P ik yl
a
, Ve onika Cicmanco a
a
,
Jhona an Rod iguez-Pe ei a
a
, Hanna Sopha
a
,
b
, 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, 530 02 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
Single-wall
A omic laye deposi ion
Pho oelec ochemis y
Pho oca alysis
ABSTRACT
A econs uc ion p ocess o TiO
2
nano ube (TNT) laye s owa ds hei supe io pho oelec ochemical pe o -
mance and pho oca aly ic ac i i y is p esen ed. A i s , TNT laye s (~5
μ
m hick, ~250 nm in diame e ) we e
p epa ed ia elec ochemical anodiza ion o ob ain double-wall (DW) TNT laye s. Second, a selec i e chemical
ea men was conduc ed o e ch he inne wall, yielding single-wall (SW) TNT laye s. Thi d, TNT laye s we e
coa ed by an addi ional app ox. 5.5 nm, 11 nm, and 16 nm hick TiO
2
coa ings, espec i ely, using A omic Laye
Deposi ion (ALD). A p onounced inc ease in he inciden pho on- o-elec on con e sion e iciency ( eaching
~85% a λ =350 nm) was achie ed on SW TNT laye s coa ed wi h 11 nm hick ALD coa ings compa ed o SW
wi hou coa ing (~35% a λ =350 nm). This is due o he op imal hickness o he ALD TiO
2
coa ing ha
passi a es su ace s a es and imp o es he sepa a ion o he pho ogene a ed cha ge ca ie s. Pho oca aly ic
pe o mance o SW TNT laye s wi h 11 nm hick ALD coa ings ( a e cons an ; k =0.1156 min
−1
) was inc eased
by app ox. 10- imes compa ed o ha o he nowadays mos epo ed blank DW TNT laye s ( a e cons an ; k =
0.0119 min
−1
).
1. In oduc ion
The pionee ing e o o Fujishima and Honda on he pho o-
elec ochemical wa e spli ing using TiO
2
[1] opened new pe spec i es
in ma e ials esea ch. E e since, hyd ogen e olu ion [2], pho o-
deg ada ion o pollu an s [3], and dye-sensi ized sola cells (DSSCs)
[4,5] a e ew o he many possible pho oelec ochemical applica ions o
me al oxide semiconduc o s ha ha e a ac ed a g ea scien i ic
a en ion. One o he mos c ucial ac o s o a semiconduc o o i s
op imal employmen in pho oelec ochemical applica ions is i s e icien
inciden ligh abso p ion and u iliza ion.
F om he a ie y o TiO
2
s uc u es, one-dimensional anodic TiO
2
nano ube (TNT) laye s [6–9], ep esen an excep ional ma e ial wi h
unique geome y and physicochemical p ope ies [10–14]. TNT laye s
can be g own [15,16] wi h con ollable dimensions (e.g., inne ube
diame e and laye hickness) [17] di ec ly om Ti by op imized
anodiza ion in sui able elec oly es con aining luo ide ions [15,16].
Th oughou he pas yea s, Ti oil [9,18,19], Ti wi e [20], Ti mesh/g id
[21–23], Ti sphe es [24], Ti alloys (e.g., Ti-Au [25], Ti-6Al-7Nb [26], Ti-
6Al-4V [26], Ti-7.5Mo [27]), magne on spu e ed Ti on Si [22,28,29],
ITO [30,31], o FTO [32–34], and elec odeposi ed Ti on Ni [35] ha e
been used as a s a ing subs a es o TNTs syn hesis. Since hei in o-
duc ion in 1984 [6] and a e pionee ing wo ks [16,18,36] ega ding
hei ad anced syn hesis and p omising applica ion p ospec s (e.g.,
pho oelec ochemis y, ba e ies, and as biomedical ma e ial) in
2005–2007, nume ous elec oly es (aqueous and/o o ganic based)
con aining HF o luo ide sal s we e employed o ob ain bo h low- and
high-aspec a io TNT laye s [15,16,18,37,38]. Among all he elec o-
ly es, e hylene glycol-based ones a e he mos popula elec oly es o
p epa a ion o such TNT laye s. Ne e heless, he e is an impo an ac
abou TNT laye s p epa ed in some o ganic-based elec oly es
(including e hylene glycol), which is o en o e looked, ye i is a
undamen al aspec o nano ube g ow h ha signi ican ly a ec s hei
mo phology and in insic p ope ies [39–42]. In hese elec oly es, a
double-wall (DW) s uc u e is ob ained du ing he anodiza ion p ocess
and he esul ing nano ubes possess an ou e and an inne wall [40,43].
* Co esponding au ho .
E-mail add ess: [email p o ec ed] (J.M. Macak).
1
P esen add ess: Depa men o Ino ganic Chemis y, Facul y o Na u al Sciences, Comenius Uni e si y in B a isla a, Ilko ico a 6, 842 15, B a isla a, Slo akia.
Con en s lis s a ailable a ScienceDi ec
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jou nal homepage: www.else ie .com/loca e/apsusc
h ps://doi.o g/10.1016/j.apsusc.2021.149306
Recei ed 4 Decembe 2020; Recei ed in e ised o m 9 Feb ua y 2021; Accep ed 10 Feb ua y 2021
Applied Su ace Science 549 (2021) 149306
2
This phenomenon can be explained ei he by he plas ic low model
[44–46] o by he bubble mold model [39,47,48].
As epo ed, by emo ing he inne wall o ob ain a single-wall (SW)
s uc u e, he e iciency o DSSCs is inc eased [49,50], he pho o-
elec ochemical [41,42,51,52] and pho oca aly ic [51,53] pe o mances
a e enhanced, and he conduc i i y [41,42] o such SW TNT laye s is
imp o ed compa ed o ha o DW ones. E en hough a la ge pa o he
o iginally pho oac i e TiO
2
ma e ial is emo ed in he SW compa ed o
DW TNT laye s, he e ec o highe TiO
2
pu i y p e ails and he cha ge
ca ie anspo in such SW laye s is mo e e icien [42,52]. Howe e , i
has ne e been in es iga ed so a , wha would an addi ion o a high
pu i y TiO
2
mass back o he emaining walls wi h high pu i y TiO
2
do
wi h he pho oelec ochemical and pho oca aly ic esponse o such
econs uc ed nano ube walls.
F om all he possible echniques a ailable o such econs uc ion,
A omic Laye Deposi ion (ALD) is conside ed o be he mos sui able
echnique [54,55]. I is especially sui ed o coa ing o highly-o de ed
TNT laye s, as i allows homogeneous coa ing h oughou he whole
nano ube olume [56]. By in oducing a seconda y ma e ial by ALD
wi hin TNT laye s (e.g., MoS
2
[57,58], MoSe
2
[59], P [60,61], ZnO
[62], Al
2
O
3
[63,64], o TiO
2
[65]), hei pho oelec ochemical and
pho oca aly ic pe o mance can be signi ican ly enhanced due o a
s onge abili y o p oduce cha ge ca ie s, an imp o ed inciden ligh
abso p ion, o by annihila ion o he TiO
2
su ace s a es. Besides he
bene icial e ec on he TNT laye s’ pho oelec ochemical and pho o-
ca aly ic pe o mance, ALD coa ings (e.g., Al
2
O
3
[66,67], MoS
2
[57], o
TiO
2
[68]) imp o e also hei he mal, chemical, and mechanical s a-
bili y [67], hei biocompa ibili y [69], p e en s he nano ubes om
unwan ed mo phological and s uc u al changes [68], and inc eases he
e iciency o Li-ion based ba e ies [57,66].
O e all, he combina ion o TNT laye s and a seconda y ma e ial
coa ed by ALD is phenomenal and mo e han p omising owa ds he
TNTs ad anced applica ion [56] and he e e inc easing numbe o e-
po s con i m his bold s a emen . Ne e heless, all epo s
[52,53,55–63,65], show ALD coa ings on DW TNT laye s (i.e., con-
aining he inne wall). The use o such ALD coa ed DW TNT laye s o
pho oelec ochemical o pho oca aly ic applica ions has led o
imp o emen in all hese cases. Howe e , he imp o emen could be
e en s onge , i a sui able ALD coa ing would be placed di ec ly and
exclusi ely on a high pu i y TiO
2
ou e wall ins ead o also he inne
wall, as i was conduc ed in all p e ious wo ks.
Based on his knowledge gap, in his wo k, we p esen a econ-
s uc ion p ocedu e o sel -o ganized DW TNT laye s (~5
μ
m hick,
~250 nm in diame e ). A selec i e chemical e ching ea men was
conduc ed o emo e he inne wall o he DW TNT laye s. Subsequen ly,
he TNT laye s we e coa ed wi h an addi ional TiO
2
coa ing (nominal
hicknesses ~5.5 nm, ~11 nm, and ~16 nm) by an op imized ALD
p ocess. The ob ained TNT laye s we e explo ed o hei pho o-
elec ochemical pe o mance and pho oca aly ic ac i i y and compa ed
wi h DW and SW TNT laye s wi hou any ALD coa ing.
2. Expe imen al
2.1. Syn hesis and e ching o TNT laye s
Acco ding o ou p e ious wo k [70], ~5 µm hick DW TNT laye s
wi h app ox. 250 nm inne nano ube diame e we e p epa ed ia elec-
ochemical anodiza ion o Ti oils. All anodiza ions we e conduc ed
using PGU-200V high- ol age po en ios a (Elek oniklabo , GmbH) a
100 V o 4 h a oom empe a u e in e hylene glycol-based elec oly e
con aining 10% wa e and 0.15 M NH
4
F.
The inne wall o he DW TNT laye s was emo ed based on ou
p e ious epo s on 1 cm
2
[42,52,53] and 2.25 cm
2
[51] a ea o ob ain
SW TNT laye s on such a eas. Fo 1 cm
2
a ea TNT laye s, i s , he as-
p epa ed DW TNT laye s we e p e-annealed a 135 ◦C o 1 h in ai
using a hea ing a e o 15 ◦C/min. Second, he p e-annealed laye s we e
imme sed in pi anha solu ion (H
2
SO
4
:H
2
O
2
=3:1) o 10 min a 70 ◦C.
Fo 2.25 cm
2
a ea TNT laye s, he as-p epa ed DW TNT laye s we e p e-
annealed a 150 ◦C o 1 h in ai (hea ing a e 15 ◦C/min). A e p e-
annealing, he laye s we e imme sed in pi anha solu ion o 5 min a
55 ◦C and addi ional 8 min a 65 ◦C. A e e ching, all laye s (1 cm
2
and
2.25 cm
2
) we e imme sed in H
2
O and E OH o 5 min, espec i ely, and
d ied wi h a ni ogen je . All TNT laye s we e annealed (400 ◦C, 1 h, ai
a mosphe e) o ob ain pho oca aly ically ac i e ana ase TiO
2
[42].
2.2. ALD p ocess
P elimina y ALD p ocesses (TFS200, Beneq) we e ca ied ou wi h
di e en Ti p ecu so s o de e mine he mos sui able Ti p ecu so ha
could be used o main ALD TiO
2
coa ings o he TNT laye s. The esul s
a e shown in Elec onic Supplemen a y In o ma ion (ESI, Fig. S1). Based
on hese p elimina y esul s, i anium (IV) isop opoxide (TTIP, min.
98.8% pu i y) and O
3
(ozone gene a o , BMT Mess echnik, S ahnsdo ,
Ge many; 8 g/h ou pu ) we e selec ed as i anium and oxygen p e-
cu so s, espec i ely, o coa ing o TNT laye s (bo h DW and SW)
employed in his wo k. Di e en numbe o ALD cycles (namely 266,
532 and 798) we e deposi ed o ob ain nominal hick 5.5 nm, 11 nm,
and 16 nm coa ings, espec i ely (g ow h a e o 0.20 Å/cycle). The
nominal hicknesses o he h ee di e en ALD TiO
2
coa ings we e
con i med by analyses o SEM images. A hea ing empe a u e o 60 ◦C
was applied o TTIP (in o de o ge enough apo p essu e) and he
deposi ion empe a u e was 250 ◦C. One ALD cycle was comp omised o
he nex s eps: TTIP (750 ms) – N
2
pu ge (5 s) – O
3
pulse (8 s) – N
2
pu ge
(15 s).
2.3. Cha ac e iza ion
A ield-emission scanning elec on mic oscope (FE-SEM, JEOL JSM
7500F), X- ay di ac ome y (XRD, PANaly ical Empy ean Cu K
α
adi-
a ion, λ =1.5418 Å), X- ay pho oelec on spec oscopy (XPS, ESCA2SR,
Scien a-Omic on) using a monoch oma ic Al K
α
(1 484.7 eV) X- ay
sou ce, and di use e lec ance UV–VIS spec a (DRS, Shimadzu UV-
36000Plus Se ies UV–VIS spec opho ome e ) we e used o cha ac e -
iza ion o TNT laye s.
2.4. Pho oelec ochemical and pho oca aly ic pe o mance measu emen s
Pho oelec ochemical pe o mance measu emen s we e ca ied ou
in a h ee-elec ode cell using Ag/AgCl as a e e ence elec ode, TNT
laye s as a wo king elec ode and P wi e as a coun e elec ode a 0.4
V
s Ag/AgCl
in an aqueous 0.1 M Na
2
SO
4
solu ion in he wa eleng h ange
om 325 nm o 450 nm acco ding o ou p e ious wo k [53]. Elec o-
chemical impedance spec a (EIS) we e eco ded a bias po en ial +0.4
V
s Ag/AgCl
in he equency ange om 0.1 Hz o 100 kHz ( ol age
pe u ba ion ampli ude o 10 mV) in he da k and unde UV ligh i a-
dia ion (λ =350 nm), espec i ely. Mo -Scho ky (MS) plo s we e
eco ded a equency o 1 kHz in he ange −0.4 V
s Ag/AgCl
o +1.0 V
s
Ag/AgCl
in he da k. Pho oca aly ic ac i i y was e alua ed acco ding o
ou p e ious epo [58]. Pho oca aly ic ac i i y measu emen s we e
epea ed h ee- imes o each sample and he di e ences in he pho o-
ca aly ic pe o mance did no exceed ±5% (based on he a e cons an ).
This con i ms a good s abili y o all TNT laye s as a UV-ligh
pho oca alys .
3. Resul s and discussion
3.1. Ma e ials cha ac e iza ion
SEM images o he as-p epa ed TNT laye s a e shown in Fig. 1A and B
( op and c oss-sec ional iew, espec i ely). Fig. 1C e eals he inne
wall in DW TNT laye s in he bo om pa s o nano ubes wi h a ypical
po ous s uc u e. E ching in pi anha solu ion emo ed he inne wall
M. Mo ola e al.
Applied Su ace Science 549 (2021) 149306
3
and he depic ed SEM image (Fig. 1D) shows no p esence o he po ous
s uc u e in SW TNT laye s.
A ep esen a i e SEM image (Fig. 2) shows blank and ALD coa ed SW
TNT laye s. A e ALD, he nano ube wall hicknesses g adually
inc eased wi h highe numbe o ALD TiO
2
cycles. Indeed, he nominal
hicknesses o he addi ional ALD TiO
2
coa ings we e ~5.5 nm, ~11 nm,
and ~16 nm a e 266, 532, and 758 ALD TiO
2
cycles, espec i ely. As a
esul , he inne ube diame e dec eased in ALD TiO
2
coa ed TNT laye s
(Fig. 2B-D) compa ed o ha o blank TNT laye s (Fig. 2A). The inne
ube diame e s o ~250 nm, ~239 nm, ~228 nm, and ~218 nm we e
ob ained o blank, +5.5 nm TiO
2
, +11 nm TiO
2
, and +16 nm TiO
2
coa ed TNT laye s, espec i ely. As p e iously epo ed [65], uni o mi y
and homogenei y o he addi ional ALD coa ings was achie ed
h oughou he whole TNT laye s. To u he con i m he uni o mi y o
he ALD TiO
2
coa ings in his wo k, a high- esolu ion SEM image o he
bo om pa o ALD TiO
2
coa ed SW TNT laye +11 nm TiO
2
is shown in
Fig. S2.
The c ys alline s uc u e and he su ace chemical composi ion o
blank and ALD TiO
2
coa ed DW and SW TNT laye s was cha ac e ized by
XRD and XPS and he esul s a e shown in ESI (Fig. S3 and Table S1),
espec i ely.
3.2. Pho oelec ochemical pe o mance
Fig. 3 shows pho ocu en densi ies (maximum a ~350 nm) and
pho on- o-elec on con e sion e iciencies (IPCE) ob ained o blank and
ALD TiO
2
coa ed DW and SW TNT laye s (λ =325–450 nm). The
eco ded pho ocu en ansien s a e shown in ESI (Fig. S4). A p o-
nounced inc ease in pho ocu en densi y (Fig. 3A) and IPCE (Fig. 3B)
was eco ded o he blank SW laye s compa ed o ha o he blank DW
ones. Mo e speci ically, he IPCE alues inc eased by ~5% o SW laye s
in he wa eleng h ange om 325 o 375 nm. Fo example, a ~350 nm,
he IPCE eached ~30% and ~35% o DW and SW TNT laye s,
espec i ely. This p onounced inc ease is due o he inne wall emo al,
which esul ed in an imp o ed cha ge ca ie anspo along he
nano ube walls in SW TNT laye s [42,52].
E en hough such inc ease among DW and SW samples is encou -
aging, much mo e p onounced inc eases we e ob ained o samples wi h
he addi ional ALD TiO
2
coa ings. Mo e speci ically, a wa eleng h o
~350 nm, 2–3 imes highe pho ocu en densi ies (Fig. 3A) and up o
85% IPCE alues (Fig. 3B) we e ob ained o ALD coa ed samples. The
maxima we e achie ed o TNT laye s +11 nm TiO
2
. Indeed, he su ace
s a es a e passi a ed and he u iliza ion o he cha ge ca ie s is mo e
e icien due o he addi ional ALD TiO
2
coa ing, as we discussed in ou
p e ious wo k on ALD TiO
2
coa ing o DW TNT laye s [65]. The hick-
ness o he addi ional ALD TiO
2
coa ing is a decisi e pa ame e
ega ding he e icien cha ge ca ie anspo and dec eased ecom-
bina ion a e o cha ge ca ie s along he ALD TiO
2
coa ing [71,72]. This
can be asc ibed o he p esence o su ace and bulk ap s a es in he ALD
TiO
2
coa ing. In a ecen epo [72], a de ailed in es iga ion o he
op imal a io o su ace/bulk aps in ALD TiO
2
coa ings wi h di e en
hicknesses owa ds pho oelec ochemical wa e spli ing was e alu-
a ed. Mo e speci ically, di e en hicknesses o ALD TiO
2
coa ings we e
coa ed on plana ITO glass and se ies o expe imen al echniques we e
conduc ed o de e mine he op imal hickness o he coa ing o pho o-
elec ochemical wa e spli ing. Based on he epo ed esul s [72], he
di e ences in he pho oelec ochemical pe o mances in ou blank and
ALD TiO
2
coa ed TNT laye s can be asc ibed o he ollowing. In hinne
ALD TiO
2
coa ings (~5.5 nm a e 266 ALD TiO
2
cycles), su ace aps
domina e he o e all cha ge ans e a e, hus he ecombina ion a e o
he cha ge ca ie s is subs an ial compa ed o ha in hicke laye s. Wi h
inc eased hickness o he ALD TiO
2
coa ing (~11 nm a e 532 ALD
TiO
2
cycles), he ecombina ion o cha ge ca ie s is dec eased due o
he inc eased amoun o he bulk ap s a es and as a esul o an
in e ac ion be ween he su ace and bulk ap s a es. Mo eo e , he bulk
aps imp o e he cha ge ca ie ans e along he ALD TiO
2
coa ing.
Ne e heless, hicke ALD TiO
2
coa ings (~16 nm a e 758 ALD TiO
2
cycles) a e de imen al o he e icien inciden ligh u iliza ion.
Al hough he cha ge ca ie ecombina ion is simila o ha in ~11 nm
hick ALD TiO
2
coa ing, he cha ge ca ie ans e along he TiO
2
coa ing is no e icien due o exceeding he c i ical hickness o he
coa ing. In gene al, he su ace aps a e esponsible o he e icien
pho ogene a ion o he cha ge ca ie s and he bulk aps a e esponsible
o he e icien cha ge ca ie anspo along TiO
2
[73,74]. To p o ide
an addi ional pho oelec ochemical insigh , cyclic ol ammog ams (CV)
we e eco ded and a e discussed in mo e de ail in ESI (Fig. S5).
The elec onic s uc u e o blank and ALD TiO
2
coa ed SW and DW
TNT laye s was in es iga ed by elec ochemical impedance spec a (EIS)
and is ep esen ed as Nyquis plo s along wi h he equi alen ci cui in
he da k (Fig. S6A) and unde UV ligh i adia ion (λ =350 nm;
Fig. S6B). O e all, he p esence o di usi e e ec s was associa ed o he
nano ubula laye whe eas he unde lying Ti subs a e was associa ed
wi h beha io close o he ideal capaci o in all SW and DW (blank and
ALD TiO
2
coa ed) TNT laye s. Rega ding he elec onic s uc u e o TNT
laye s, lowe esis ance alue was ob ained o blank SW compa ed o
ha o DW ones as he esul o he inne wall emo al. A e ALD TiO
2
coa ings, he esis ance o bo h SW and DW TNT laye s inc eased o-
wa ds highe alues. Indeed, by adding an addi ional TiO
2
as an n- ype
Fig. 1. SEM images o blank TiO
2
nano ube laye s annealed a 400 ◦C (A) op
iew, (B) c oss-sec ional iew, (C) double-wall s uc u e, and (D) single-wall.
Fig. 2. Top iew SEM images o blank and ALD TiO
2
coa ed single-wall TiO
2
nano ube laye s (A) blank, (B) +5.5 nm TiO
2
, (C) +11 nm TiO
2
, and (D) +16
nm TiO
2
. The “Xnm” speci ies he hickness o he addi ional TiO
2
ALD coa ing.
M. Mo ola e al.
Applied Su ace Science 549 (2021) 149306
4
semiconduc o on TNT laye s, such inc ease is expec ed, and he esis-
ance alues inc eased in pa allel wi h hicke coa ing due o addi ional
TiO
2
ma e ial. Al hough he highe esis ances esul in lowe elec onic
conduc i i y in gene al in hicke ALD TiO
2
coa ings [72], he di e -
ences be ween he e epo ed blank and ALD TiO
2
coa ed TNT laye s a e
measu able, bu no e y signi ican (app ox. 30 Ωcm
2
be ween blank
and +16 nm TiO
2
). Ne e heless, u he coa ing wi h hicke ALD TiO
2
coa ings (>16 nm) would be p obably accompanied by a subs an ial
inc ease in esis ance (in o de s o magni ude), as ecen ly epo ed o
plana coa ings [72].
To ob ain addi ional insigh s ega ding he ma e ial om he semi-
conduc o poin o iew, Mo -Scho ky (MS) analyses (Fig. 4) we e
Fig. 3. (A) Pho ocu en densi y s wa eleng h and (B) co esponding IPCE s wa eleng h ob ained o blank and ALD TiO
2
coa ed double- and single-wall TiO
2
nano ube laye s. Guiding lines a λ =350 nm a e in en ionally d awn o show he IPCE alues a maximum pho ocu en s (shown in (A)). The “Xnm” speci ies he
hickness o he addi ional TiO
2
ALD coa ing. All da a we e eco ded in an aqueous 0.1 M Na
2
SO
4
a +0.4 V
s Ag/AgCl
.
Fig. 4. MS plo s wi h ob ained V
b
and calcula ed N
D
o blank and ALD TiO
2
coa ed double- and single-wall TiO
2
nano ube laye s. The “Xnm” speci ies he hickness
o he addi ional TiO
2
ALD coa ing. All da a we e eco ded in an aqueous 0.1 M Na
2
SO
4
a 1 kHz and +0.4 V
s Ag/AgCl
.
M. Mo ola e al.
Applied Su ace Science 549 (2021) 149306
5
eco ded and he dono densi y N
D
and he la band po en ial V
b
o
blank and ALD TiO
2
coa ed DW and SW TNT laye s we e e alua ed by
ollowing he equa ion:
1
C2=2
ε
ε
0qND
[(Vappl −V b) − kT
q](1)
whe e C is he capaci ance,
ε
he ela i e dielec ic cons an o TiO
2
(
ε
=41.4 a 1 kHz),
ε
0
he acuum dielec ic cons an , q he elec on
cha ge, V
appl
he applied po en ial (+0.4 V
s Ag/AgCl
), k he Bol zmann
cons an , and T he absolu e empe a u e. The in e cep poin wi h he x-
axis o he plo in Fig. 4 ep esen s V
b
whe eas N
D
was calcula ed using
Eq. (1). The equency o 1 kHz was applied du ing he measu emen s,
which is gene ally conside ed su icien o he MS analysis o po ous
s uc u es, such as TNT laye s [75–77].
No signi ican di e ences in he dono densi y we e obse ed o
blank and ALD TiO
2
coa ed DW and SW TNT laye s and he calcula ed
N
D
alues anged om app ox. 5.8 ×10
19
cm
−3
o 2.28 ×10
20
cm
−3
o
all TNT laye s. Exac numbe s a e shown in Fig. 4. No ably, he e was an
inc easing end in N
D
alues wi h added TiO
2
, eaching hei maxima a
1.25 ×10
20
o DW wi h 5.5 nm and a 2.28 ×10
20
cm
−3
SW wi h 11 nm
ALD coa ed TNT laye s, espec i ely. Howe e , he N
D
alue o DW TNT
laye s a 11 nm (1.24 ×10
20
) was also e y close o maximum (1.25 ×
10
20
). These ends a e in a e y good ma ch wi h pho ocu en and IPCE
alues shown in Fig. 3.
O e all, he ob ained N
D
alues a e also in acco d wi h p e ious
epo s [75–78] on anodic oxides and TNT laye s, whe e he dono
densi ies we e ypically wi hin he ange o 10
19
– 10
20
cm
−3
. The
calcula ed N
D
alues sugges ha all TNT laye s con ain a simila
amoun o oxygen acancies and de ec s in gene al and he addi ional
ALD TiO
2
coa ing is o compa able quali y (in e ms o pu i y) as he
TiO
2
p epa ed by anodiza ion o Ti. This is in con as wi h ou p e ious
epo [65], whe e he ALD TiO
2
coa ed TNT laye s possessed N
D
alue
o ~10
18
cm
−3
, which was lowe han o blank TNT laye . Howe e , in
ha wo k, di e en p ecu so s we e used o he ALD p ocess (TiCl
4
as
Ti and H
2
O as O p ecu so ) ha esul ed in coa ings wi h high pu i y
TiO
2
(i.e., educed numbe o oxygen acancies compa ed o ha o
blank TNT laye s). Mo eo e , in ha wo k he pho oelec ochemical and
pho oca aly ic pe o mances we e no as good as in his wo k, because
he TiCl
4
de i ed ALD coa ings we e almos oo pu e. In o he wo ds,
some op imal amoun o de ec s is needed in TiO
2
in gene al o i s
op imal pho oelec ochemical and pho oca aly ic pe o mance. The
mo e e icien ans e o he pho ogene a ed cha ge ca ie s is due o
he addi ional ALD TiO
2
coa ings desc ibed he e (by using TTIP and O
3
as Ti and O p ecu so s, espec i ely). Indeed, he op imal numbe o
oxygen acancies and ecombina ion si es in gene al (e.g., su ace
s a es) in TiO
2
enhances he cha ge ca ie anspo and dec eases he
cha ge ca ie ecombina ion [10,79].
Di e ences we e obse ed in la band po en ials V
b
o di e en
TNT laye s used in his wo k (i.e., SW s DW), as shown in Fig. 4. A
p onounced shi o V
b
was obse ed o blank SW TNT laye s (+0.155
V) compa ed o ha o DW ones (-0.025 V). Howe e , he shape o he
MS cu e was simila in bo h cases. This sugges ha he V
b
po en ial
change is ela ed o he Fe mi le el shi in TiO
2
[78] due o i) an
inc eased conduc i i y o TiO
2
o ii) an al e a ion o he po en ial d op a
he ma e ial/elec oly e in e ace due o change in he su ace chemis-
y. Indeed, p e ious epo s [41,42] show inc eased conduc i i y in SW
TNT laye s. A e ALD, he ends in V
b
we e di e en among SW and
DW coa ed TNT laye s. Fo DW and SW laye s he highes V
b
we e
eached o 16 nm and 11 nm o ALD TiO
2
coa ings, espec i ely. The
shi o posi i e alues o ALD coa ed samples, compa ed o uncoa ed
ones, can be explained by a modi ica ion o he TNT su ace by TiO
2
o
di e en na u e and composi ion. In pa icula , i is belie ed ha
hickening o he a ailable TiO
2
and changing he mass a io be ween
anodic and ALD-based TiO
2
causes his shi owa ds posi i e alues.
Howe e , his in e play is mo e complex and as he composi ional
di e ences a e no aceable (e.g., by XPS), we canno explici ly be su e
abou he mos dominan e ec s ha in luence his end.
3.3. Pho oca aly ic ac i i y
Pho odeg ada ion o MB solu ion was explo ed unde UV ligh i a-
dia ion (λ =365 ±5 nm) on blank and ALD TiO
2
coa ed DW and SW
TNT laye s. I ollowed he i s -o de eac ion ypical o he TiO
2
pho oca alys and an o ganic dye [8]. The co esponding deg ada ion
a es a e shown in Fig. 5A ( epea ed pho oca aly ic uns a e shown in
ESI, Fig. S7 and Table S3). The ob ained kine ic a e cons an s along
wi h he deg ada ion a es a e shown in Fig. 5C. The ob ained pho o-
ac i i ies o blank and ALD TiO
2
coa ed SW and DW TNT laye s a e in
line wi h he eco ded pho oelec ochemical pe o mances (Fig. 3 and
Fig. S4-S5). The highes decomposi ion a e was ob ained using SW TNT
laye s +11 nm TiO
2
(k =0.1156 min
−1
), which is ~9.7 imes highe
compa ed o ha o blank DW TNT laye s (k =0.0119 min
−1
), i.e., he
mos commonly used TNT laye s nowadays. In gene al, he SW TNT
laye s, bo h blank and ALD TiO
2
coa ed, possess enhanced pho o-
ca aly ic ac i i y compa ed o he DW TNT laye s. Indeed, he inne wall
in DW TNT laye s (Fig. 1C) con ibu es nega i ely o he cha ge ca ie
ecombina ion [43,53]. Rega ding he h ee di e en hicknesses o he
addi ional ALD TiO
2
coa ing, he pho oac i i y ollowed he end o
TNT laye s +11 nm TiO
2
>+16 nm TiO
2
>+5.5 nm TiO
2
. This end is
due o he op imal hickness o he ALD coa ing along wi h he mos
e icien inciden ligh u iliza ion o TNT laye s +11 nm TiO
2
, dis-
cussed ea lie in his wo k. A subs an ial pho oac i i y imp o emen
was ob ained also o TNT laye s +16 nm TiO
2
. This is in co ela ion
wi h he ob ained pho ocu en densi y (Fig. 3A) and IPCE (Fig. 3B)
da a. The inciden ligh u iliza ion in TNT laye s +16 nm TiO
2
is
enhanced due o he addi ional ALD TiO
2
coa ing, i.e., diminished
cha ge ca ie ecombina ion and imp o ed he ans e along TiO
2
.
Ne e heless, bo h pho oelec ochemical (Fig. 3) and pho oca aly ic
(Fig. 5) pe o mances o TNT laye s +16 nm TiO
2
we e dec eased
compa ed o ha o TNT laye s +11 nm TiO
2
. Indeed, hickening o he
ALD TiO
2
coa ing is esponsible o such dec eased pe o mances. In
hicke coa ings, he cha ge ca ie ’s ecombina ion occu ed in he bulk
o he ALD TiO
2
coa ing hus dec eased he ma e ials pe o mances.
Mo eo e , as he pho oca aly ic deg ada ion o o ganic pollu an s p o-
ceeds on he in e ace (i.e., TNT laye s/dye), he a ailable su ace a ea
in luences he pho oca aly ic ac i i y o a ma e ial. Indeed, in TNT
laye s +16 nm TiO
2
, he o al a ailable su ace a ea is dec eased due o
he addi ional ALD TiO
2
coa ing (si ua ed on bo h ou e and inne ube
wall) and he inne ube diame e is dec eased (diame e o ~218 nm;
Fig. 2D) compa ed o ha o blank TNT laye s (diame e o ~250 nm;
Fig. 2A).
A las , DRS we e eco ded o de e mine he e lec ance and he in-
di ec op ical band gap ene gy o blank and ALD TiO
2
coa ed SW and
DW TNT laye s was calcula ed om Kubelka-Munk heo y (Fig. 5B). The
e lec ance edge (a λ =~400 nm) is asc ibed o ana ase TiO
2
(E
BG
=
3.0–3.2 eV) [42,80]. The op ical band gap (Fig. 5C) g adually dec eased
wi h he inc eased hickness o he addi ional ALD TiO
2
coa ing om
3.1 eV (blank) o 2.99 eV (+16 nm TiO
2
). This is due o he pene a ion
dep h (de i ed om he DRS spec a) ha is highe han 100 nm. Thus,
i is a deepe han he combined hickness o he nano ube wall (10–60
nm) and he addi ional ALD TiO
2
coa ing (~5.5 nm, ~11 nm, and ~16
nm a e 266, 532, and 758 ALD TiO
2
cycles, espec i ely). The e o e,
he amoun o pho ons abso bed by he ma e ial is inc easing wi h he
hicke ALD TiO
2
coa ings. As a esul , he op ical band gap shi s o-
wa ds lowe alues ( om 3.1 eV o 2.99 eV).
4. Conclusions
Two-s ep mo phology econs uc ion p ocess o anodic DW TNT
laye s owa ds supe io pho oelec ochemical and pho oca aly ic pe -
o mance was conduc ed. The inne wall was emo ed ia a selec i e
M. Mo ola e al.

Applied Su ace Science 549 (2021) 149306
6
chemical e ching ea men . A mo e e icien cha ge ca ie anspo
was achie ed in such SW TNT laye s. This led o 5–10% inc ease in
pho oelec ochemical and pho oca aly ic pe o mances o SW TNT
laye s. To u he enhance he inciden ligh u iliza ion o he SW TNT
laye s, coa ing by an addi ional ALD TiO
2
( hickness o ~5.5 nm, ~11
nm, and ~16 nm, espec i ely) was conduc ed. The addi ional TiO
2
coa ing imp o ed u iliza ion o he pho ogene a ed cha ge ca ie s and
passi a ed he su ace s a es on TiO
2
. The ~11 nm hick addi ional TiO
2
coa ing was op imal o inc eased pe o mances o TNT laye s due o
op imal a io o su ace and bulk aps. Indeed, he IPCE alues
inc eased by app ox. 2.5- imes and eached ~85% a he wa eleng h o
350 nm o SW TNT laye s +11 nm TiO
2
compa ed o ha o blank DW
TNT laye s (IPCE alue o ~30% a λ =350 nm). Mo eo e , he pho-
oca aly ic ac i i y inc eased by app ox. 10- imes o SW TNT laye s +
11 nm TiO
2
wi h a a e cons an o 0.1156 min
−1
compa ed o ha o
blank DW TNT laye s (k =0.0119 min
−1
). O e all, he he e p esen ed
mo phology econs uc ion p ocess signi ican ly imp o ed he TNT
laye s’ pe o mance in pho oelec ochemical applica ions.
Au ho s con ibu ions
MM and JMM designed expe imen s; MM, HS – syn hesis o nano-
ubes; LH – XRD and SEM cha ac e iza ions; RZ and JP – a omic laye
deposi ion; JP – UV/VIS di use e lec ance; JRP – XPS; MM – (pho o)
elec ochemical pe o mance measu emen s; MM – pho oca aly ic ac-
i i y measu emen s; MM, HS and JMM w o e he manusc ip and ca -
ied ou e isions; all o he s e iewed and edi ed he manusc ip ; JMM
supe ised he eam and p o ided suppo . All au ho s ha e ead and
ag eed he published e sion o he manusc ip .
Decla a ion o Compe ing In e es
The e a e no con lic s o decla e.
Acknowledgemen s
The au ho s acknowledge he inancial suppo om he Minis y o
Educa ion, You h and Spo s o he Czech Republic (p ojec s
LM2018103, LQ1601, CZ.02.1.01/0.0/0.0/17_048/0007421).
Appendix A. Supplemen a y ma e ial
Supplemen a y da a o his a icle can be ound online a h ps://doi.
o g/10.1016/j.apsusc.2021.149306.
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