Ce amics In e na ional 49 (2023) 34712–34725
A ailable online 12 Augus 2023
0272-8842/© 2023 The Au ho s. Published by Else ie L d. This is an open access a icle unde he CC BY license (h p://c ea i ecommons.o g/licenses/by/4.0/).
Se-doped Nb
2
O
5
–Al
2
O
3
composi e-ce amic nanoa ays ia he anodizing o
Al/Nb bilaye in selenic acid
Ki ill Kamne
b
, Ma ia Bendo a
a
,
b
, Zdenek Py licek
b
, Jan P asek
a
,
b
, Luk´
aˇ
s Kejík
b
, F ank Güell
c
,
Edua d Llobe
d
, Alexande Mozale
a
,
b
,
*
a
Depa men o Mic oelec onics, Facul y o Elec ical Enginee ing and Communica ion, B no Uni e si y o Technology, Technicka 10, 616 00, B no, Czech Republic
b
CEITEC – Cen al Eu opean Ins i u e o Technology, B no Uni e si y o Technology, Pu kyno a 123, 61200, B no, Czech Republic
c
ENFOCAT-IN2UB, Uni e si y o Ba celona, Ma i i F anques 1, 08028, Ba celona, Ca alonia, Spain
d
MINOS, Uni e si y Ro i a i Vi gili, A . Paisos Ca alans 26, 43007, Ta agona, Ca alonia, Spain
ARTICLE INFO
Handling Edi o : D P. Vincenzini
Keywo ds:
Al
2
O
3
–Nb
2
O
5
nanocomposi e
Anodizing
Selenic acid
T ansi ion me al oxides
Po ous anodic alumina
SERS
ABSTRACT
No el a ays o Nb
2
O
5
-based ce amic nanos uc u es o a ious sizes (9–210 nm) and mo phologies (do s,
goble s, ods) aligned on subs a es a e ab ica ed ia he anodizing o a hin Nb ilm h ough he ini ially o med
po ous anodic alumina (PAA) ilm in 1.5 M selenic acid (H
2
SeO
4
) – a new aqueous elec oly e gene a ing
ex ao dina ily hinne PAA po es han any o he solu ions. Acco dingly, he nanos uc u es o med in he
selenic acid a e 1.3- old hinne and be e sel -o de ed han hei coun e pa s o med om he same Al/Nb
p ecu so bilaye in a e e ence oxalic-acid elec oly e. The nanos uc u es ha e a dual (co e/shell) composi ion:
he inne ma e ial ( he co e) is s oichiome ic Nb
2
O
5
, whe eas he ou e laye ( he shell) is a ew nm- hick
subs oichiome ic NbO
x
mixed wi h Al
2
O
3
. The composi e-ce amic nanoa ays g ow doped wi h selenium spe-
cies such as selena e (SeO
4
2−
) and selenide (Se
2−
) anions o igina ing om he elec oly e and mig a ing inwa d
unde he high elec ic ield. The inco po a ed Se species do no con ibu e o pho oluminescence emission no
hinde he Raman signal om he nanoa ays, which makes hem highly p omising as Nb
2
O
5
-based SERS bio-
sensing subs a es. The plana PAA-inbuil Se-doped Nb
2
O
5
–Al
2
O
3
nanos uc u ed ce amic ilm pe o ms like a
high-k low-loss low-leakage-cu en dielec ic p omising o on-chip in eg a ion. Mo e po en ial applica ions o
he Se-doped ce amic nanoa ays de eloped he e include biomedical an ibac e ial coa ings, ad anced supe -
hyd ophobic su aces, gas-sensing, and ca aly ic laye s.
1. In oduc ion
Niobium pen oxide (Nb
2
O
5
) and i s compounds wi h o he
ansi ion-me al oxides a e a ac i e ce amic ma e ials o ca alysis,
biomedicine, and su ace p o ec ion applica ions [1–3]. Niobium pen -
oxide is also ac i ely u ilized as a building block o pho o ol aic,
sensing, ene gy s o age, elec och omic, elec onic, and op ical de ices
[4–9]. Fo many mode n applica ions, nanos uc u ing o Nb
2
O
5
-based
ce amics is equi ed o enhance he ma e ial’s cha ac e is ics, inc ease
he su ace- o- olume a io, and possibly in oduce no el p ope ies.
Commonly used nanos uc u ing echniques a e he sol-gel, ion-beam
spu e ing, chemical apo deposi ion, and a omic laye deposi ion [7,
10,11]. A me hod based on elec ochemical anodizing o a hin laye o
aluminum (Al) supe imposed on a laye o niobium (Nb) has ecen ly
been de eloped o syn hesize sel -o de ed a ays o Nb
2
O
5
nano-
s uc u es wi h con ollable dimensions [12]. Fi s , he Al laye is
anodized o o m a po ous anodic alumina (PAA) ilm. The unde lying
Nb is hen anodized h ough he supe imposed PAA laye ( he so-called
PAA-assis ed anodizing), o ming Nb
2
O
5
nuclei wi hin and benea h he
ba ie laye o he PAA ilm. Subsequen PAA-assis ed high- ol age
e-anodizing ans o ms he Nb
2
O
5
nuclei in o longe oxide nano-
s uc u es such as columns o ods ia he g ow h o niobium oxide in-
side he PAA nanopo es [13]. Selec i e dissolu ion o he PAA o e laye
yields a ays o up igh -s anding and spa ially sepa a ed Nb
2
O
5
semi-
conduc o nanos uc u es ad an ageous o gas-sensing and
su ace- inishing applica ions [2,4,14].
P ecise con ol o e he su ace mo phology and chemical compo-
si ion o Nb
2
O
5
nanos uc u es is c ucial o ad ancing exis ing and
* Co esponding au ho . Depa men o Mic oelec onics, Facul y o Elec ical Enginee ing and Communica ion, B no Uni e si y o Technology, Technicka 10, 616
00, B no, Czech Republic.
E-mail add ess: [email p o ec ed] (A. Mozale ).
Con en s lis s a ailable a ScienceDi ec
Ce amics In e na ional
jou nal homepage: www.else ie .com/loca e/ce amin
h ps://doi.o g/10.1016/j.ce amin .2023.08.134
Recei ed 25 Ap il 2023; Recei ed in e ised o m 30 July 2023; Accep ed 11 Augus 2023
Ce amics In e na ional 49 (2023) 34712–34725
34713
de eloping new applica ions o niobium-oxide-based nanoma e ials
[15]. The undamen al and unc ional p ope ies o he PAA-assis ed
Nb
2
O
5
ce amic nanoa ays can be con olled by simply adjus ing he
elec ical and elec oly ic o ma ion condi ions [12,14,16]. The na u e
o he anodizing elec oly e and he o ma ion ol age decide he a eal
densi y and diame e s o he PAA nanopo es. In u n, hese PAA p op-
e ies go e n he popula ion densi y and diame e s o he PAA-assis ed
niobium-oxide nanos uc u es. The ela ion has been expe imen ally
p o ed in se e al anodizing elec oly es such as sulphu ic, oxalic,
phospho ic, and ci ic acids [2,4,12,13].
An aqueous solu ion o selenic acid (H
2
SeO
4
) has ecen ly been
explo ed as a no el PAA- o ming elec oly e gene a ing subs an ially
hinne po es han any o he acid elec oly e a he same o ma ion
ol ages [17]. Consequen ly, he po osi y o he PAA ilms g own in
selenic acid (2–3%) dec eases by 3- old compa ed wi h o he anodizing
elec oly es (6–10%). The uniquely low po osi y o PAA o med in
selenic acid appea ed highly ad an ageous o ab ica ing pho onic
c ys als and syn hesizing me ama e ials [18,19].
Suppose niobium can be anodized h ough he PAA ilms o med in
selenic acid. In ha case, he unique po ous-cellula s uc u e o he PAA
ilms migh be ’ ans e ed’ o he anodic niobium oxide and p omo e
he g ow h o ex emely hin bu long-dis anced niobium-oxide nano-
columns o ods. Such nano-mo phology could be in demand o
modula ing Nb
2
O
5
su ace-plasmon esonance (SPR) because he e ec
la gely depends on he ela ion be ween he wid h and spa ial sepa a ion
o he nanos uc u es [19]. The possibili y o une he SPR o he Nb
2
O
5
nanoa ays makes hem pa icula ly p omising in he eme ging
niobium-oxide applica ion as a subs a e o su ace-enhanced Raman
spec oscopy (SERS) [20]. Mo eo e , due o he lowe ed con ac a ea
be ween he liquid and he solid, he ex emely hin and long-dis anced
Nb
2
O
5
nanos uc u es a e p e e ed o enginee ing supe hyd ophobic
ce amic coa ings [2,21]. Fu he , he ex ao dina ily hin PAA nano-
po es o med in selenic acid may e ec i ely supp ess he ield c ys al-
liza ion o g owing niobium oxide by limi ing he space a ailable o
c ys al o ma ion. Such a ays o amo phous Nb
2
O
5
nanos uc u es can
ha dly be syn hesized by o he me hods and a e in demand o a ious
elec ic/dielec ic applica ions due o iso opy o p ope ies, lack o
in e acial de ec s, and absence o leakage pa hs h ough he c ys alline
g ain bounda ies, as epo ed elsewhe e [22].
I is known ha elec oly e-de i ed species eadily inco po a e in o
bo h PAA and PAA-assis ed niobia nanos uc u es du ing anodiza ion
and a ec he oxides’ chemical composi ion and unc ional p ope ies
[13]. Fo example, he inco po a ion o phospho ous (P), ca bon (C),
sul u (S), ch omium (C ), o bo on (B) om acid elec oly es may esul
in oxide colo a ion, educed op ical anspa ency, and wide-spec um
luo escence [23]. On he o he hand, he PAA o med in selenic acid
is epo edly highly anspa en and p oduces no unwan ed luo escence
[24–26]. The e o e, i is an icipa ed ha PAA-assis ed niobium anod-
izing in selenic acid may esul in nanos uc u ed Nb
2
O
5
ce amics wi h
simila ly unhinde ed op ical p ope ies. The lack o op ical-signal
con amina ion is especially desi ed o he eme ging applica ion o
Nb
2
O
5
in SERS due o he undamen al in e sec ion o Raman sca e ing
wi h luo escence [27].
Fu he mo e, Se species a e known o ha e po en an ibac e ial and
an i-oncogenic p ope ies. Inco po a ing Se species in o PAA-assis ed
Nb
2
O
5
ce amic nanos uc u es ia anodizing in selenic acid can boos
he oxide’s biomedical cha ac e is ics and acili a e i s applica ion as
implan coa ing [28,29]. High an i-oxida i e p ope ies o adso bed o
inco po a ed Se species can also enhance he ac i i y o he nano-
s uc u ed Nb
2
O
5
and Nb
2
O
5
/Al
2
O
3
ca alys s [30–32]. Las ly, Se migh
bond wi h Nb o o m a NbSe
2
compound wi h he nanoscale pe iodic
mo phology desi ed o eme ging elec onic applica ions [33].
In he p esen wo k, o he i s ime, Nb
2
O
5
-based ce amic nano-
a ays we e ab ica ed ia he PAA-assis ed anodizing and highe -
ol age e-anodizing o a hin Nb ilm in selenic acid. The in luence o
he o ma ion condi ions on he mo phology, chemical composi ion,
mixing wi h Al
2
O
3
, selenium inco po a ion, and se e al p ac ically
impo an unc ional p ope ies o he Se-doped niobium-oxide-based
nanoce amics we e s udied by high- esolu ion scanning elec on mi-
c oscopy (SEM), X- ay pho oelec on spec oscopy (XPS), Raman spec-
oscopy, pho oluminescence (PL) spec oscopy, and elec ical/
dielec ic measu emen s. To p o e he mo phological ad an ages o he
PAA-assis ed ce amic nanoa ays o med in he selenic-acid elec oly e,
hey we e di ec ly compa ed wi h hose syn hesized in an oxalic-acid
elec oly e a he same o ma ion ol ages. A mechanism o he PAA-
assis ed selenic-acid niobium anodizing was p oposed and expe imen-
ally jus i ied. Po en ial applica ions o he Se-doped Nb
2
O
5
-based
ce amic nanoa ays as ad anced SERS biosensing subs a es, supe -
hyd ophobic su aces, biomedical coa ings, ca aly ic, gas-sensing, and
high-k plana nanocomposi e dielec ic laye s o on-chip in eg a ion
we e expe imen ally and heo e ically app aised.
2. Expe imen al sec ion
2.1. Sample p epa a ion
A s anda d 4
″
Si wa e co e ed by a 380-nm SiO
2
laye was used as
he s a ing subs a e. A 140-nm niobium laye ollowed by a 1000-nm
aluminum laye was deposi ed on he subs a e by he magne on
spu e ing o Nb (99.95%) and Al (99.999%) a ge s o make a p ecu so
Al/Nb bilaye . The wa e wi h he deposi ed bilaye was cu in o ca. 1
cm ×1 cm pieces. The indi idual samples we e anodized in 1.5 M
H
2
SeO
4
aqueous solu ion a oom empe a u e (21 ◦C). A cus om-made
poly e a luo oe hylene wo-elec ode cell secu ing a 0.8 cm
2
ci cula
wo king a ea was used as he anodizing ba h [34]. A 6 mm in diame e
glassy ca bon od, dis anced by 3 mm om he sample su ace, was
employed as he ca hode. The PAA ilms we e o med a a cons an
cu en densi y o ei he 5.0 o 0.5 mA cm
−2
, esul ing in a s eady-s a e
ol age o 42 o 12 V, espec i ely. Selec ed samples we e e-anodized
using a high-speed (10 V s
−1
) po en iodynamic pola iza ion up o 120
o 80 V, ollowed by a po en ios a ic pola iza ion o ensu e uni o m
oxide g ow h ac oss he sample su ace. The PAA o e laye was dis-
sol ed om selec ed samples in an aqueous solu ion o 0.2 М C
2
O
3
and
0.45 М H
3
PO
4
a 65 ◦C o 1 h [35]. He ea e , samples wi h he dis-
sol ed PAA laye a e named he ’PAA- ee’ samples. A e he
anodizing/ e-anodizing and PAA dissolu ion, he samples we e ho -
oughly insed in Milli-Q® ul apu e wa e , d ied in a ni ogen s eam,
and hea ed in an o en a 393 K o 1 h o deso b he wa e molecules
om he oxide su ace.
2.2. Sample analysis
The su aces and c oss- ac u es o PAA- ee samples we e examined
in an FEI Ve ios 460L High-Resolu ion SEM ope a ed a 15 kV accele -
a ing ol age and 0.8 nA p obe cu en . The chemical composi ion o he
ilm su aces was analyzed by XPS in a K a os Axis Ul a DLD spec-
ome e using a monoch oma ic Al K
α
sou ce. The X- ay emission
powe was 150 W wi h a 15 kV accele a ing ol age ocused o a 300
μ
m
×700
μ
m spo . Typical ope a ing p essu es we e be e han 10
−9
To .
The emi ed elec ons we e de ec ed by a hemisphe ical analyze a
ixed pass ene gies o 160 eV o he su ey and 20 eV o he high-
esolu ion spec a. The K a os cha ge neu alize sys em was used o
all specimens. The dep h p o iling was pe o med using an a gon-ion
beam o 5 kV p o ided by a s anda d ion gun wi h a sample cu en o
750 nA scanned o e a su ace window o 2 mm ×2 mm o 90 and 270
s, esul ing in sligh su ace e ching. The spu e ing-beam incidence
angle was 45◦ ela i e o he subs a e. Spec a we e analyzed using
CasaXPS ( e . 2.3.17) so wa e. GL(30) p o iles, de ined in CasaXPS,
we e used o all componen s besides he me allic co e lines o Nb 3d, o
which asymme ic p o iles in he o m o LA(1.2,5,12) we e used. A
s anda d Shi ley backg ound was used in all i ed spec a. Spec a om
all samples we e cha ge co ec ed o gi e he ad en i ious C 1s spec al
K. Kamne e al.
Ce amics In e na ional 49 (2023) 34712–34725
34714
componen (C–C, C–H) binding ene gy o 284.8 eV. The decon olu ion
o C 1s spec a was pe o med as desc ibed in p e ious wo ks [13].
Quan i a i e analysis was pe o med using he na ow spec a and
ela i e sensi i i y ac o s om CasaXPS sui able o K a os Axis Ul a,
e e enced o F 1s.
Selec ed PAA- ee samples we e analyzed by PL spec oscopy using
a chopped Kimmon IK Se ies He–Cd lase (325 nm and 40 mW). The
luo escence was dispe sed wi h an O iel Co ne S one 1/8 74000
monoch oma o , de ec ed using a Hamama su H8259-02 wi h a socke
assembly E717-500 pho omul iplie , and ampli ied h ough a S an o d
Resea ch Sys ems SR830 DSP. Selec ed PAA- ee samples we e analyzed
by Raman spec oscopy using he WITec con ocal Raman imaging
sys em alpha300 R. The samples we e illumina ed by a 532 nm lase
wi h powe educed o 1 mW and ocused by 100×objec i e (NA 0.9).
2.3. Elec ical cha ac e iza ion
Elec ical measu emen s we e conduc ed using es me al-
insula o -me al (MIM) de ices. A 100 nm- hick Nb laye ollowed by a
65 nm- hick Al laye (Al/Nb bilaye ) we e deposi ed by he magne on
spu e ing on o a SiO
2
-coa ed Si wa e . A piece o 1.5 cm ×1.5 cm cu
om he wa e was anodized a 0.5 mA cm
−2
and hen e-anodized o 70
V a a a e o 10 V s
−1
using he me hodology and anodizing se up
desc ibed in sec ion 2.1. Such anodizing was expec ed o gene a e a
plana ilm whe e he PAA-assis ed Nb
2
O
5
nanos uc u es g ow up o he
PAA-su ace le el. Fo making he op elec odes, a laye o ~100 nm-
hick pla inum (P ) was spu e -deposi ed h ough an a ay o 10
μ
m ×
10
μ
m squa e openings in a hin ce amic pla e used as he shadow mask.
The unoxidized Nb laye ha emained unde he anodic ilm a e he
e-anodizing se ed as he bo om elec ode. The assembled MIM
mic ode ices we e moun ed o Cascade Mic o ech MPS 150 P obe S a-
ion connec ed o an Agilen Technologies P ecision LCR Me e E4980A
and a Kei hley 4200A-SCS Pa ame e Analyze . Unless o he wise spec-
i ied, he op P con ac s we e used as a ca hode in elec ical es s.
3. Resul s and discussion
3.1. Anodizing beha io
Typical ol age- and cu en - ime esponses du ing he 5.0 and 0.5
mA cm
−2
anodizing o he spu e -deposi ed Al/Nb bilaye in 1.5 M
H
2
SeO
4
aqueous solu ion, ollowed wi hou in e up ion by 120 and 80
V high-speed e-anodizing in he same elec oly e, a e p esen ed in
Fig. 1. Bo h esponses gene ally exhibi simila beha io and can be
di ided in o h ee dis inc s ages. 1s s age: The ol age- ime cu es a e
ypical o he po ous anodizing o aluminum in acid elec oly es,
including he PAA o ma ion on aluminum oils in selenic-acid elec o-
ly es o compa able concen a ion epo ed elsewhe e [24,36]. Sys-
ema ic g ow h o PAA ilm is associa ed wi h he pe iods when he
ol age a ains he s eady-s a e alues, U
s
, o ~42 V (he ea e he
’high- ol age’ sample) o ~12 V ( he ’low- ol age’ sample). Once he
anodizing on eaches he niobium unde laye , he ol age begins o
inc ease, and he p ocess is au oma ically swi ched in o he po en io-
s a ic mode a 45 o 13 V, espec i ely. 2nd s age: The beha io is
de e mined by he ’sma ’ PAA-assis ed anodizing o he niobium
unde laye : a ays o niobium-oxide nanos uc u es nuclea e and g ow
downwa d in o he Nb me al and upwa d in o he PAA ba ie laye
[13]. The applied ol age and he leng h o he cu en decay impac he
oxide g ow h. 3 d s age: The high-speed po en iodynamic e-anodizing
up o 120 V (Fig. 1a) o 80 V (Fig. 1b) is ollowed wi hou in e up-
ion by a sho po en ios a ic pola iza ion wi h a cu en decay. The
niobium oxide is expec ed o g ow u he and pa ially ill he PAA
po es a his s age [13]. This p ocess will likely compe e wi h expanding
he nanos uc u es’ bases, which can me ge in a con inuous bo om
oxide laye . The me ging o he bases depends on a combina ion o
ac o s like he nanos uc u es’ dimensions, he a io o ionic esis ances
be ween alumina and niobia, ionic anspo numbe s, anion inco po-
a ion, oxide c ys alliza ion beha io , e c.
In he ollowing ex , he samples will be dis inguished by hei U
s
du ing he 1s s age (Fig. 1): The high- and low- ol age samples p e-
pa ed by he selenic-acid anodizing o he end o he 2nd s age will be
named SE42
A
and SE12
A
, espec i ely. The high- and low- ol age
samples p epa ed by he selenic-acid anodizing ollowed by e-
anodizing o he end o he 3 d s age will be named SE42
R
and
SE12
R
, espec i ely.
3.2. Film mo phology
Fig. 2 shows SEM images o he PAA- ee SE42
A
and SE42
R
samples.
The SE42
A
su ace (Fig. 2a) e eals an a ay o up igh -s anding,
spa ially sepa a ed nanosized p o usions, p esumably o niobium
oxide. The p o usions a e su ounded by an unoxidized me al, likely Al.
F om Fig. 2c, he shape o he p o usions esembles a ’goble ’. The
Fig. 1. Vol age- and cu en - ime esponses du ing he anodic p ocessing o Al
(1000 nm)/Nb (140 nm) bilaye in 1.5 M H
2
SeO
4
aqueous elec oly e a 21 ◦C
o o ming Nb
2
O
5
nanoa ays: 1s s age – he gal anos a ic aluminum anod-
izing o o ming po ous anodic alumina (PAA) a (a) 5 mA cm
−2
(s eady-s a e
ol age U
s
=42 V) and (b) 0.5 mA cm
−2
(U
s
=12 V); 2nd s age – he
po en ios a ic PAA-assis ed niobium anodizing a (a) 45 V and (b) 13 V; 3 d
s age – he po en iodynamic PAA-assis ed niobium e-anodizing wi h a a e o
10 V s
−1
o (a) 120 V and (b) 80 V ollowed by po en ios a ic pola iza ion a he
same ol ages. The samples a e named ela ing o he selenic (SE) acid, U
s
-
alues (12 o 42 V), and he o ma ion s ages (A: anodizing ill he end o 2nd
s age, R: e-anodizing: ill he end o 3 d s age), as speci ied abo e he plo s.
K. Kamne e al.
Ce amics In e na ional 49 (2023) 34712–34725
34715
nanogoble s’ apexes g adually ansi o ela i ely hinne middle pa s,
o en composed o se e al dis inguished nano oo s ha expand o o m
b oade bases. The bases a e sepa a ed om each o he by unoxidized
niobium me al.
Fig. 2d– depic s he SE42
R
ilm mo phology. F om he su ace iew,
he ho izon al p ojec ions o he p o usions hicken wi h e-anodizing.
The p o usions become subs an ially longe and acqui e he shape o
ods, each ha ing a single oo ha is ela i ely hinne han he od’s
body. Al hough mo e expanded, he bases o he nano ods a e s ill
sepa a ed by he unoxidized Nb me al. The SEM analysis canno eliably
con i m he p esence o unoxidized aluminum.
SEM images o he SE12
A
and SE12
R
samples a e shown in Fig. 3. The
up igh -s anding sub-10 nm oxide p o usions popula e he SE12
A
su -
ace (Fig. 3a). They ha e signi ican ly smalle diame e s and highe
popula ion densi y han he SE42
A
nanogoble s. F om Fig. 3c, he
nanop o usions a e do -shaped. The indi idual nanodo s seem sepa-
a ed by he unoxidized Nb me al. The insu icien SEM esolu ion
makes i impossible o judge he p esence o an unoxidized aluminum.
The su ace iew in Fig. 3d shows ha he oxide p o usions expand wi h
e-anodizing. The apexes o he p o usions end o conglome a e in
small g oups, likely due o an de Waals o ces. F om Fig. 3 , he e-
anodizing no iceably elonga es he SE12
A
oxide nanodo s, esul ing in
an a ay o od-like nanos uc u es. A subs an ial di e ence be ween
SE12
R
and he o he ilms is ha he bases o he nano ods me ge in a
con inuous 30 nm- hick oxide laye ha bu e s he ods om he
unoxidized Nb me al. Appa en ly, he e is no unoxidized Al a he ilm
su ace.
3.3. Compa ison be ween selenic- and oxalic-acid anodizing
The PAA-assis ed niobium-oxide nanos uc u es g ow inside and
benea h he po es du ing he anodizing and e-anodizing [13]. The e-
o e, he PAA po ous-cellula s uc u e decides he diame e s and su -
ace dis ibu ion o he niobium-oxide nanop o usions. The po e
cen e - o-cen e dis ance, D
in
, is known o depend on U
s
( he 1s
s ages in Fig. 1a and b) h ough a cons an k =2.5 nm V
−1
pe linea
ela ion D
in
=kU
s
[37]. Consequen ly, he cen e - o-cen e dis ance
be ween he niobium-oxide nanos uc u es, D
cen
, should be dependen
on D
in
and U
s
:
Dcen =Din =kUs (1)
Since he PAA laye can be g own in he selenic- and oxalic-acid
elec oly es a he same U
s
, one may expec ha he subsequen ly
Fig. 2. SEM (a and d) e ical su ace iews, (b and e) 30
◦- il ed su ace iews, and (c and ) 3D c oss- ac u e iews o niobium-oxide nanoa ays de i ed om an
Al/Nb bilaye anodized and e-anodized in 1.5 M selenic acid as de ined and named in Fig. 1a. The PAA laye s we e selec i ely dissol ed away be o e SEM
obse a ion. All unde ined scale ba s a e 400 nm.
Fig. 3. SEM (a and d) e ical su ace iews, (b and e) 30
◦- il ed su ace iews, and (c and ) 3D c oss- ac u e iews o niobium-oxide nanoa ays de i ed om an
Al/Nb bilaye anodized and e-anodized in 1.5 M selenic acid as de ined and named in Fig. 1b. The PAA laye s we e selec i ely dissol ed away be o e SEM
obse a ion. All unde ined scale ba s a e 400 nm.
K. Kamne e al.
Ce amics In e na ional 49 (2023) 34712–34725
34716
o med niobium-oxide nanos uc u es would ha e simila D
cen
. On he
o he hand, he PAA-assis ed selenic-acid anodizing is expec ed o lead
o niobium-oxide nanos uc u es wi h no iceably smalle diame e s, d
n
,
due o he smalle PAA po e diame e s, d
p
, compa ed wi h he PAA-
assis ed oxalic-acid anodizing. Al hough he nanos uc u es o med in
he selenic acid may ha e a D
cen
simila o hose o med in he oxalic
acid, hey should possess a smalle d
n
and, he e o e, acqui e an
ad anced su ace s uc u ing, which is no a ainable in o he elec o-
ly es. Since sensing, SPR, su ace p o ec ion, sel -cleaning, and
biomedical p ope ies o he oxide depend on he nanos uc u es’ ge-
ome y and hei mu ual a angemen , he new mo phology o he PAA-
assis ed nanos uc u es o med in he selenic acid could be especially
ele an o he eme ging applica ions o Nb
2
O
5
ce amics.
Fo di ec compa ison, samples we e p epa ed in 0.6 M oxalic acid
(H
2
C
2
O
4
) ia he anodizing/ e-anodizing o he same Al/Nb p ecu so
bilaye . To his end, he cu en densi ies o PAA o ma ion we e
adjus ed o allow U
s
o 42 and 12 V – he same as o he selenic-acid
anodizing. Simila ly, he niobium unde laye was e-anodized in he
oxalic acid ia he po en iodynamic pola iza ion a 10 V s
−1
up o 120 o
80 V. The samples o med by he PAA-assis ed oxalic-acid anodizing will
be named OX42
A
, OX42
R
, OX12
A
, and OX12
R
, co esponding o hei
coun e pa s SE42A, SE42
R
, SE12
A
, and SE12
R
o med in he selenic
acid.
Fig. S1–S4 shows a side-by-side compa ison o he nanos uc u es
o med in he wo elec oly es. The SEM da a we e quan i ied using
ImageJ so wa e o assess he mean alues o nanos uc u es’ di-
mensions summa ized in Table 1. The D
cen
and d
n
alues we e plo ed
and bes i ed wi h a no mal dis ibu ion (Figs. S5–S7). D
cen
and pop-
ula ion densi y,
ρ
N
, we e e alua ed only o he anodized samples
because he e-anodizing elonga es he niobium-oxide nuclei wi hou
a ec ing hei mu ual a angemen . Mo eo e , he hin SE12
R
and
OX12
R
nano ods end o agglome a e, complica ing image e alua ion.
F om Figs. S1, S3, S6, and S7, one may see ha he nanodo s and
nanogoble s o med in he selenic acid a e subs an ially be e sel -
o de ed han he co esponding nanos uc u es o med in he e e -
ence oxalic-acid elec oly e.
Despi e he same U
s
, he nanos uc u es o med in he selenic acid
e eal abou 10% smalle D
cen
and co espondingly sligh ly la ge
ρ
N
han he nanos uc u es o med in he oxalic acid. Simila ly, a ~10%
smalle D
in
was epo ed by Kikuchi e al. o he PAA o med on
aluminum oil in selenic-acid elec oly es o compa able concen a ions
[24].
No elec oly e-dependen di e ences we e e ealed o he leng h o
he niobium-oxide nanop o usions, h (Table 1, Figs. S1–S4). The SE12
A
and OX12
A
nanodo s ha ing h =20 nm a e bo h elonga ed o he 125
nm- all SE12
R
and OX12
R
nano ods by he 80 V e-anodizing. Simila ly,
he SE42
A
and OX42
A
nanogoble s ha ing h =75 nm g ow u he o
become he 210 nm- all SE42
R
and OX42
R
nano ods a e he 120 V e-
anodizing. The SE12
R
and OX12
R
nano ods a e ancho ed o a con inuous
~30 nm- hick bo om-oxide laye . In con as , he SE42
A
and OX42
A
nanogoble s and he SE42
R
and OX42
R
nano ods eside on con ex-
shaped bases sepa a ed by he unoxidized Nb. The bases o he OX42
R
nano ods seem sligh ly la ge han hose o he SE42
R
nano ods.
Fu he mo phological di e ences be ween he nanos uc u es
o med in he selenic and oxalic acids a e associa ed wi h hei di-
ame e s d
n
. Based on he SEM analysis, we no e he ollowing majo
ends: (1) d
n
dec eases by ~30% o he nanos uc u es o med in he
selenic acid. The e ec is appa en ly due o he ela i ely hinne PAA
nanopo es g own by selenic-acid anodizing. (2) d
n
inc eases by ~35%
due o he e-anodizing in bo h acids. This e ec could be explained by
he pa ial g ow h o he ods wi hin he inne pa o he PAA cell walls,
which is enabled by he upwa d mig a ion o Nb ca ions h ough he
PAA cell walls [13]. The cell walls may p o ide an e ec i e pa hway o
ion anspo due o de ec s such as acancies and elec oly e-de i ed
impu i ies, as desc ibed elsewhe e [13,38].
The dimensions o PAA ilms o med in bo h acids we e assumed
based on he measu ed D
cen
and heo e ical conside a ions summa ized
in Fig. S8. One may see ha d
n
sys ema ically exceeds d
p
in all he
samples (Table 1, Fig. S9). The deg ee o niobium-oxide expansion in o
he PAA cell walls was assessed by compa ing he po osi y [37] o he
PAA ilms, P =0.907d
p
2
/D
in
2
, wi h he niobium-oxide nanos uc u es’
p ojec ed a ea, A =0.907d
n
2
/D
cen
2
. I was e ealed ha A consis en ly
exceeds P. In addi ion, A inc eases app oxima ely by 2- old a e he
e-anodizing. Ano he impo an e ela ion is ha he A- alue o he
low- ol age samples (12/80 V) is abou wo imes highe compa ed wi h
he high- ol age samples (42/120 V) ega dless o he elec oly e. Such a
di e ence in A- alues may a ise because he anodic-oxide g ow h wi hin
he hinne po es o he low- ol age PAA is mo e obs uc ed. In such hin
po es, he mig a ion o Nb ca ions becomes mo e p e alen wi hin he
cell walls unde he high elec ic ield. Addi ionally, he e ec may be
due o he di e en a ios o he e-anodizing and s eady-s a e ol age:
o he low- ol age samples (80/12 V), he ol age a io is 6.7, while o
he high- ol age samples (120/42 V), he a io is only 2.9. I is assumed
ha he highe he a io, he mo e in ensi e he ca ion mig a ion wi hin
he PAA cell walls.
The mos p ac ically impo an inding is ha he nanos uc u es
o med in he selenic acid exhibi , on a e age, a 1.5- old lowe A han
hei coun e pa s o med in he oxalic acid. Since he nanos uc u es’
p ojec ion a ea (A) is he c i e ion o access he undamen al, s uc u al,
and unc ional bene i s o he PAA-assis ed oxide nano ods, he selenic-
acid anodizing is expe imen ally jus i ied. The high- ol age samples
look mos ad an ageous o niobium-oxide nanoa ay applica ions, such
as SPR and sel -cleaning su aces, whe e he lowes possible A is desi ed.
The s uc u al, composi ional, and applica ion- ela ed bene i s o he
niobium-oxide nanos uc u ed ce amics o med by he PAA-assis ed
selenic-acid anodizing a e u he discussed in sec ion 3.6.
3.4. Nanos uc u es’ chemical composi ion
The examina ion o chemical composi ion and bonding s a es in he
Table 1
Compa ison o he PAA-assis ed niobium-oxide nanoa ays syn hesized in 1.5 M selenic acid (H
2
SeO
4
) and 0.6 M oxalic acid (C
2
H
2
O
4
) elec oly es unde he same
anodizing/ e-anodizing ol ages.
1.5 M H
2
SeO
4
(selenic acid) 0.6 M C
2
H
2
O
4
(oxalic acid – e e ence)
SE42
A
SE42
R
SE12
A
SE12
R
OX42
A
OX42
R
OX12
A
OX12
R
Cu en densi y /mA cm
−2
5.0 0.5 12.2 1.3
S eady-s a e ol age (U
s
) /V 42 12 42 12
Re-anodizing ol age /V – 120 – 80 – 120 – 80
Cen e - o-cen e (D
cen
) /nm 96 ±9 27 ±6 104 ±14 30 ±7
Popula ion densi y (
ρ
N
) /cm
−2
1.2 ⋅ 10
10
1.6 ⋅ 10
11
1.1 ⋅ 10
10
1.3 ⋅ 10
11
Leng h (h) /nm 75 210 20 125 75 210 20 125
Nanos uc u e diame e (d
n
) /nm 24 ±4 35 ±6 9 ±2 13 ±3 32 ±6 45 ±7 12 ±3 17 ±4
PAA po e diame e (d
p
) /nm 16.2 4.2 24.2 7.2
Nanos uc u e p ojec ed a ea (A) /% 5.7 12.0 10.0 21.0 8.6 17.0 14.5 29.1
PAA po osi y (P) /% 2.5 2.2 4.9 5.2
K. Kamne e al.

Ce amics In e na ional 49 (2023) 34712–34725
34717
niobium-oxide nanoa ays o med by he PAA-assis ed selenic-acid
anodizing was pe o med by XPS (Fig. 4), mainly o he PAA- ee
samples. In addi ion, a SE42
R
sample wi h pa ially dissol ed PAA, as
ske ched in he inse o Fig. 4m, was analyzed. In such a sample, abou
~40 nm o he uppe pa s o he ods p o ude ou wa ds om he PAA
su ace le el. This sample wi h he pa ially-dissol ed PAA is he ea e
named SE42
Rpd
.
Fo compa ison, a compac niobium-oxide ilm was p epa ed by
po en iodynamic anodiza ion o an Nb laye in 1.5 M selenic acid a a
a e o 0.1 V s
−1
up o 10 V. The ini ial Nb laye was ob ained by dis-
sol ing he Al laye om he Al/Nb p ecu so bilaye in 1 w % NaOH
solu ion. XP spec a we e eco ded on he as-p epa ed (no spu e -
cleaned) su aces o all samples. In addi ion, he SE42
R
sample was
analyzed ollowing wo sho A -ion spu e cycles o emo e su ace
ca bon con amina ion and a po ion o he ou e ilm ma e ial.
The p esence o C, Nb, O, Al, and Se was iden i ied in he su ey
spec a o all samples; addi ionally, P and C we e de ec ed on he PAA-
ee su aces. Na ow-scan C 1s, Nb 3d, O 1s, Al 2p, and Se 3d spec a
we e collec ed o analyze hei co e le els and bonding s a es. The
expe imen al and i ed Nb 3d, Al 2p, and Se 3d spec a o he as-
ecei ed su aces a e shown in Fig. 4. The co esponding spec a o
he A -ion-spu e ed SE42
R
ilm a e p esen ed in Fig. S10.
Fig. 4a shows he Nb 3d spec um o he SE42
A
su ace. Th ee double s
o app op ia ely cons ained peaks a e used o i he spec um (Nb 3d
5/2
and d
3/2
wi h he ixed peak-sepa a ion ene gy o 2.75 eV, he ixed
in ensi y a io o 3:2, and he ull wid h a hal maximum (FWHM) equal
o oxide componen s) [13]. The highes binding-ene gy (BE) double
(Nb 3d
5/2
a 206.85 eV) is due o Nb
5+
ca ions in he oxide [13]. The
lowes BE double (Nb 3d
5/2
a 201.2 eV) o a compa able in ensi y is
associa ed wi h me allic Nb
0
ha sepa a es he nanogoble s’ bases
Fig. 4. Expe imen al and cu e- i ed na ow-scan ( i s ow) Nb 3d, (second ow) Al 2p, and ( hi d ow) Se 3d XP spec a o PAA- ee (a–c) SE42
A
, (d– ) SE42
R
, (g–i)
SE12
A
, (j–l) SE12
R
su aces. (m) Nb 3d, (n) Se 3d, and (n-inse ) Al 2p XP su ace spec a o SE42
R
sample ha ing pa ially-dissol ed PAA laye . (o) Nb 3d and (p) Se 3d
XP su ace spec a o a compac niobium-oxide ilm p epa ed o compa ison by anodizing niobium a 10 V in 1.5 M selenic acid.
K. Kamne e al.
Ce amics In e na ional 49 (2023) 34712–34725
34718
(Fig. 2a–c). The amoun o me allic Nb
0
is assessed as 53% o all Nb
species. The hi d, lowes -in ensi y double , shi ed o −1.35 eV om he
Nb
5+
componen (wi h Nb 3d
5/2
a 205.5 eV), is due o he p esence o
Nb
4+
ca ions in he ilm, wi h a Nb
5+
:Nb
4+
a io o 97:3. Due o he high
amoun o Nb
5+
ca ions and he low amoun o Nb
4+
ca ions in he
su ace composi ion, he ou mos ilm laye is assessed as s oichiome ic
Nb
2
O
5
mixed wi h a mino amoun o oxygen-de icien niobia.
The Nb 3d spec um o he SE42
R
su ace (Fig. 4d) is i ed wi h h ee
double s (wi h Nb 3d
5/2
a 207.1, 205.75, and 201.5 eV), which a e
simila ly associa ed wi h Nb
5+
, Nb
4+
, and Nb
0
species. The Nb
5+
:Nb
4+
a io is 95:5, close o ha o he SE42
A
su ace. The e o e, he SE42
R
nano ods also con ain subs oichiome ic (oxygen-de icien ) niobium
pen oxide. The amoun o Nb
0
a he SE42
R
su ace is much lowe han
ha a he SE42
A
su ace (12% s. 53% o all Nb species). The wo- imes
spu e ing o he sample su ace wi h weak-ene gy A ions (Fig. S10)
educes he con ibu ion o Nb
0
om 12 o 8%. This means ha a po ion
o he ou e ma e ial was spu e ed away along wi h he ca bon
con amina ion. Spu e ing also changes he Nb
5+
:Nb
4+
a io o 93:7 by
inc easing he amoun o Nb
4+
ca ions, which may be a ibu ed o
p e e en ial oxygen spu e ing. The p esence o Nb
4+
ca ions (and
lowe -oxida ion-s a e me al ca ions in gene al) a he su ace laye is a
common phenomenon o he oxides o med by PAA-assis ed anodizing
[13,38,39].
The Nb 3d spec um o he SE12
A
su ace (Fig. 4g) is i ed wi h h ee
double s (Nb 3d
5/2
a 207.15, 203.6, and 201.5 eV), he high- and low-
BE double s being due o Nb
5+
and Nb
0
. The amoun o me allic niobium
is ela i ely highe in his sample (70% o all Nb species). The lowes -
in ensi y double shi ed o −3.55 eV om he Nb
5+
peak is assigned
o Nb
2+
ca ions in he anodic oxide [13]. The Nb
5+
:Nb
2+
a io is 85:15,
indica ing he p esence o s oichiome ic Nb
2
O
5
mixed wi h a ela i ely
high amoun o NbO suboxide.
The spec um o he SE12
R
su ace (Fig. 4j) consis s o a high-
in ensi y double wi h BE o 207.15 eV o Nb 3d
5/2
associa ed wi h
Nb
5+
ca ions and a low-in ensi y double wi h BE o 204.7 eV o Nb 3d
5/
2
. The 2.45 eV dis ance be ween hem indica es ha he low-in ensi y
double is due o Nb
3+
ca ions in he oxide. As he Nb
5+
:Nb
3+
a io is
low (98:2), he low-in ensi y peak is associa ed wi h small amoun s o
Nb
2
O
3
suboxide mixed wi h Nb
2
O
5
. No me allic niobium species a e
de ec ed a he SE12
R
su ace. The high deg ee o oxide s oichiome y
and he lack o me allic Nb on he su ace may a o he applica ion o
such ilm as dielec ic, which will be conside ed in Sec ion 3.6.2.
The Nb 3d spec um o he SE42
Rpd
su ace (Fig. 4m) is i ed wi h
wo double s (Nb 3d
5/2
a 207.4 and 206.25 eV), associa ed wi h Nb
5+
and Nb
4+
ca ions, espec i ely. The lowe -in ensi y peak, shi ed o only
−1.15 eV om he Nb
5+
one, may be due o a mo e subs oichiome ic
Nb
2
O
5
owing o oxygen acancies [40]. Since only he uppe 40 nm-long
pa s o he ods a e exposed o analysis ( he lowe pa s o he ods a e
s ill co e ed by he emaining po ion o PAA, as shown in he inse o
Fig. 4m), he dec eased Nb
5+
:Nb
4+
a io o 66:34 indica es ha he
apexes o he nano ods con ain a subs an ial amoun o educed oxide.
This implies ha he niobium oxide becomes ela i ely mo e educed
owa ds he uppe pa s o he ods. As expec ed, no me allic Nb is
de ec ed because he PAA laye co e s he me allic esidues be ween he
ods’ bases.
Fo compa ison, he Nb 3d spec um o he compac niobium-oxide
su ace is shown in Fig. 4o. A single double ep oduces he spec um
wi h Nb 3d
5/2
a 207.3 eV a ibu ed o Nb
5+
ca ions. The su ace laye
o he compac anodic niobium oxide is he e o e composed o ully
s oichiome ic Nb
2
O
5
.
Summa izing he Nb 3d spec a analysis, he su aces o he anodized
and e-anodized nanos uc u es a e p edominan ly composed o s oi-
chiome ic Nb
2
O
5
mixed wi h a small po ion o subs oichiome ic
Nb
2
O
5
( educed o oxygen-de icien ) o niobium suboxide (NbO o
Nb
2
O
3
). The oxides could be mixed a he molecula le el o p esen as
nanoinclusions and une enly dis ibu ed along he nanos uc u es. The
mos educed oxide o ms in he SE12
A
nanodo s. Compa ing he
ela i e amoun o he educed oxide in he SE42
R
and SE42
Rpd
su aces,
we assume ha he oxide subs oichiome y inc eases owa d he ops o
he p o usions. All samples, excep SE12
R
, a e con i med o ha e
me allic Nb sepa a ing he bases o he oxide nanos uc u es. The
absence o Nb me al a he SE12
R
su ace implies ha he bases o he
oxide nano ods ully me ge o o m a con inuous bo om-oxide laye
du ing e-anodizing, as also seen in he c oss- ac u e SEM images in
Fig. 3 .
The Al 2p spec a o he SE42
A
and SE42
R
su aces (Fig. 4b and e) a e
i ed wi h wo single symme ical peaks: The high-in ensi y peak is
associa ed wi h Al
2
O
3
(74.5 and 74.65 eV), and he low-in ensi y peak is
due o me allic Al (71.9 and 72.15 eV). The me allic Al o igina es om
he ne wo k o Al esidues emaining a ound he niobium-oxide nano-
s uc u es o e he me allic Nb (Fig. 2). A e wo imes A -ion spu -
e ing o he SE42
R
su ace, he amoun o me allic Al dec eases om 18
o 13%, which indica es he emo al o a hin su ace laye . In he Al 2p
spec um o SE42
Rpd
( he inse in Fig. 4n), only he peak associa ed wi h
Al
2
O
3
is p esen (74.55 eV).
The Al 2p spec um o he SE12
A
su ace (Fig. 4h) is simila ly i ed
wi h a domina ing peak assigned o Al
2
O
3
(74.45 eV) and a lowe -
in ensi y peak assigned o me allic Al (71.85 eV). Howe e , he e he
me allic con ibu ion is subs an ially mino compa ed wi h he SE42
A
su ace. This implies ha aluminum emains a he SE12
A
su ace in a
subs an ially smalle amoun han a he SE42
A
su ace. The Al 2p
spec um o SE12
R
(Fig. 4k) has a single Al
2
O
3
- ela ed peak (74.45 eV),
indica ing he absence o me allic Al.
The me allic con ibu ions o he Nb 3d and Al 2p spec a o igina ing
om he co esponding me allic esidues a ound he nanos uc u es’
bases we e sub ac ed om he o e all con en o he elemen s o ob ain
an app oxima e su ace amoun o Nb and Al in he oxides. Fo Al, he
co esponding na i e oxide was also conside ed and sub ac ed [41].
P o ided ha a .%(Al +Nb) =100%, he SE42
A
and SE42
R
nano-
s uc u es a e es ima ed o ha e 65 and 61 a .% o Al, espec i ely, while
he SE12
A
and SE12
R
coun e pa s show 81 and 47 a .% o Al, espec-
i ely. The e o e, he su ace o he nanos uc u es is composed o mixed
Nb
2
O
5
–Al
2
O
3
, whe eas hei in e io is mos likely alumina- ee Nb
2
O
5
[13]. Simila ly, a la ge amoun o su ace Al (abou 75 a .%) has p e-
iously been epo ed o he PAA-assis ed N-doped TiO
2
nanocolumns
[39].
The noisy Se 3d spec a, associa ed wi h he low Se concen a ion,
and hei o e lapping wi h he Nb 4s peaks complica es he analysis o
he Se 3d spec a. The p esence o mul iple componen s is assumed and
con i med by compa ing he spec a o he a ious samples, i ing hem,
and e i ying he peak posi ions wi h he li e a u e- epo ed alues: Nb
4s peak o Nb
5+
ca ions, Nb 4s peak o Nb
0
a oms, Se 3d peaks o selena e
anions (SeO
4
2−
), and Se 3d peaks o selenide anions (Se
2−
). A single peak
o Nb 4s ansi ion o Nb
5+
was assumed a ~60.5 eV [42]. The Nb
0
line
was assumed a ~56.0 eV [42]. The i ing leads o 60.9 ±0.2 and 56.4
±0.1 eV o he Nb
5+
and Nb
0
peaks, espec i ely. The a ea below he
Nb 4s peaks was conside ed p opo ional o he a ea o he co e-
sponding Nb 3d peaks (based on he ela i e sensi i i y ac o s o 0.126
and 2.921 o Nb 4s and Nb 3d, espec i ely). Howe e , he i ing e-
sul s in he 2.0 ±0.1- old lowe -in ensi y o Nb 4s peaks han heo-
e ically expec ed. The a ea o he Nb
0
peak was cons ained as a
mul iple o he Nb
5+
peak o ag ee wi h he co esponding Nb 3d
spec a. The Se 3d ansi ions we e i ed as double s wi h a cons ained
spin-o bi spli ing o 0.86 eV [43], he a io o he Se 3d
3/2
and 3d
5/2
componen s was ixed a 0.72 [43], and FWHMs we e cons ained o be
equal wi hin each componen . The BEs o Se 3d
5/2
peaks o ~60 eV and
~54.5 eV we e expec ed o SeO
4
2−
and Se
2−
anions, espec i ely [43,
44]. The i ed BEs o he Se 3d
5/2
peaks a e 58.55–59.6 (a g. 58.8) eV
o he selena e and 53.55–54.5 (a g. 54.0) eV o he selenide ions,
ag eeing wi h he li e a u e.
The Se 3d spec a a e shown in he 3 d ow o Fig. 4. They a e all
i ed wi h Nb 4s peaks o Nb
5+
and possibly Nb
0
, co ela ing wi h he
co esponding Nb 3d spec a, and wi h wo double s o Se 3d peaks o
K. Kamne e al.
Ce amics In e na ional 49 (2023) 34712–34725
34719
SeO
4
2−
and Se
2−
ions. P o ided ha a .%(SeO
4
2−
+Se
2−
) =100%, he
amoun s o Se
2−
anions a e 61 a .% o he anodized (SE12
A
and SE42
A
)
and 49 a .% o he e-anodized samples (SE12
R
and SE42
R
). A e he
wo- imes spu e ing o he SE42
R
su ace, he amoun o Se
2−
ions in-
c eases om 49 o 64 a .%. On he o he hand, a domina ing amoun o
SeO
4
2−
ions is ound in he Se 3d spec um o he SE42
Rpd
sample
(Fig. 4n), wi h only a mino con ibu ion o Se
2−
ions (~3 a .%). No
selenides a e assumed om he Se 3d spec um o he compac niobium
oxide (Fig. 4p), which is i ed only wi h one SeO
4
2−
double (in addi ion
o an Nb 4s peak o Nb
5+
).
The peak a ea o Se 3d componen s (i.e., wi hou he con ibu ion o
Nb 4s) was used o calcula e he amoun o Se a he sample su aces
(p o ided a .%(Al +Nb +Se) =100%), a e sub ac ing he me allic
con ibu ion om he Al 2p and Nb 3d spec a, as desc ibed abo e. The
SE42
A
and SE42
R
su aces ha e 5.3 and 3.2 a .% o Se, espec i ely,
whe eas he SE12
A
and SE12
R
su aces ha e 3.9 and 4.9 a .% o Se,
espec i ely. Spu e ing he SE42
R
su ace a ec s he Se con en sligh ly
(2.8 a .%). The SE42
Rpd
su ace shows a li le highe Se amoun (4.3 a .
%) han he PAA- ee SE42
R
. On he o he hand, he su ace o he
compac niobium oxide has a subs an ially mo e conside able Se amoun
(9.6 a .%) ela i e o he nanos uc u ed samples.
In summa y, he chemical composi ion o he ou mos nano-
s uc u es’ ma e ial can be exp essed as a mix u e o Al
2
O
3
and niobium
oxides: s oichiome ic Nb
2
O
5
mixed wi h subs oichiome ic Nb
2
O
5
o
wi h a suboxide exp essed on a e age as NbO
2.5
⋅xNb
n +
O
n/2
. A po ion
o he O
2−
anions in bo h oxides is eplaced by SeO
4
2−
and possibly Se
2−
anions, as summa ized in Table 2. The e is 47–81 a .% o Al
2
O
3
a he
sample su aces, and abou 1.5–3.0% o he O
2−
anions a e eplaced by
selenium species. The SE42
Rpd
su ace con ains much mo e alumina on
a e age (95 a .%), appa en ly due o he p esence o he PAA ma ix
su ounding he ops o he ods, and 2.9% o he O
2−
anions a e
eplaced by selenium species. Con a ily o he nanos uc u ed su aces,
he compac anodic niobium-oxide ilm con ains nei he Al
3+
no Se
2−
ions. I mainly comp ises s oichiome ic Nb
2
O
5
, in which 4.2% o he
O
2−
anions a e eplaced by SeO
4
2−
anions.
3.5. Oxide g ow h model
3.5.1. Fo ma ion-mo phology ela ionship
Based on he p esen expe imen al indings and wi h e e ence o
p e ious publica ions [12–14], we de eloped a model o he
PAA-assis ed g ow h o nanos uc u ed niobium-oxide-based ce amic
nanoa ays in he selenic-acid elec oly e, as ske ched in Fig. 5. The
g ow h s eps depic ed in Fig. 5 co espond o he ilm- o ma ion s ages
in Fig. 1: (a and ) he s eady-s a e PAA g ow h du ing he 1s s age, (b
and g) he anodizing o he Nb unde laye du ing he 2nd s age, and (c
and h) he e-anodizing o he Nb unde laye du ing he 3 d s age.
Ini ially, he uppe Al laye is anodized [45] a he high (42 V) o low
(12 V) s eady-s a e ol ages. The SeO
4
2−
elec oly e-de i ed anions a e
inco po a ed in he g owing PAA. A small po ion o he PAA a he
ba ie laye /aluminum in e ace, abou 10% o he ba ie -laye
hickness, ypically emains impu i y- ee [45,46]. When he PAA cells
ouch he Nb me al, niobium oxide begins o g ow inside he PAA ba ie
laye , accompanied by he pa ial dissocia ion o he Al–O bonds unde
he high elec ic ield, which is ypical o some o he al e me als
[47–49]. Nanochannels o niobium oxide g ow wi hin he PAA ba ie
laye ia he c oss-mig a ion o ca ions and anions. Finally, he g owing
oxide p o udes in o he po es, and he nanochannels wi hin he PAA
ba ie laye expand o o m solid niobium-oxide bulges (Fig. 5b and g).
The oxide also g ows wi hin he PAA cell walls due o he enhanced ionic
anspo h ough a ious de ec s in he ou e cell-wall ma e ial: he
inco po a ed elec oly e species, s uc u al impe ec ions, acancies,
e c. [13]. Due o his phenomenon, he niobium-oxide nanos uc u es
g ow hicke han he PAA po es, and a mixed alumina-niobia egion
o ms along he Nb
2
O
5
/Al
2
O
3
in e ace. The anodizing o he unde lying
Nb compe es wi h he anodizing o he esidual Al ne wo k su ounding
he bo oms o he PAA cells. The Al esidues may emain a e
comple ing he anodizing s ep o may be ully oxidized – he beha io
depends upon he a io o he ionic esis i i ies o anodic alumina and
niobia.
Re-anodizing o a highe ol age (Fig. 5c,h) esul s in p opo ionally
elonga ing and hickening he niobium-oxide p o usions, u he illing
he po es and occupying mo e space wi hin he PAA cell walls. New
niobium oxide also keeps g owing benea h he PAA cells due o he O
2−
mig a ion. The nano ods’ bases expand and deepen bu do no me ge in
SE42
R
(Fig. 5c). Howe e , hey me ge in o a con inuous bo om-oxide
laye in SE12
R
(Fig. 5h).
The di e en manne s o he bo om-oxide g ow h in SE42
A
-SE42
R
and SE12
A
-SE12
R
pai s a ise om dissimila ionic esis i i ies o he wo
ypes o niobium-oxide nanop o usions. The highe he ionic esis i i y
o he anodic niobium oxide (close o ha o alumina), he mo e he
bo om oxide expands unde he po es. The di e ences in he niobium-
oxide esis i i ies a e mos ly ela ed o he a ia ions in chemical
composi ions and dissimila alumina-niobia mixing wi hin he PAA
ba ie laye and cell walls. The second p ac ically essen ial and closely
ela ed phenomenon is he p esence o absence o unoxidized aluminum
a e comple ing he anodizing and e-anodizing s ages. The ionic e-
sis i i ies o he SE12
A
and SE12
R
niobium-oxide nanop o usions a e
highe han hose o he SE42
A
and SE42
R
nanop o usions since only
small aces o esidual Al me al emain in SE12
A
, while SE12
R
is en i ely
Al- ee. Opposi ely, he ela i ely lowe esis i i y o he SE42
A
and
SE42
R
niobium-oxide nanop o usions esul s in mo e aluminum
emaining a ound he p o usions.
No ably, he ionic esis i i y o he Nb
2
O
5
nanos uc u es can be
con olled by adjus ing he anodizing p ocess pa ame e s and may be
conside ed o enginee ing niobium-oxide nanoa ays o pa icula
applica ions. I is possible o modula e he esidual Al ne wo k on he
sample su ace o me ge he nanos uc u es’ bases in o a con inuous
Nb
2
O
5
bo om laye . Fo applica ions o he ee-s anding Nb-me al-
suppo ed Nb
2
O
5
nanos uc u es wi hou any aces o Al me al, a NaOH
Table 2
Su ace chemical composi ion o he niobium-oxide nanos uc u es o med ia he PAA-assis ed anodizing/ e-anodizing in 1.5 M selenic acid.
Subs oichiome ic niobium pen oxide
a
NbO
2.5
⋅xNb
n+
O
n/2
Al:Nb a io Rela ed o Se
a .% o Se (Se +Al +Nb =100 a .%) % o O
2−
eplaced by Se species SeO
4
2−
: Se
2−
n x
SE42
A
4 0.034 65:35 5.3 3.0 39:61
SE42
R
4 0.057 61:39 3.2 1.8 51:49
SE42
R
a e 1s spu e ing 4 0.074 63:37 2.8 1.5 40:60
SE42
R
a e 2nd spu e ing 4 0.081 62:38 2.8 1.5 36:64
SE42
Rpd
4 0.52 95:5 4.3 2.9 97:3
SE12
A
2 0.18 81:19 3.9 2.5 38:62
SE12
R
3 0.024 47:53 4.9 2.6 51:49
compac niobium oxide n/a n/a 0:100 9.6 4.2 100:0
a
The ou e ma e ial o he nanos uc u es was exp essed as a mix u e o Al
2
O
3
wi h niobium oxides (s oichiome ic Nb
2
O
5
mixed wi h subs oichiome ic Nb
2
O
5
o
wi h a suboxide, in a e age NbO
2.5
⋅xNb
n+
O
n/2
), in which some o he O
2−
anions a e eplaced by SeO
4
2−
(selena e) and Se
2−
(selenide) anions.
K. Kamne e al.
Ce amics In e na ional 49 (2023) 34712–34725
34720
e chan may be used o dissol e bo h he PAA and he emaining Al.
Addi ionally, a e dissol ing he PAA and/o he emaining Al, he
unoxidized Nb me al su ounding he nano ods may be anodized and
ans o med in o a con inuous compac Nb
2
O
5
laye o a desi ed hick-
ness, as desc ibed elsewhe e [50].
3.5.2. Ionic anspo and oxide g ow h
Ano he essen ial ea u e ha should be conside ed is he inco po-
a ion o SeO
4
2−
(selena e) and Se
2−
(selenide) anions in he nanoa ays.
The SeO
4
2−
anions a e p esen in he anodizing elec oly e because o he
comple e dissocia ion o H
2
SeO
4
. These ions adso b on he anode su ace
and mig a e inwa d unde he high elec ic ield du ing he anodizing
and e-anodizing (wi h a slowe a e ela i e o O
2−
(OH
−
) anions). The
ollowing anode eac ions exp ess he niobium-oxide o ma ion and
selena e-ion inco po a ion:
2Nb
5+
+5O
2−
→ Nb
2
O
5
(2)
2Nb
5+
+5(1−y
1
)O
2−
+5y
1
SeO
4
2−
→ Nb
2
O
5(1−y1)
(SeO
4
)
5y1
(3)
whe e coe icien y
1
ep esen s he ac ion o O
2−
anions eplaced by
selena e anions in he niobium oxide.
The inco po a ion o selenide, o possibly hyd ogen selenide (HSe
−
)
anions [51,52], in he ou e nanos uc u es’ laye occu s a e such
anions o m in he selenic-acid elec oly e o he gi en concen a ion
and he co esponding pH due o he educ ion o selena e anions on he
ca hode ia a se ies o s ep eac ions:
SeO
4
2−
+3H
+
+2e
−
→ HSeO
3
−
+H
2
O [51,52,53] (4)
HSeO
3
−
+5H
+
+4e
−
→ Se
0
+3H
2
O [51,52,53] (5)
HSeO
3
−
+6H
+
+6e
−
→ HSe
−
+3H
2
O [51] (6)
Se
0
+H
+
+2e
−
→ HSe
−
[51,52] (7)
Reac ions (4–7) can be summa ized by he ollowing o e all ca hode
eac ion:
SeO
4
2−
+9H
+
+8e
−
→ HSe
−
+4H
2
O (8)
The educ ion o he nega i ely cha ged species a he ca hode is
enabled by he di usion o anions om he bulk elec oly e ac oss he
Helmhol z laye , as explained elsewhe e [54]. A ed powde , p esum-
ably elemen al Se, is obse ed on he ca hode du ing he
anodizing/ e-anodizing. Conside ing he 3-mm dis ance be ween he
elec odes in he anodizing se up, he o med selenide anions a e ex-
pec ed o be anspo ed om he ca hode o he anode by di usion
wi hin a ew minu es, which happens al eady du ing he g ow h o he
PAA o e laye . The ac ha selenide anions a e inco po a ed in he
PAA-assis ed niobium oxides (Fig. 4c, ,i,l) bu a e no p esen in he
compac niobium oxide (Fig. 4p) con i ms ha he selenide anions did
no di use owa d he compac niobium oxide be o e i s g ow h was
inished (wi hin ~2 min a e applying he pola iza ion). Howe e , since
he PAA-assis ed niobium-oxide nanoa ays only s a o g ow a e he
PAA o e laye is ully o med, a subs an ial amoun o selenide anions is
al eady p oduced a he ca hode. These selenide anions ha e enough
ime o di use owa d he anode and en e he p e o med PAA po es
well be o e he beginning o anodizing and, especially, e-anodizing he
niobium unde laye . Assuming ha HSe
−
anions d op hei p o ons a
he anodic-oxide su ace be o e he mig a ion, he ollowing anode e-
ac ion summa ises he inco po a ion o selenide anions in o he nano-
s uc u ed niobium oxide:
2Nb
5+
+5(1−y
2
)O
2−
+5y
2
Se
2−
→ Nb
2
O
5(1−y2)
Se
5y2
(9)
whe e coe icien y
2
ep esen s he ac ion o O
2−
anions eplaced by
selenide anions in he anodic niobium oxide. The sum o y
1
and y
2
in
eac ions (3) and (9) co esponds o he ac ion o O
2−
anions in he
oxide eplaced by all- ype Se species, simila o he alues summa ized
in Table 2 as he pe cen ages. Anode eac ions (3) and (9) can be sum-
ma ized by he ollowing o e all eac ion o he PAA-assis ed niobium-
oxide g ow h in selenic acid:
2Nb
5+
+5(1−y
1
−y
2
)O
2−
+5y
1
SeO
4
2−
+5y
2
Se
2−
→ Nb
2
O
5
(1−y1)
(SeO
4
)
5y1
Se
5y2
(10)
Conclusi ely, he expe imen al indings o his wo k sugges ha he
nanos uc u ed ce amics o med ia he PAA-assis ed selenic-acid
niobium anodizing ha e a dual (co e/shell) composi ion: he inne
Fig. 5. Schema ic g ow h model du ing he PAA-assis ed anodizing and e-anodizing o niobium in 1.5 M selenic acid o o ming (le panel) SE42
A
(42 V) and
SE42
R
(120 V) samples and ( igh panel) SE12
A
(12 V) and SE12
R
(80 V) samples: (a and ) he o ma ion o PAA laye (1s s ages in Fig. 1 jus be o e he ol age
begins o inc ease), (b and g) he g ow h o niobium-oxide nanos uc u es wi hin he PAA ba ie laye du ing he anodizing (2nd s ages in Fig. 1), and (c and h) he
de elopmen o niobium-oxide nano ods du ing he e-anodizing (3 d s ages in Fig. 1). The dashed whi e lines di e en ia e he ’co e/shell’ s uc u e o he p o-
usions. The pa allel- o-subs a e sec ion iews show compa a i ely he cell-po e- od ela ionship o (d) SE42
R
and (e) SE12
R
samples.
K. Kamne e al.