Se-doped Nb2O5-Al2O3 composite-ceramic nanoarrays via the anodizing of Al/Nb bilayer in selenic acid

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.