Fast response hydrogen microsensor based on semiconductor niobium-oxide nanostructures via smart anodizing of Al/Nb metal layers

P ocedia Enginee ing 87 ( 2014 ) 811 – 814 A ailable online a www.sciencedi ec .com 1877-7058 © 2014 Published by Else ie L d. This is an open access a icle unde he CC BY-NC-ND license (h p://c ea i ecommons.o g/licenses/by-nc-nd/3.0/). Pee - e iew unde esponsibili y o he scien i ic commi ee o Eu osenso s 2014 doi: 10.1016/j.p oeng.2014.11.674 ScienceDi ec EUROSENSORS 2014, he XXVIII edi ion o he con e ence se ies Fas esponse hyd ogen mic osenso based on semiconduc o niobium-oxide nanos uc u es ia sma anodizing o Al/Nb me al laye s R.M. Vázqueza, A. Mozale b, E. Llobe a, * aMINOS-EMaS, Uni e si y Ro i a i Vi gili, Ta agona, Spain bCen al Eu opean Ins i u e o Technology (CEITEC), B no Uni e si y o Technology, B no, Czech Republic Abs ac Nanos uc u ed niobium oxide semiconduc o is gaining inc easing a en ion as elec o-op ic and gas sensing ma e ial. Howe e , he p epa a ion o s able niobium oxide nano ilm wi h ep oducible mo phology and beha iou emains a challenge. He e we desc ibe a apid and well-con olled app oach o syn hesize a niobium oxide ilm wi h he columnlike nanos uc u ed mo phology ia anodic p ocessing o Al/Nb me al laye s spu e ed on o an oxide-coa ed Si wa e . The ilm is de eloped due o he g ow h o a nanopo ous anodic alumina laye ollowed by po e-di ec ed oxida ion o he Nb unde laye . The pos -anodizing ea men esul s in he con olled o ma ion o Nb2O5 c ys al phase, which causes he ans o ma ion om dielec ic o n- ype semiconduc o beha io o he ilm. A labo a o y gas senso ab ica ed by uni ing he anodizing app oach de eloped he e wi h s anda d mic omachining echnologies shows supe io cha ac e is ics o hyd ogen gas de ec ion, he esponse- eco e y ime being among bes e e epo ed. © 2014 The Au ho s. Published by Else ie L d. Pee - e iew unde esponsibili y o he scien i ic commi ee o Eu osenso s 2014. Keywo ds: niobium oxide, anodizing, po ous alumina, nanos uc u e, hyd ogen senso 1. In oduc ion Nanos uc u ed niobium oxide semiconduc o is o g owing in e es as ac i e laye o chemi esis i e gas sensing applica ions [1,2]. Recen epo s on he ad anced gas sensing p ope ies o nanopo ous ilms ia di ec anodiza ion *Co esponding au ho . Tel.: +34-977558502 E-mail add ess: edua d.llobe @u .ca © 2014 Published by Else ie L d. This is an open access a icle unde he CC BY-NC-ND license (h p://c ea i ecommons.o g/licenses/by-nc-nd/3.0/). Pee - e iew unde esponsibili y o he scien i ic commi ee o Eu osenso s 2014 812 R.M. Vázquez e al. / P ocedia Enginee ing 87 ( 2014 ) 811 – 814 and he mal oxida ion o Nb oils [2] inspi ed us o de elop an al e na i e elec ochemis y-based echnology o well-con olled niobium oxide nanos uc u es wi h ailo ed mo phologies and ad anced unc ional p ope ies in pu sui o ab ica ing compe i i e niobium oxide ac i e laye s o chemi esis i e sensing applica ions. 2. Expe imen al The ilm o ma ion begins wi h spu e -deposi ion o an Al/Nb (aluminium-on-niobium) laye s on o an oxidized Si wa e . A e he Al laye is con e ed in o nanopo ous alumina by anodizing in an aqueous acid solu ion (Fig. 1a) [3], an a ay o niobium oxide nanosized p o usions o ms o he Nb laye unde he po es (Fig. 1b). Then he sample is eanodized o a highe po en ial [4] so as o achie e po e-di ec ed g ow h o long aspec a io niobium oxide nanocolumns (Fig. 1c). A e selec i e dissolu ion o he alumina laye (Fig. 1d), he columnlike nano ilm is annealed a 650oC o achie e c ys alliza ion and phase ansi ion e ec s in he nanos uc u ed oxide. The o ma ion-mo phology-s uc u e ela ionship o he ilm g own was examined by scanning elec on mic oscopy (SEM) and X- ay di ac ion (XRD) analysis. A labo a o y scale mic osenso ab ica ed o gas sensing es s comp ised pai s o 80-μm spaced in e digi a ed P elec odes placed o e he SiO2/Si-suppo ed niobium oxide nanocolumn a ay p epa ed by magne on spu e -deposi ion and li -o pho oli hog aphy. The sample was cu in se e al pieces ha we e hen a ached o a minia u ized ho pla e allowing he ope a ing empe a u es up o 350°C (Fig. 1e) and assembled o a TO-8 package (Fig. 2). Gas sensing es s we e pe o med wi h a ully au oma ed sys em by measu ing he chemi esis i e esponse o he ilms o hyd ogen gas o a iable concen a ions. Fig. 1: Schema ic o o ming a niobium-oxide-based mic osenso : (a) o ma ion o po ous alumina, (b) anodizing he Nb unde laye , (c) eanodizing he Nb unde laye , (d) dissolu ion o he alumina o e laye , (e) p epa a ion o a es mic osenso and ( ) esis ance- empe a u e esponse o he es mic osenso . 813 R.M. Vázquez e al. / P ocedia Enginee ing 87 ( 2014 ) 811 – 814 Fig. 2: Digi al pho og aphs o samples wi h he anodic niobium oxide sensing laye s moun ed on a s anda d TO- 8 package (le ) and he TO-8 packaged senso se up in an expe imen al s ainless s eel chambe ( he lid is emo ed o pic u e aking) ins alled on a p in ed ci cui boa d and used o gas sensing es s ( igh ). 3. Resul s Typical SEM iews o he ilm su ace mo phology a e shown in Fig. 3. The ilm is composed o an a ay o long aspec a io sel -o de ed nanocolumns a ached o an anodic oxide compac laye . The leng h and diame e o nanocolumns may a y, i necessa y, in he ange o 450-600 nm and 30-50 nm espec i ely. F om he XRD analysis, he ini ially amo phous niobium oxide ilm c ys allized o o ho hombic Nb2O5 phase (space g oup Pbam) due o he annealing in ai , wi h he ans o ma ion o n- ype semiconduc o beha io (Fig. 3). The gas sensing p ope ies we e ca ied ou by measu ing he esis ance be ween he 80 μm in e digi a ed elec odes o med o e he sensing ilm in he p esence o di e en concen a ions o hyd ogen gas using d y ai as a balance gas. F om he measu emen s, senso esponse was calcula ed as he a io o he senso esis ance in ai (Rai ) o e he esis ance in he p esence o gas (Rgas). The senso esponse o 1000 ppm H2 a ope a ing empe a u e o 250 ºC and he esponse-concen a ion beha iou a e shown in Fig. 4. Fig. 3: SEM images o a niobium oxide gas sensing ilm (a, b) oge he wi h in e digi a ed P elec odes and (c, d) be ween he elec odes. The ilm was syn hesized ia anodizing o spu e -deposi ed Al/Nb me al laye s a 45 V in 0.9 M H4C3O4 aqueous solu ion ollowed by eanodizing o 300 V. 814 R.M. Vázquez e al. / P ocedia Enginee ing 87 ( 2014 ) 811 – 814 Fig. 4: a) Senso esponse o 1000 ppm H2 a ope a ing empe a u e o 250oC, de ined as he a io o he senso esis ance in a d y ai a mosphe e (Rai ) o he senso esis ance in he p esence o H2 (Rgas); b) Senso esis ance as a unc ion o hyd ogen concen a ion (100, 500, 1000 ppm) measu ed a ope a ing empe a u e o 250°C. The as senso esponse (< 1 min), wi h a apid eco e y and a s able baseline, cha ac e izes he niobium oxide based senso as one o he as es among nanos uc u ed me al-oxide H2-sensing ilms [1,2]. Fu he , he senso esis ance appea ed o be well-dependen on he hyd ogen concen a ion, wi h he de ec ion limi no wo se han 100 ppm; no measu emen s ha e ye been pe o med in he lowe concen a ion ange. The achie emen is due o he pe iodic 2-D nanosized mo phology o he niobium oxide ilm, he nanoc ys alline s uc u e and he g aded chemical composi ion, complemen ed by a subs an ially enla ged su ace- o- olume a io o he ilm, all con ibu ing o inc easing he a e o adso p ion and di usion p ocesses and enhancing he edox eac ions be ween he oxide and gaseous species and making he whole ilm olume in ol ed in in e ac ion wi h hyd ogen. 4. Conclusions In his wo k, a c ys alline sel -o de ed niobium oxide nanos uc u ed ilm has been syn hesized ia he sma anodizing o spu e -deposi ed Al/Nb me al laye s combined wi h he high empe a u e annealing. A es mic osenso employing he ilm showed as esponse and eco e y o he ini ial baseline in a hyd ogen a mosphe e. The senso is being es ed in mo e gases o examine he abili y o he ilm o disc imina e he di e en gaseous species, which a e p esen , o ins ance, in di ec -e hanol uel cells. Acknowledgemen s Pa ial suppo s o his esea ch h ough MICINN g an No. TEC2009-07107 and BES-2010-036183, MINECO g an No. TEC2012-32420, he EU G an s COST MP-0901 “NanoTP”, COS TD-1105 “EuNe Ai ” and he Czech Science Founda ion (GAČR) g an No. 14-29531S a e g a e ully acknowledged. E. L. is suppo ed by he ICREA Academia awa d. Re e ences [1] Z. Wang, Y. Hu, W. Wang, X. Zhang, B. Wang, H. Tien, Y. Wang, J. Guan and H. Gu, Fas and highly-sensi i e hyd ogen sensing o Nb2O5 nanowi es a oom empe a u e, In . J. Hyd ogen Ene gy, 37 (2012), pp. 4526-4532. [2] R. A. Rani, A. S. Zool aka , J. Z. Ou, M. R. Field, M. Aus in and K. Kalan a -Zadeh, Nanopo ous Nb2O5 hyd ogen gas senso , Sens. Ac ua o s B, 176 (2013), pp. 149-156. [3] V. Su gano , P. Mo gen, J. G. Nielsen, G. Go okh and A. Mozale , S udy o he ini ial s age o aluminium anodiza ion in malonic acid solu ion, Elec ochim. Ac a, 32 (1987) pp. 125-1127. [4] A. Mozale , G. Go okh, M. Sakai i and H. 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