Micromachined Gas Sensors Based on Au-functionalized SnO2 Nanorods Directly Integrated without Catalyst Seeds via AA-CVD

P ocedia Enginee ing 168 ( 2016 ) 1078 – 1081 1877-7058 © 2016 The Au ho s. 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/4.0/). Pee - e iew unde esponsibili y o he o ganizing commi ee o he 30 h Eu osenso s Con e ence doi: 10.1016/j.p oeng.2016.11.344 ScienceDi ec A ailable online a www.sciencedi ec .com 30 h Eu osenso s Con e ence, EUROSENSORS 2016 Mic omachined gas senso s based on Au- unc ionalized SnO 2 nano ods di ec ly in eg a ed wi hou ca alys seeds ia AA-CVD S. Vallejos, a* S. Selina,b F. E. Annanouch,c, e I. G àcia,d E. Llobe ,c C. Blackmanb a SIX Resea ch Cen e, B no Uni e si y o Technology, B no, Czech Republic b Depa men o Chemis y, Uni e si y College London, London, UK c MINOS-EMaS, Depa amen d’Enginye ia Elec ònica, Uni e si a Ro i a i Vi gili, Ta agona, Spain d Ins i u o de Mic oelec ónica de Ba celona (IMB-CNM, CSIC), Ba celona, Spain e Aix Ma seille Uni e si é, Uni e si é de Toulon, IM2NP UMR 7334, Ma seille, F ance Abs ac Tin oxide nano ods unc ionalized wi h Au nanopa icles a e apou syn hesised a ela i ely lowe empe a u es han p e iously epo ed and wi hou he need o ca alys seeds using co-deposi ion me hod ia ae osol-assis ed chemical apou deposi ion. These unc ionalized s uc u es o med di ec ly, in a single-s ep p ocess, on silicon mic omachined pla o ms a e es ed owa d H 2 , showing 12- old g ea e esponse, 6- old as e esponse ime and be e selec i i y o CO compa ed o a simila non- unc ionalized sys em. Resul s show he signi icance o hese me hod o o m highly gas sensi i e nanos uc u es compa ible wi h he complemen a y elec onic o he ab ica ion o gas mic osenso de ices. © 2016 The Au ho s. Published by Else ie L d. Pee - e iew unde esponsibili y o he o ganizing commi ee o he 30 h Eu osenso s Con e ence. Keywo ds: Nanos uc u es; in oxide; unc ionaliza ion; AACVD; gas senso s 1. In oduc ion Tin oxide is an in insic n- ype wide-bandgap semiconduc ing me al oxide wi h applica ions in anspa en conduc ing elec odes, sola cells and gas senso s.[1, 2] In pa icula , in oxide is used in mos cu en comme cial esis i e gas senso s and is he mos s udied ma e ial in he gas sensing li e a u e, wi h demons a ed sensi i i y o * Co esponding au ho . Tel.: +420 541 146 153; ax: +420 541 146 2198. E-mail add ess: a gas@ eec. u b .cz © 2016 The Au ho s. 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/4.0/). Pee - e iew unde esponsibili y o he o ganizing commi ee o he 30 h Eu osenso s Con e ence 1079 S. Vallejos e al. / P ocedia Enginee ing 168 ( 2016 ) 1078 – 1081 ca bon monoxide, hyd ogen, e hanol, and ni ogen dioxide, amongs o he s.[2-4] Whils in oxide nano ods (NRs) ha e been syn he ized (o en wi h ca alys seeds) using a ious ou es, including chemical apou deposi ion (CVD), he o ma ion o hese s uc u es ia ae osol-assis ed (AA) CVD wi hou ca alys seeds, i.e., ia apo -solid (VS) mechanism, and hei in-si u unc ionaliza ion wi h Au nanopa icles (NPs) ha e no been de eloped be o e. AA-CVD wo ks a a mosphe ic p essu e and elies on a solu ion-based deli e y app oach, p o iding ad an ages o e adi ional CVD as i allows o a wide ange o p ecu so s o be u ilised. I also allows o he unc ionaliza ion o SMOx nanos uc u es wi h me al NPs in a single p ocessing s ep ia co-deposi ion, as demons a ed p e iously o inco po a ion o gold o pla inum NPs seg ega ed a he su ace o ungs en oxide nanos uc u es.[5] He e we epo he AA-CVD o Au- unc ionalised and non- unc ionalized in oxide NRs (Au@SnO2 and SnO2, espec i ely) di ec ly on silicon mic omachined pla o ms (ȝMP) o he ab ica ion o chemo esis i e gas senso s. 2. Ma e ials and Me hods Au@SnO2 and SnO2NRs we e deposi ed di ec ly on ȝMP a 620 °C ia AA-CVD o a mix u e o in (IV) chlo ide pen ahyd a e (30 mg, SnCl4.5H2O, Sigma-Ald ich, 98%) and e achlo oau ic acid ihyd a e (4.2 mg, HAuCl4·3H2O, Sigma-Ald ich, 99.9%) dissol ed in ace one (15 ml, Sigma-Ald ich, 99.6%) o only in (IV) chlo ide pen ahyd a e (30 mg, SnCl4.5H2O, Sigma-Ald ich, 98%) dissol ed in ace one (15 ml, Sigma-Ald ich, 99.6%), espec i ely, using he me hod and sys em epo ed p e iously.[5] The ȝMP consis ed o an a ay o ou SiO2/Si3N4/SiO2 memb ane, each o hem wi h isola ed polysilicon hea e s and pla inum elec odes (gap: 50 ȝm, hick: 0.2 ȝm).[6] The sensing ilms we e deposi ed on he ȝMP using a shadow mask in o de o p o ec he con ac s and subsequen ly bonded in a TO-8 package (inse in Fig. 1). The mo phology o he samples was examined using SEM (Tescan FE Mi a II LMU) and TEM (JEOL JEM-100CX II, 100 kV). The s uc u e using XRD (Rigaku Sma lab 9kW) and he chemical composi ion using WDX (Philips, XL30ESEM). Gas senso s we e es ed in a con inuous low (200 sccm) es chambe (280 cm3) comp ised o a mass low sys em (B onkho s hi- ech 7.03.241) and calib a ed cylinde s o hyd ogen (H2, P axai , 1000 ppm), ca bon monoxide (CO, P axai , 1000 ppm) and syn he ic ai (Ca bu os Me álicos, 99.99%) as desc ibed p e iously.[6] The senso esponse was de ined as R= Ra/Rgas, whe e Ra is he senso esis ance in ai and Rgas he senso esis ance a e 10 min o he analy e exposu e. The esponse ime ( R) was de ined as he ime equi ed o he senso o each 90% o he senso esponse, and he eco e y ime ( ec) as he ime equi ed o each 10% o he ini ial baseline esis ance a e he analy e was pu ged. Fig. 1. XRD o a mic osenso based on Au@SnO 2 NRs g own ia AA-CVD. Di ac ion peaks a e indexed o a e agonal phase (P42/mnm, ICCD ca d no. 41-1445) o in oxide and ace-cen e ed cubic Au (Fm3m, ICCD ca d no. 4-0784). Pla inum (P ) and bo on ni ide (BN) di ac ion peaks coming om he mic osenso pla o m a e also iden i ied. 1080 S. Vallejos e al. / P ocedia Enginee ing 168 ( 2016 ) 1078 – 1081 3. Resul s and discussion XRD o he ilms composed o Au@SnO2 and SnO2NRs indica ed he p esence o e agonal SnO2 (P42mnm space g oup, a=4.7382 Å, c=3.1871 Å; ICCD ca d no. 411-1445) wi h a week di ac ion a 44.3 deg ees in he Au@SnO2 ilms co esponding o he (200) e lec ions o ace-cen e ed cubic Au (Fm3m space g oup, a=4.07860 Å; ICCD ca d no. 04-0784) (Fig. 1). XPS o he ilms indica ed a (0.9 a .%) 3.7 w .% Au in he ilms wi h he cha ac e is ics o Au 4 co e le el spec a being in ag eemen wi h ha epo ed o gold me al,[5] which sugges s he gold NPs inco po a ed a he su ace o he in oxide NRs a e in he me allic s a e. SEM o he ȝMP showed ilms composed o non-aligned NRs, g own uni o mly on he elec odes (Fig. 2a and 2b). Simila ly, TEM con i med he o ma ion o p ism-like NRs e mina ed in a py amidal cap and he inco po a ion o Au NPs (׽35 nm) wi h sphe ical mo phologies a he NR su ace (Fig. 2c and 2d), p o ing consis ency wi h ou p e ious obse a ions o he co-deposi ion o ungs en oxide and gold ia AA-CVD.[5] The non- unc ionalized NRs g own on he ȝMP ia AA-CVD showed simila mo phological, chemical and c ys al s uc u e o SnO2. Fig. 2. Typical low (a) and high (b) magni ica ions SEM imaging o he SnO 2 and Au@SnO 2 NRs g own ia AACVD on he ȝMP. TEM o a single SnO 2 (c) and Au@SnO 2 (d) NR. Tes o he samples owa ds H2 and CO egis e ed s able signal and e y low d i o elec ical esis ance o e he es ing pe iod, wi h he Au@SnO2senso s showing enhanced sensing cha ac e is ics compa ed o SnO2senso s, which included highe and as e esponse and lowe -c oss esponse (Table 1 and Fig. 3). A compa ison o hese esul s wi h hose eco ded o simila sys ems syn hesised ia sol-gel in he li e a u e[7] e ealed nea ly 7 imes highe alues o ou Au@SnO2NRs, sugges ing he AA-CVD me hod p esen ed he e allows o no only a di ec in eg a ion wi h ȝMP, bu also o an e ec i e unc ionaliza ion o SnO2. Table 1. Senso esponse and c oss- esponse (ǻR) owa ds H 2 and CO o he SnO 2 and Au@SnO 2 senso s ope a ing a 290 °C. Analy es & c oss- esponse Response, R a /R gas SnO 2 Au@SnO 2 250 ppm H 2 3.3 35.4 500 ppm H 2 3.8 41.1 250 ppm CO 1.1 1.6 500 ppm CO 1.2 1.9 ǻR500 ppm= (R H2 -R CO ) 2.6 39.2 1081 S. Vallejos e al. / P ocedia Enginee ing 168 ( 2016 ) 1078 – 1081 Fig. 3. Maximum senso esponse eco ded wi h he SnO 2 and Au@SnO 2 NRs owa ds 500 ppm o ei he H 2 o CO a 290 °C o 320 °C, espec i ely. 4. Conclusion Mic omachined chemo esis i e gas senso s based on SnO2 and Au@SnO2 NRs we e de eloped using AA-CVD o SnCl4·5H2O a 620 °C, a much lowe onse empe a u e compa ed o o he CVD me hods based on a VS mechanism, which ypically equi es empe a u es exceeding 850 °C. The gas mic osenso s we e alida ed owa ds H2 and CO and show sensing p ope ies ha a e in ag eemen wi h he li e a u e, wi h no able enhancemen o sensing p ope ies o Au@SnO2 NRs which showed 12- old highe esponse wi h 6- old as e esponse and imp o ed selec i i y o H2 compa ed o he gas senso s based on in insic SnO2 NRs. Acknowledgemen s S.V. is suppo ed by he SoMoP o II P og amme, co inanced by he Eu opean Union and he Sou h-Mo a ian Region, ia G an 4SGA8678. E.L. is suppo ed by he Ca alan Ins i u ion o Resea ch and Ad anced S udies ia he ICREA Academia Awa d. This wo k was unded in pa by MINECO unde g an no TEC2013-48147 and TEC2015- 71663-R, and was ca ied ou using he in as uc u es o he SIX Resea ch Cen e and he co e acili ies o CEITEC - Cen al Eu opean Ins i u e o Technology unde CEITEC - open access p ojec , ID numbe LM2011020, by he Minis y o Educa ion, You h and Spo s o he Czech Republic. Re e ences [1] M. Ba zill, U. Diebold, The su ace and ma e ials science o in oxide, P og. Su . Sci. 79 (2005) 47-154. [2] J. Pan, H. Shen, S. Ma hu , One-Dimensional SnO 2 Nanos uc u es: Syn hesis and Applica ions, J. Nano hecnology 2012 (2012) 12. [3] J. F. McAlee , P. T. Moseley, J. O. W. No is, D. E. Williams, B. C. To ield, Tin Dioxide Gas Senso s. Pa 2.-The Role o Su ace Addi i es, J. Chem. Soc., Fa aday T ans. 1 84 (1988) 441-457. [4] S. Vallejos, F. Di Maggio, T. Shujah, C. Blackman, Chemical apou deposi ion o gas sensi i e me al oxides, Chemosenso s 4 (2016) 4. [5] S. Vallejos, P. Umek, T. S oyche a, F. Annanouch, E. Llobe , X. Co eig, P. De Ma co, C. Bi encou , C. Blackman, Single-s ep deposi ion o Au- and P -nanopa icle- unc ionalized ungs en oxide nanoneedles syn hesized ia ae osol-assis ed CVD, and used o ab ica ion o selec i e gas mic osenso a ays, Ad . Func . Ma e . 23 (2013) 1313-1322. [6] S. Vallejos, V. Kha ko, J. Calde e , I. G acia, C. Canè, E. Llobe , X. Co eig, Mic o-machined WO 3 -based senso s selec i e o oxidizing gases, Sens. Ac ua o s B 132 (2008) 209-215. [7] X.-T. Yin, X.-M. Guo, Sensi i i y and selec i i y o (Au, P , Pd)-loaded and (In, Fe)-doped SnO 2 senso s o H 2 and CO de ec ion, J Ma e Sci: Ma e Elec on 25 (2014) 4960-4966.