Sub PPM detection of NO2 using strontium doped bismuth ferrite nanostructures

Ci a ion: Dmon e, D.J.; Bha dwaj, A.; Wilhelm, M.; Fische , T.; Kuˇ i ka, I.; Ma hu , S. Sub PPM De ec ion o NO2Using S on ium Doped Bismu h Fe i e Nanos uc u es. Mic omachines 2023,14, 644. h ps://doi.o g/10.3390/ mi14030644 Academic Edi o : Dimi is Tsoukalas Recei ed: 27 Feb ua y 2023 Re ised: 6 Ma ch 2023 Accep ed: 9 Ma ch 2023 Published: 12 Ma ch 2023 Copy igh : © 2023 by he au ho s. Licensee MDPI, Basel, Swi ze land. This a icle is an open access a icle dis ibu ed unde he e ms and condi ions o he C ea i e Commons A ibu ion (CC BY) license (h ps:// c ea i ecommons.o g/licenses/by/ 4.0/). mic omachines A icle Sub PPM De ec ion o NO2Using S on ium Doped Bismu h Fe i e Nanos uc u es Da id John Dmon e 1, Aman Bha dwaj 2, Michael Wilhelm 2, Thomas Fische 2, I o Kuˇ i ka 1,* and Sanjay Ma hu 2,* 1 Cen e o Polyme Sys ems, Tomas Ba a Uni e si y in Zlin, T . Tomase Ba i 5678, 760 01 Zlín, Czech Republic; [email p o ec ed] 2Ins i u e o Ino ganic Chemis y, Uni e si y o Cologne, G eins . 6, 50939 Cologne, Ge many *Co espondence: [email p o ec ed] (I.K.); sanjay[email p o ec ed] (S.M.); Tel.: +420-576038049 (I.K.); +49-221-470-4107 (S.M.) Abs ac : The p esen wo k in es iga es he NO 2 sensing p ope ies o accep o -doped e i e pe - o ski e nanos uc u es. The S -doped BiFeO 3 nanos uc u es we e syn hesized by a sal p ecu so - based modi ied pechini me hod and cha ac e ized by X- ay di ac ion (XRD), scanning elec on mic oscopy (SEM), and X- ay pho oelec on spec oscopy (XPS). The syn hesized ma e ials we e d op coa ed o ab ica e chemo esis i e gas senso s, deli e ing a maximum sensi i i y o 5.2 owa ds 2 ppm NO 2 a 260 ◦ C. The eco ded alues o esponse and eco e y ime a e 95 s and 280 s, e- spec i ely. The senso based on Bi 0.8 S 0.2 FeO 3–δ (BSFO) ha was ope a ed was shown o ha e a LOD (limi o de ec ion) as low as 200 ppb. The senso p o ed o be p omising o epea abili y and selec i i y measu emen s, indica ing ha he S doping Bismu h e i e could be a po en ially compe i i e ma e ial o sensing applica ions. A ele an gas-sensing mechanism is also p oposed based on he su ace adso p ion and eac ion beha io o he ma e ial. Keywo ds: gas sensing; bismu h e i e; ppb 1. In oduc ion Ni ogen dioxide (NO 2 ), one o he mos e iden ai pollu an s, ends o ha e a e y pungen , bi ing scen . This well-known ai pollu an a ec s lung unc ion, inc easing he isk o espi a o y in lamma ions [ 1 ]. The Wo ld Heal h O ganiza ion (WHO) has ecen ly emphasized a mo e s ingen annual mean limi o ambien NO 2 , educing he exposu e limi om 40 µ g/m 3 o 10 µ g/m 3 [ 2 ]. I is shown no jus o con ibu e as hma ic symp oms bu is connec ed o signi ican mo bidi y in as hma pa ien s [3]. I s p esence in he a mosphe e is key as a p ecu so in he o ma ion o oposphe ic ozone, a mosphe ic ae osols, and acid ain [ 4 ]. In sec o s ha use indus ial plan s, en i onmen al moni o ing, and ai quali y assu ance o clinical p ac ices and heal hca e, accu a e oxic gas de ec ion a low concen a ions is essen ial o he g ow h in hese ields. E en lowe concen a ions o NO X a pa s pe billion (ppb) a e equi ed, as hey can be used o de ec lung issue in ec ions [ 5 ]. Many ypes o senso s exis , such as ampe ome ic and elec ochemical, bu hei cu bed LOD o highe ope a ing empe a u e is shown o be limi ing o ex emely sensi i e applica ions [ 6 , 7 ]. One o he many d i ing o ces, apa om he many inhe en ad an ages, a e me al oxide semiconduc o -based senso s, which ha e been he mos esea ched ype o gas sensing among all he o he ypes [ 8 ]. The wide ange o me al oxides wi h di e se mo phologies, including NiO, In 2 O 3 , WO 3 , ZnO, and SnO 2 , ha e been explo ed o gas-sensing applica ions [ 9 – 13 ]. ZnO is he mos widely used sensi i e ma e ial o he de ec ion o NO 2 , hanks o i s wide band gap (~3.37 eV), non- oxici y, obus chemical, and elec ical s abili y [ 14 ]. Howe e , a ious o he nanoma e ials such as spinel me al oxides a e being ac i ely esea ched as sensing ma e ials. Mic omachines 2023,14, 644. h ps://doi.o g/10.3390/mi14030644 h ps://www.mdpi.com/jou nal/mic omachines Mic omachines 2023,14, 644 2 o 14 In gene al, spinel s uc u es a e well- esea ched in he ield o gas s o age, sepa a ion, and ba e y ca alysis. Thei use has been e iewed in li e a u e and hei special s uc u e has spa ked enough in e es in he gas-sensing ield [ 15 ]. Pe o ski e oxides ha e become pa icula ly a ac i e o se e al applica ions, such as pho oca alys s [ 16 , 17 ]. In he high acancy egion, oxygen can be di used in o he oc ahed a [BO 6 ], which co ela es wi h hole densi y, inc easing conduc i e pa hways. They can p o ide mic os uc u al and mo - phological s abili y, unable wi h a a ie y o dopan s due o hei lexibili y in subs i u ion o A and/o B posi ion o u he imp o e hei exquisi e elec onic s uc u e and elec- on mobili y [ 18 , 19 ]. Doping enables gas senso s owa ds NO 2 based on such pe o ski e s uc u e ha e been a new di ec ion o esea ch. Yoshi e u e al. ha e used a ime allic oxides SmFe 1–x Co x O 3 (x = 0 − 1.0) p epa ed by py olysis was used in a conduc ome ic senso [ 20 ]. Ano he in e es ing esea ch di ec ion was he p epa a ion o a highly selec i e o ganic–ino ganic he e ojunc ion, which used a halide pe o ski e and InGaZnO o de ec low concen a ions o NO 2 .by Mani e al. [ 21 ]. A mo e sus ainable app oach was aken by Juan e al. o in es iga e he gas-sensing mechanism o wo lead ee pe o ski es Cs 3 CuB 5 and Cs2AgBiB 6suppo ed on G aphene o ul asensi i e de ec ion o 500 ppb NO2[22]. In his e iew by Rahul e al., hey men ioned bismu h e i e-based pe o ski e gas senso s showing excellen s abili y o o e 1 yea , al hough he esponse alue and ope a - ing empe a u e could be imp o ed. They also men ion ha he e a e mo e publica ions employing his ma e ial, which may be because hey can now ope a e a lowe empe a u es and may soon become comme cially signi ican [ 23 ]. The e o e, i is essen ial o esea ch his ma e ial. On aking he elec on s uc u e o su ace Fe ions (i.e., he ela i ely high numbe o coo dina ely unsa u a ed d-o bi als due o he lack o oxygen bonding) in o conside a ion, bismu h e i e nanopa icles a e expec ed o ha e excellen gas sensi i i y because he dangling bonds can p o ide mo e geome ically and elec ically sui able lo- ca ions o molecula chemiso p ion in compa ison o a simple bime allic oxide, which is e y essen ial o enhancing i s gas-sensing p ope y. This can be inc eased much u he by aising he g ain su ace- o- olume a io, and doping will ha e signi ican impac s on he sensing p ope y [ 24 ]. Simila esea ch was published by Xu H, bu wi h a low doping pe cen age showing he ma e ial o be sensi i e o e hanol and ace one gas [ 25 ]. I is also well-es ablished ha he modi ica ions induced in he p ope ies o S -doped BiFeO 3 change wi h an inc ease in he dopan [ 26 ]. Hussain e al. showed ha bismu h e i e changes i s phase om hombohed al (R3c) o pseudo-cubic (Pm-3m) wi h a S doping amoun o 25%. Wi h S doping, oxygen acancies a e gene a ed in he c ys al la ice which changes he elec ical p ope ies o he bismu h e i e, possibly due o he change o Fe coo dina ion om oc ahed al o e ahed al [ 27 ]. Bismu h e i e, in i s p is ine o m, is shown o sense NO 2 and, upon u he doping wi h palladium, inc eases he po en ial ba ie on he su ace. This esul s in he inc emen o he esis ance o he senso ma e ial, he eby imp o ing esponse and eco e y imes, as shown by Shi aji e al. [ 28 ]. Ano he esea ch using a simila pe o ski e BaTiO 3 shows ha doping wi h S on ium also inc eases elec on densi y and he eby induces s ong pola iza ion on he su ace. I is shown o acili a e NO 2 and NH 3 eac ing wi h posi i ely cha ged ( δ +) O and H a oms on he su ace. No e ha he esponse owa ds he o me is wo imes highe han he la e [ 29 ]. The e o e, as explained and poin ed ou om he li e a u e e iew and p e ious s udies abo e, he inc ease in S on ium doping in BiFeO 3 would be e y sui able o he applica ion o gas sensing, as he inc eased dis o ions and newly c ea ed oxygen acancies can be e y i al o sensing oxidizing gases, such as NO2in low concen a ions. In his s udy, based on esea ch [ 30 , 31 ], we ha e in es iga ed he gas-sensing p ope ies o BiFeO 3 (BFO) and S doped BiFeO 3 (BSFO) owa ds sensi i i y and selec i i y o NO 2 . The p epa ed ma e ials we e cha ac e ized, hen u he used in senso s. These senso s we e de eloped using a hea e subs a e ollowing a s anda d p ocedu e, a e which i was subjec ed o p econdi ioning. The ocus o his s udy is he ma e ials ha we e p epa ed o be used as senso s in gas-sensing measu emen s o selec i i y, sensi i i y, and epea abili y o e y low concen a ions. Mic omachines 2023,14, 644 3 o 14 2. Ma e ials and Me hods 2.1. P epa a ion o Bi1−xS xFeO3(x = 0, 0.20) Nanopowde Bi 1−x S x FeO 3 (x = 0, 0.20) was syn hesized by a modi ied pechini me hod [ 32 ]. Bi(NO 3 ) 3 .5H 2 O (The mo Fishe Scien i ic Inc., Uni ed S a es) was dissol ed in dilu ed ni ic acid. Fe(NO 3 ) 3 .9H 2 O and S (NO 3 ) 2 we e g adually added o he p e iously de- sc ibed solu ion when he bismu h ni a e was comple ely dissol ed. Subsequen ly, ci ic acid and hen a polyme izing agen , e hylene glycol, was added in a mola a io o 1:1:2. The abo e solu ion becomes a clea anspa en sol. This solu ion was d ied a 80 ◦ C o 24 h o emo e mos o he wa e con en , esul ing in he o ma ion o a gel. The gel is he esul o anses e i ica ion be ween he me al–ci a e complex and he polyme agen , which was hen hea - ea ed a 700 ◦ C o 4 h in ai and cooled down o oom empe a u e. This way, he BFO and BSFO nanoma e ials we e success ully syn hesized. 2.2. Cha ac e iza ion The powde sample phase was s udied by powde X- ay di ac ion (XRD). I was ca ied ou using a Cu-K α adia ion as an elec on sou ce on a Rigaku D/max ULTIMA III di ac ome e (Japan). The su ace mo phology and pa icle shape we e in es iga ed by ield emission scanning elec on mic oscope (FE-SEM) and elemen al analysis was done by ene gy dispe si e X- ay spec oscopy (EDX) using he scanning elec on mic oscope No a NanoSEM450 (FEI company). Addi ionally, he su ace composi ions and hei co esponding elemen alences we e cha ac e ized by X- ay pho oelec on spec um (XPS) a a p essu e in he 10 −9 mba ange, using an ESCA M-p obe Spec ome e om Su ace Science Ins umen s, equipped wi h a monoch oma ic Al K α exci a ion sou ce ( λ= 8.33 Å ). The su ey spec a we e measu ed wi h 0.5 eV esolu ion, and he s ep size o high- esolu ion spec a was se o 0.05 eV s ep size. The spec a we e cha ge-co ec ed and binding ene gies (BE) we e assigned using ad en i ious ca bon C 1s line posi ion. The co ec ion (BE es ima ion) p ocess has an associa ed e o o a leas ± 0.1 eV up o 0.2 ±eV o insula ing samples [ 33 ]. High- esolu ion spec a decon olu ed peaks we e i ed wi h GL(30) line shapes and Shi ley- ype backg ounds. U ilizing CasaXPS so wa e (Casa So wa e), spec al adjus men s, and peak decon olu ion we e ca ied ou . 2.3. Senso Fab ica ion The powde sample was made in o a 10-weigh % solu ion wi h α - e pinol. This solu ion was u he dispe sed om a mic opipe e, and 10 µ L was d opped on he p e ab- ica ed senso subs a e 4 pin-TO39 om UST Umwel senso Technik GmbH. This senso was hen allowed o d y a 80 ◦ C o 18 h o achie e a uni o mly smoo h su ace laye . This simple cons uc ion allowed us o he es he limi o he ma e ial’s sensi i i y a he han o he ac o s such as hickness, e c. The senso was hen sin e ed a 600 ◦ C o 2 h be o e s a ing measu emen s o gas sensing. Th ee senso s o each ma e ial we e p epa ed o u he measu emen s. 2.4. Gas-Sensing Sys em Gas-sensing measu emen s we e in es iga ed in a cus om se up using an ai igh chambe o abou 250 mL wi h p o ision o gas injec ion and exhaus . This schema ic is shown in Figu e 1, and he desi ed concen a ion (in ppb and ppm) was achie ed by mixing he analy e gas low wi h syn he ic ai gas low using wo mass low con olle s (Ae a FC- 7700C) simul aneously. To ule ou any su ace in e ac ions wi h mois u e, he backg ound en i onmen was p oduced using a d y ai gas cylinde (Ai P oduc s and Chemicals, Inc., ce i ied o 0% humidi y). The ce i ied gas cylinde s o a ge gases (NH 3 , NO 2 , CO, CH 4 , and C 2 H 6 O) we e used o dilu ion in he measu emen s. The esis ance in exposu e o ai and analy e mixes was con inuously measu ed om he senso using a Kei hley K2700 da a acquisi ion sys em. A calib a ion cu e be ween ol age and empe a u e was p epa ed o accu a e hea ing. The powe sou ce me e used was model ES-PS 3065-03B om Elek o Au oma ik, and i was used o powe he pla inum hea e in senso subs a e TO-39. In Mic omachines 2023,14, 644 4 o 14 Figu e 1, he blue a ows ep esen he low o da a be ween he componen s o he sys em. The black a ows ep esen he gas low h ough he chambe . Mic omachines 2023, 14, 644 4 o 15 CO, CH4, and C2H6O) we e used o dilu ion in he measu emen s. The esis ance in ex- posu e o ai and analy e mixes was con inuously measu ed om he senso using a Kei hley K2700 da a acquisi ion sys em. A calib a ion cu e be ween ol age and empe - a u e was p epa ed o accu a e hea ing. The powe sou ce me e used was model ES-PS 3065-03B om Elek o Au oma ik, and i was used o powe he pla inum hea e in senso subs a e TO-39. In Figu e 1, he blue a ows ep esen he low o da a be ween he com- ponen s o he sys em. The black a ows ep esen he gas low h ough he chambe . Figu e 1. Schema ic o gas-sensing sys em. E e y sys em and uni in his se up we e con olled by cus om p og ammed Lab- VIEW p og am o un and ob ain expe imen al da a. The gas-sensing se up is desc ibed in he supplemen a y documen (Figu e S1). The sensi i i y o he esponses was calcu- la ed by he ollowing o mula. 𝑆 = 𝑅𝑎 −𝑅𝑔 𝑅𝑎 (1) 𝑆 = 𝑅𝑔 −𝑅𝑎 𝑅𝑎 (2) Equa ion (1) is o educing gases and Equa ion (2) is o oxidizing gases. Rg and Ra s and o he esis ance o he senso in he p esence o he analy e and he esis ance o he senso in syn he ic ai . The amoun o ime needed o he senso o sa u a e o 90% o he o al esis ance is wha is mean by T90 esponse and T90 eco e y ime [34–36]. 3. Resul s and Discussion 3.1. Physical P ope ies The XRD pa e ns in Figu e 2a o he BFO ma ch he peaks o powde di ac ion ile no: 01-070-5668, which belongs o he R3c space g oup wi h a hombohed al s uc u e. The main di ac ion peaks a 2θ (2- he a) o 22.4, 31.72, 37.61, 38.91, 39.45, 45.72, and 51.26 well-ma ched he (0 1 2), (1 0 4), (1 1 0), (1 1 3), (0 0 6), (2 0 2), and (0 2 4) c ys al planes [37]. The nanoma e ial displayed almos iden ical peaks wi h no a ia ion, and hei c ys al s uc u es we e well-indexed. The eplacemen o he Bi3+ in he BSFO spec a ypically leads o some o he di ac ion peaks disappea ing, a ibu ed o he ela i ely g ea e size o he S 2+ (0.112 nm) as con o e ing o Bi3+ (0.103 nm), and o he di e ence in he size o c ys als [25]. Figu e 2b shows he enla ged pa o he g aph be ween 31° and 34°, which shows ha in BSFO he doubly spli peaks me ge o ally o o m a single peak. This Figu e 1. Schema ic o gas-sensing sys em. E e y sys em and uni in his se up we e con olled by cus om p og ammed LabVIEW p og am o un and ob ain expe imen al da a. The gas-sensing se up is desc ibed in he supplemen a y documen (Figu e S1). The sensi i i y o he esponses was calcula ed by he ollowing o mula. S=Ra −Rg Ra (1) S=Rg −Ra Ra (2) Equa ion (1) is o educing gases and Equa ion (2) is o oxidizing gases. R g and R a s and o he esis ance o he senso in he p esence o he analy e and he esis ance o he senso in syn he ic ai . The amoun o ime needed o he senso o sa u a e o 90% o he o al esis ance is wha is mean by T90 esponse and T90 eco e y ime [34–36]. 3. Resul s and Discussion 3.1. Physical P ope ies The XRD pa e ns in Figu e 2a o he BFO ma ch he peaks o powde di ac ion ile no: 01-070-5668, which belongs o he R3c space g oup wi h a hombohed al s uc u e. The main di ac ion peaks a 2 θ (2- he a) o 22.4, 31.72, 37.61, 38.91, 39.45, 45.72, and 51.26 well-ma ched he (0 1 2), (1 0 4), (1 1 0), (1 1 3), (0 0 6), (2 0 2), and (0 2 4) c ys al planes [ 37 ]. The nanoma e ial displayed almos iden ical peaks wi h no a ia ion, and hei c ys al s uc u es we e well-indexed. The eplacemen o he Bi 3+ in he BSFO spec a ypically leads o some o he di ac ion peaks disappea ing, a ibu ed o he ela i ely g ea e size o he S 2+ (0.112 nm) as con o e ing o Bi 3+ (0.103 nm), and o he di e ence in he size o c ys als [ 25 ]. Figu e 2b shows he enla ged pa o he g aph be ween 31 ◦ and 34 ◦ , which shows ha in BSFO he doubly spli peaks me ge o ally o o m a single peak. This u he con i ms he p esence o ou doping and simila phase ans o ma ion beha io has been discussed by Hussain a al [ 27 ]. The c ys alli e size is gi en by Debye Sche e equa ion = kλ/B cos θ . The alue o k is se o 0.94 as we conside an app oxima ely sphe ical shape and λ is se o 1.54 Å. The mean c ys alli e size is calcula ed o he peaks abo e in BFO o be 34 nm, and o BSFO i is 24 nm. They a e also in close ag eemen wi h he SEM esul s. Mic omachines 2023,14, 644 5 o 14 Mic omachines 2023, 14, 644 5 o 15 u he con i ms he p esence o ou doping and simila phase ans o ma ion beha io has been discussed by Hussain a al [27]. The c ys alli e size is gi en by Debye Sche e equa ion = kλ/B cos θ. The alue o k is se o 0.94 as we conside an app oxima ely sphe - ical shape and λ is se o 1.54 Å. The mean c ys alli e size is calcula ed o he peaks abo e in BFO o be 34 nm, and o BSFO i is 24 nm. They a e also in close ag eemen wi h he SEM esul s. (a) (b) Figu e 2. XRD pa e ns o (a) pu e BFO and BSFO samples. (b) Peak me ging o BSFO. Scanning elec on mic oscopy was used o con i m he ma e ials’ nanoc ys alline s uc u e o BFO and BSFO. The SEM elec og aph in Figu e 3a,b show he e a e mic om- e e -sized p ima y lakes ha a e agglome a ed wi h i egula ly shaped nanopa icles on op o hem. Thei pa icle size is be ween 30 and 100 nanome e s in size. The doping has con ibu ed o he o e all size by educing he size o he pa icles o e all. S on ium oxide has a g ea e mel ing poin (2531 °C) han bismu h oxide (817 °C), which may be inhibi ing g ain de elopmen and causing he g ain size o dec ease [38]. EDX was also measu ed, and he alues a e lis ed below in he Table 1 o BFO and BSFO. The comple e da ase is shown in he supplemen a y (Figu es S2 and S3). They a e in close ag eemen wi h he alues published be o e, showing success ul doping [26]. (a) (b) Figu e 3. SEM images o (a) BFO lakes and (b) BSFO nanopa icles. Figu e 2. XRD pa e ns o (a) pu e BFO and BSFO samples. (b) Peak me ging o BSFO. Scanning elec on mic oscopy was used o con i m he ma e ials’ nanoc ys alline s uc u e o BFO and BSFO. The SEM elec og aph in Figu e 3a,b show he e a e mic ome e - sized p ima y lakes ha a e agglome a ed wi h i egula ly shaped nanopa icles on op o hem. Thei pa icle size is be ween 30 and 100 nanome e s in size. The doping has con ibu ed o he o e all size by educing he size o he pa icles o e all. S on ium oxide has a g ea e mel ing poin (2531 ◦ C) han bismu h oxide (817 ◦ C), which may be inhibi ing g ain de elopmen and causing he g ain size o dec ease [ 38 ]. EDX was also measu ed, and he alues a e lis ed below in he Table 1 o BFO and BSFO. The comple e da ase is shown in he supplemen a y (Figu es S2 and S3). They a e in close ag eemen wi h he alues published be o e, showing success ul doping [26]. Mic omachines 2023, 14, 644 5 o 15 u he con i ms he p esence o ou doping and simila phase ans o ma ion beha io has been discussed by Hussain a al [27]. The c ys alli e size is gi en by Debye Sche e equa ion = kλ/B cos θ. The alue o k is se o 0.94 as we conside an app oxima ely sphe - ical shape and λ is se o 1.54 Å. The mean c ys alli e size is calcula ed o he peaks abo e in BFO o be 34 nm, and o BSFO i is 24 nm. They a e also in close ag eemen wi h he SEM esul s. (a) (b) Figu e 2. XRD pa e ns o (a) pu e BFO and BSFO samples. (b) Peak me ging o BSFO. Scanning elec on mic oscopy was used o con i m he ma e ials’ nanoc ys alline s uc u e o BFO and BSFO. The SEM elec og aph in Figu e 3a,b show he e a e mic om- e e -sized p ima y lakes ha a e agglome a ed wi h i egula ly shaped nanopa icles on op o hem. Thei pa icle size is be ween 30 and 100 nanome e s in size. The doping has con ibu ed o he o e all size by educing he size o he pa icles o e all. S on ium oxide has a g ea e mel ing poin (2531 °C) han bismu h oxide (817 °C), which may be inhibi ing g ain de elopmen and causing he g ain size o dec ease [38]. EDX was also measu ed, and he alues a e lis ed below in he Table 1 o BFO and BSFO. The comple e da ase is shown in he supplemen a y (Figu es S2 and S3). They a e in close ag eemen wi h he alues published be o e, showing success ul doping [26]. (a) (b) Figu e 3. SEM images o (a) BFO lakes and (b) BSFO nanopa icles. Figu e 3. SEM images o (a) BFO lakes and (b) BSFO nanopa icles. Acco ding o he elemen al analysis, he a omic a io o me als Bi: Fe in he o iginal ma e ial is 4:1 which is qui e a om he expec ed s oichiome y o BFO, indica ing mo e Bi 2 O 3 componen s in he s uc u e. Howe e , he e is no such p ominen c ys alline phase in he X- ay di ac og am as one would expec i his we e a bulk composi ion ea u e. Mo eo e , EDX analysis showed an a omic a io o 1: 1. The p obing dep h o XPS may Mic omachines 2023,14, 644 6 o 14 be es ima ed o be se e al nanome e s a maximum, whe eas he pene a ion dep h o he e-beam in SEM is mo e han one mic ome e . In he case o such a ine-g ain ma e ial, EDX may be conside ed a bulk analysis me hod. Al oge he , i means ha he s uc u e o he pa icles’ su ace is i on de icien and is a om being s oichiome ic, whe eas he co e o he pa icles consis s o a BFO phase. I may also ha e an impac on he beha io o he semiconduc ing ma e ial, including a change be ween he band-bending beha io and bulk conduc i i y [39]. Table 1. EDX shows he elemen al analysis lis ed below. BFO BSFO Elemen Weigh % A omic % Elemen Weigh % A omic % O 12.93 55.15 O 12.59 47.13 Bi 69.71 22.43 Bi 49.59 14.21 Fe 18.36 22.42 Fe 32.92 35.3 -- - S 4.9 3.35 The composi ions and chemical alence s a es o su ace elemen s a e cha ac e ized us- ing X- ay pho oelec on spec oscopy. The su ey spec um is displayed in Figu es S4 and S5 in he supplemen a y documen . The doping o he ma e ial by s on ium induced signi i- can changes in elemen al a ios. Acco ding o XPS, he su ace- ela ed a omic concen a- ions a e 5.4: 2.5: 1 o Bi: Fe: S , indica ing he p esence o S in he su ace laye a he expense o Bi concen a ion. The bulk-sensi i e EDX showed a a io o 10.5: 4.2: 1 o Bi: Fe: S , which means ha he a ios be ween Bi and Fe became compa able in bulk (2.5:1) and a he su ace (2.2: 1). The esul s also indica e abou wo imes highe s on ium doping le el a he su ace han in he co es o he pa icles. The high- esolu ion XPS measu emen is shown in Figu es 3and 4. The peaks in he g aph in Figu e 4a a 158.5 eV and 163.9 eV a e ela ed o Bi 4 7/2 and Bi 4 5/2, espec i ely, indica ing posi i ely cha ged Bi in mixed oxide compounds [ 40 ]. A close analysis shows wo Bi 4 componen s es i ying o wo binding si es, one majo componen a 158.5 eV and 163.9 eV ( ed cu e), which may be associa ed wi h Fe 3+ –O–Bi 3+ en i onmen in he c ys alline la ice, and he mino componen a 159.9 eV and 165.3 eV (blue cu e), which may be associa ed wi h he Bi 3+ –O–Bi 3+ en i onmen in he c ys alline la ice [ 41 ]. The a io o co esponding a eas is 3:1. The doping o BFO by 20% s on ium caused a easonable dec ease in he ela i e signal in ensi y o he blue componen o a a io ca 6:1. I indica es he eplacemen o he Bi ions in he highe binding ene gy posi ions (blue) by S 2+ , while Bi ions in he lowe binding ene gy posi ions ( ed) a e no in luenced. The spin-o bi spli ing ene gy be ween Bi 4 7/2 and Bi 4 5/2 is 5.40 eV, a ypical alue o Bi. G aph (b) in Figu e 4shows he peak a 529.3 eV, which is a no ed signa u e o skele al oxygen ions O 2− in he la ice o c ys alline oxides. The peak componen a 531 eV is possibly due o he con ibu ion o adso bed ca bon moie ies con aining he C-O g oup o due o hyd oxyl g oups, while he smalles componen a 532.5 eV is associa ed wi h he ca bonyl g oup (C=O) o ca bona es. The ela i e signal in ensi y o he O 1s componen a 531.0 eV dec eased sligh ly wi h doping by s on ium. Howe e , his change migh be due o he su ace con amina ion moie ies. These obse a ions show a desi able change in he ma e ial su ace s uc u e due o he doping o S 2+ , as i could be e y bene icial o he applica ion o ou aim in his wo k. To con i m he doping o S 2+ , i can be seen in Figu e 5a ha a ain peak wi h wo componen s a 132.8 eV and 134.7 eV es i ies he p esence o he S 3d 5/2 and S 2d 3/2 and co esponding ypical o bi al spli ing [ 25 ]. Figu e 5b shows high- esolu ion spec a o bo h o iginal and doped ma e ials in he ypical ange o Fe 2p lines. The analysis o i on pho oelec on spec a is ex emely peculia , and he Gup a and Sen (GS) mul iple s a e o be used as shown in [ 42 ]. We unde ook he analysis using such a mul iple app oach, howe e , we eel he quali y o da a as well as he ma e ial c ys allini y and homogenei y is oo low Mic omachines 2023,14, 644 7 o 14 and he i ing p ocedu e may be p one o inaccu acies o exagge a ed in e p e a ion. To a oid i , we adop ed and e ised he common iew on he BFO nanoma e ials iden i ying wo main componen s in he ma e ials as Fe 3+ and Fe 2+ [ 25 , 41 ]. Unlike in hese e e ences, we did no i only a ew peaks o he o iginal da a ob aining hus a ious Fe 3+ /Fe 2+ a ios, bu he whole GS mul iple was i ed. Mic omachines 2023, 14, 644 7 o 15 (a) (b) Figu e 4. XPS high esolu ion scans o (a) Bi 4 o BFO and BSFO (b) O1s o BFO and BSFO. To con i m he doping o S 2+, i can be seen in Figu e 5a ha a ain peak wi h wo componen s a 132.8 eV and 134.7 eV es i ies he p esence o he S 3d 5/2 and S 2d 3/2 and co esponding ypical o bi al spli ing [25]. Figu e 5b shows high- esolu ion spec a o bo h o iginal and doped ma e ials in he ypical ange o Fe 2p lines. The analysis o i on pho oelec on spec a is ex emely peculia , and he Gup a and Sen (GS) mul iple s a e o be used as shown in [42]. We unde ook he analysis using such a mul iple ap- p oach, howe e , we eel he quali y o da a as well as he ma e ial c ys allini y and ho- mogenei y is oo low and he i ing p ocedu e may be p one o inaccu acies o exagge - a ed in e p e a ion. To a oid i , we adop ed and e ised he common iew on he BFO nanoma e ials iden i ying wo main componen s in he ma e ials as Fe3+ and Fe2+ [25,41]. Unlike in hese e e ences, we did no i only a ew peaks o he o iginal da a ob aining hus a ious Fe3+/Fe2+ a ios, bu he whole GS mul iple was i ed. On he o he hand, we me ged he i ed peaks o Fe3+ and Fe2+ mul iple s o a ain he desi ed le el o cla i y and simplici y, which is app op ia e o he da a quali y. I one ocuses on he Fe 2p 3/2 peak a 710.8 eV, wo main componen s can be iden i ied. One is cen e ed a 709.8 eV and he o he is cen e ed a 711.2 eV. The componen a lowe binding ene gy can be associa ed wi h Fe2+ ions, whe eas he one a highe binding ene gy can be associa ed wi h Fe3+ ions in he c ys alline s uc u e o he ma e ial. The a io be ween he co esponding a eas o he componen s is 5.0: 1 in a o o he highe binding ene gy si es. A e doping, he a io changes o 3.3: 1, bu he highe binding ene gy si es s ill p e ail. I was shown ha he su ace s uc u e di e s signi ican ly om BFO s oichiome y, and i is no easonable o expec only Bi+3, Fe+3, and O3−2 o o m oxida ion s a es. On he o he hand, simila ly o Bi, which is no expec ed in he o m o Bi2+ a all, he conside a ion o Fe3+ and Fe2+ ions in well-de ined s a es in he de ec s uc u e migh be an exagge a ion. Ra he han ha , we suppose he p esence o i on in wo di e en binding si es, dis in- guished by lowe and highe binding ene gy depending on speci ic a angemen s o oxy- gen, bismu h and i on a oms, and acancies o impu i ies in hei coo dina ion sphe es. Figu e 4. XPS high esolu ion scans o (a) Bi 4 o BFO and BSFO (b) O1s o BFO and BSFO. Mic omachines 2023, 14, 644 8 o 15 (a) (b) Figu e 5. XPS high esolu ion scans. (a) S 3d o BSFO. (b) Fe 2p o BFO and BSFO. 3.2. Gas-Sensing P ope ies The sensing p ope ies o bo h he BFO and he BSFO a e e alua ed by esis ance change in he p esence o ei he a educing gas o oxidizing gas. The dynamic esponse o all he senso s was es ed ac oss a empe a u e ange om 150 °C o 500 °C owa ds ou a ge gas o NO2. A e a p econdi ioning pe iod o 2 h, a s able baseline was eached. The i s basic es is o analyze he ansien s o dynamic sensing o bo h he senso s o he NO2 gas low a all possible he mal kine ics by a ying he empe a u e. I is un- de s ood ha he esponse o he senso s o NO2 gas depends on he ela i e balance be- ween he adso p ion and he deso p ion o he NO2 gas molecules and he eac ion wi h he adso bed oxygen [43]. This adso p ion lacks he equi ed amoun o ene gy in low empe a u es, showing slow sensing kine ics. The inc ease in empe a u e p o ides mo e ene gy o his eac ion, enhancing he esponse bu mo e han adequa ely inc easing he deso p ion. The BSFO senso shows a esponse o 4.7 imes o gas injec ion o 100 s 2 ppm NO2 o he baseline a he op imal empe a u e o 260 °C. This es is depic ed in Figu e 6a using a o al low a e o 200 sccm. The i ing o non-linea o m using a gaussian unc ion wi h an o hogonal dis ance eg ession as an i e a ion algo i hm is g aphed on he sensi- i i y cha o mani es he op imal ange o he highes sensi i i y. The e e ence un- doped BFO senso was e y esis an o eco d any baseline esis ance alues below 300 °C. I can be seen ha he d op in ope a ing empe a u e migh be a ibu ed o he in o- duc ion o new ene gy le els caused by he S doping elemen as well as he nano-size e ec [44]. This es is epea ed wi h a small di e ence o ex ending he ime o gas injec- ion o 300 s o each maximum sa u a ion and calcula e maximum esponse and eco e y imes o 2 ppm o NO2. The sensi i i y eaches 5.2 imes and is ela i ely s able. I espec- i e o looking a he physicochemical esul s, we p esume o ha e success ully c ea ed abundan oxygen acancy de ec s in he pe o ski e la ice, which compensa ed he accep- o doping o S on ium a om a A si es. This means ha he p- ype conduc i e beha io o ou ma e ial is enhanced and would help us ha e a su icien measu able esis ance a ela i ely low empe a u e. Addi ionally, he gas adso p ion is highe a lowe empe a- u es, and he e o e we see an enhancemen in he sensing esponse. This es is depic ed in Figu e 6b. Figu e 5. XPS high esolu ion scans. (a) S 3d o BSFO. (b) Fe 2p o BFO and BSFO. On he o he hand, we me ged he i ed peaks o Fe 3+ and Fe 2+ mul iple s o a ain he desi ed le el o cla i y and simplici y, which is app op ia e o he da a quali y. I one ocuses on he Fe 2p 3/2 peak a 710.8 eV, wo main componen s can be iden i ied. One is cen e ed a 709.8 eV and he o he is cen e ed a 711.2 eV. The componen a lowe binding ene gy can be associa ed wi h Fe 2+ ions, whe eas he one a highe binding ene gy can be associa ed wi h Fe 3+ ions in he c ys alline s uc u e o he ma e ial. The a io be ween he co esponding a eas o he componen s is 5.0: 1 in a o o he highe binding ene gy si es. A e doping, he a io changes o 3.3: 1, bu he highe binding ene gy si es s ill p e ail. I was shown ha he su ace s uc u e di e s signi ican ly om BFO s oichiome y, and i is no easonable o expec only Bi +3 , Fe +3 , and O 3−2 o o m oxida ion s a es. On he o he hand, simila ly o Bi, which is no expec ed in he o m o Bi 2+ a all, he conside a ion o Fe 3+ and Fe 2+ ions in well-de ined s a es in he de ec s uc u e migh be an exagge a ion. Ra he han ha , we suppose he p esence o i on in wo di e en binding si es, dis inguished by lowe and highe binding ene gy depending on speci ic Mic omachines 2023,14, 644 8 o 14 a angemen s o oxygen, bismu h and i on a oms, and acancies o impu i ies in hei coo dina ion sphe es. 3.2. Gas-Sensing P ope ies The sensing p ope ies o bo h he BFO and he BSFO a e e alua ed by esis ance change in he p esence o ei he a educing gas o oxidizing gas. The dynamic esponse o all he senso s was es ed ac oss a empe a u e ange om 150 ◦ C o 500 ◦ C owa ds ou a ge gas o NO2. A e a p econdi ioning pe iod o 2 h, a s able baseline was eached. The i s basic es is o analyze he ansien s o dynamic sensing o bo h he senso s o he NO 2 gas low a all possible he mal kine ics by a ying he empe a u e. I is unde s ood ha he esponse o he senso s o NO 2 gas depends on he ela i e balance be ween he adso p ion and he deso p ion o he NO 2 gas molecules and he eac ion wi h he adso bed oxygen [ 43 ]. This adso p ion lacks he equi ed amoun o ene gy in low empe a u es, showing slow sensing kine ics. The inc ease in empe a u e p o ides mo e ene gy o his eac ion, enhancing he esponse bu mo e han adequa ely inc easing he deso p ion. The BSFO senso shows a esponse o 4.7 imes o gas injec ion o 100 s 2 ppm NO 2 o he baseline a he op imal empe a u e o 260 ◦ C. This es is depic ed in Figu e 6a using a o al low a e o 200 sccm. The i ing o non-linea o m using a gaussian unc ion wi h an o hogonal dis ance eg ession as an i e a ion algo i hm is g aphed on he sensi i i y cha o mani es he op imal ange o he highes sensi i i y. The e e ence undoped BFO senso was e y esis an o eco d any baseline esis ance alues below 300 ◦ C. I can be seen ha he d op in ope a ing empe a u e migh be a ibu ed o he in oduc ion o new ene gy le els caused by he S doping elemen as well as he nano-size e ec [ 44 ]. This es is epea ed wi h a small di e ence o ex ending he ime o gas injec ion o 300 s o each maximum sa u a ion and calcula e maximum esponse and eco e y imes o 2 ppm o NO 2 . The sensi i i y eaches 5.2 imes and is ela i ely s able. I espec i e o looking a he physicochemical esul s, we p esume o ha e success ully c ea ed abundan oxygen acancy de ec s in he pe o ski e la ice, which compensa ed he accep o doping o S on ium a om a A si es. This means ha he p- ype conduc i e beha io o ou ma e ial is enhanced and would help us ha e a su icien measu able esis ance a ela i ely low empe a u e. Addi ionally, he gas adso p ion is highe a lowe empe a u es, and he e o e we see an enhancemen in he sensing esponse. This es is depic ed in Figu e 6b. Mic omachines 2023, 14, 644 9 o 15 (a) (b) Figu e 6. (a) Sensi i i y cha o BSFO and BFO. (b) Response cu e o BSFO a 260 °C. The Figu e 7a BSFO senso was u he es ed o epea abili y by using he NO2 gas, injec ion pe iod o 100 s, and concen a ion o 2 ppm. I was seen ha he senso esponse s ays cons an o e i e cycles, showing ha all senso s a e eliable wi h consis en e- sponse and eco e y imes. The inse g aphs show some le el o sa u a ion in his gas injec ion and show ha as kine ics is achie ed. This is clea ly due o he la ice dis o ion and phase ans o ma ion. The inc ease in ca ie concen a ion and elec on mobili y leads o a subs an ially bigge hole accumula ion laye a he su ace o he nanopa icle, he e o e imp o ing he sensing pe o mance [45]. Ano he es o e alua e he lowes de- ec ion limi o his senso is by a ying he concen a ion o he a ge gas. The sample is es ed wi h he NO2 gas, which is injec ed in o he gas chambe om 200 ppb o 1 ppm o a pe iod o 150 s. As pe ou sys em design, a highe low a e o 800 sccm was used o each such low concen a ions o he a ge gas. In Figu e 7b his measu emen is epea ed un il 1 ppm and he no iceable lowes esponse is om 200 ppb, bu he esponse om 400 ppb, 600 ppb, 800 ppb, and 1 ppm a e ela i ely highe and mo e consis en . U ilizing he lowes allo ed se ings in he mass low con olle o ob ain ou LOD alue o 200 ppb elec onically means ha mechanical unc ion should be aken in o conside a ion because i is an icipa ed ha he elec onic con ol o he o i ice will ake mo e ime o econ igu e i sel . We do no , howe e , es ablish an exemp ion speci ically o his si ua ion. This could be one o a numbe o con ibu ing causes o he ela i ely weake esponse a he lowes concen a ion. O he wise, in addi ion o he e ec s o doping, he inc ease in he su ace a ea due o he nano-size g ains also c ea es mo e ac i e eac ion si es wi h he NO2 gas molecules. Ene ge ically, doping also educes he band gap ene gy o ampli y su ace eac ions, allowing o mo e easible elec onic eac ions and be e sensi i i y, which we hold accoun able o ou sub ppm gas sensing [45]. Figu e 6. (a) Sensi i i y cha o BSFO and BFO. (b) Response cu e o BSFO a 260 ◦C. The Figu e 7a BSFO senso was u he es ed o epea abili y by using he NO 2 gas, injec ion pe iod o 100 s, and concen a ion o 2 ppm. I was seen ha he senso esponse s ays cons an o e i e cycles, showing ha all senso s a e eliable wi h consis en esponse Mic omachines 2023,14, 644 9 o 14 and eco e y imes. The inse g aphs show some le el o sa u a ion in his gas injec ion and show ha as kine ics is achie ed. This is clea ly due o he la ice dis o ion and phase ans o ma ion. The inc ease in ca ie concen a ion and elec on mobili y leads o a subs an ially bigge hole accumula ion laye a he su ace o he nanopa icle, he e o e imp o ing he sensing pe o mance [ 45 ]. Ano he es o e alua e he lowes de ec ion limi o his senso is by a ying he concen a ion o he a ge gas. The sample is es ed wi h he NO 2 gas, which is injec ed in o he gas chambe om 200 ppb o 1 ppm o a pe iod o 150 s. As pe ou sys em design, a highe low a e o 800 sccm was used o each such low concen a ions o he a ge gas. In Figu e 7b his measu emen is epea ed un il 1 ppm and he no iceable lowes esponse is om 200 ppb, bu he esponse om 400 ppb , 600 ppb, 800 ppb, and 1 ppm a e ela i ely highe and mo e consis en . U ilizing he lowes allo ed se ings in he mass low con olle o ob ain ou LOD alue o 200 ppb elec onically means ha mechanical unc ion should be aken in o conside a ion because i is an icipa ed ha he elec onic con ol o he o i ice will ake mo e ime o econ igu e i sel . We do no , howe e , es ablish an exemp ion speci ically o his si ua ion. This could be one o a numbe o con ibu ing causes o he ela i ely weake esponse a he lowes concen a ion. O he wise, in addi ion o he e ec s o doping, he inc ease in he su ace a ea due o he nano-size g ains also c ea es mo e ac i e eac ion si es wi h he NO 2 gas molecules. Ene ge ically, doping also educes he band gap ene gy o ampli y su ace eac ions, allowing o mo e easible elec onic eac ions and be e sensi i i y, which we hold accoun able o ou sub ppm gas sensing [45]. Mic omachines 2023, 14, 644 10 o 15 (a) (b) Figu e 7. T ansien esponse cu es o BSFO. (a) Repea abili y owa ds 2 ppm o NO2 a 260 °C. (b) Dynamic sensing cha ac e is ics o di e en concen a ion o NO2 a 260 °C. The T90 esponse and T90 eco e y a e calcula ed and plo ed in Figu e 8a. The ob- se able dec easing end in T90 esponse ime and inc easing ime in T90 eco e y ime is expec ed, as i explains he easibili y o he chemical eac ion as he amoun o a ge gas analy e inc eases in he sys em. A calib a ion cu e is plo ed in Figu e 8b using he expe imen al da a. In e e ence d awn om he g aph shows ha he calib a ion cu e has a linea dependency be ween he sensi i i y and loga i hm o NO2 concen a ion un- de cons an condi ions. The exponen ial inc ease in he gas senso esponse usually de- pic s a possibly cascading edox eac ion which could be due o he in luence o he do- pan and he eby i s new lowe wo k unc ion. The linea i equa ion is Y = 311.64X − 689.94 wi h R2 = 0.98209. Howe e , u he s udies a e necessa y o es i s long- e m s a- bili y. (a) (b) Figu e 8. (a) Response/ eco e y dynamics. (b) Loga i hmic dependency o he BSFO senso . Finally, he BSFO specimen was in es iga ed o i s c oss-sensi i i y p ope ies in he same condi ions and using he same se up wi h he ope a ing empe a u e o 260 °C. The p epa ed senso s we e es ed in he same manne o a ansien dynamic esponse wi h Figu e 7. T ansien esponse cu es o BSFO. ( a ) Repea abili y owa ds 2 ppm o NO 2 a 260 ◦ C. (b) Dynamic sensing cha ac e is ics o di e en concen a ion o NO2a 260 ◦C. The T90 esponse and T90 eco e y a e calcula ed and plo ed in Figu e 8a. The obse able dec easing end in T90 esponse ime and inc easing ime in T90 eco e y ime is expec ed, as i explains he easibili y o he chemical eac ion as he amoun o a ge gas analy e inc eases in he sys em. A calib a ion cu e is plo ed in Figu e 8b using he expe imen al da a. In e e ence d awn om he g aph shows ha he calib a ion cu e has a linea dependency be ween he sensi i i y and loga i hm o NO 2 concen a ion unde cons an condi ions. The exponen ial inc ease in he gas senso esponse usually depic s a possibly cascading edox eac ion which could be due o he in luence o he dopan and he eby i s new lowe wo k unc ion. The linea i equa ion is Y = 311.64X − 689.94 wi h R2= 0.98209. Howe e , u he s udies a e necessa y o es i s long- e m s abili y.