senso s
A icle
Wa e -Based Indium Tin Oxide Nanopa icle Ink o
P in ed Toluene Vapou s Senso Ope a ing a
Room Tempe a u e
Jan Maslik, I o Ku i ka * , Pa el U banek , Pe K cma , Pa ol Suly, Milan Masa and
Michal Macho sky
Cen e o Polyme Sys ems, Uni e si y Ins i u e, Tomas Ba a Uni e si y in Zlin, ida Tomase Ba i 5678,
760 01 Zlin, Czech Republic; [email p o ec ed] (J.M.); [email p o ec ed] (P.U.); [email p o ec ed] (P.K.);
[email p o ec ed] (P.S.); [email p o ec ed] (M.M.); [email p o ec ed] (M.M.)
*Co espondence: [email p o ec ed] o [email p o ec ed]
Recei ed: 21 Augus 2018; Accep ed: 24 Sep embe 2018; Published: 27 Sep embe 2018


Abs ac :
This s udy is ocused on he de elopmen o wa e -based ITO nanopa icle dispe sions and
ink-je ab ica ion me hodology o an indium in oxide (ITO) senso o oom empe a u e ope a ions.
Dimensionless co ela ions o ma e ial- ool-p ocess a iables we e used o map he p in ing p ocess
and se e al in e p e a ional amewo ks we e e-examined. A educ ion o he p oblem o he
New onian luid app oach was applied o he sake o simplici y. The ink p ope ies as well as he
p ope ies o he deposi ed laye s we e es ed o a ious nanopa icles loading. High-quali y ilms
we e p epa ed and annealed a di e en empe a u es. The bes pe o ming ma e ial composi ion,
p ocess pa ame e s and pos -p in ea men condi ions we e used o p epa ing he es ing senso
de ices. P in ed specimens we e exposed o oluene apou s a oom empe a u e. Good sensi i i y,
as esponses and eco e ies we e obse ed in ambien ai al hough he n- ype esponse mechanism
o oluene is in luenced by mois u e in ai and baseline d i was obse ed. Sensing esponse
in e sion was obse ed in an oxygen and mois u e- ee N
2
a mosphe e which is explained by he
cha ge- ans e mechanism be ween he adso ben and adso ba e molecules. The sensi i i y o he
de ice was sligh ly be e and he esponse was s able showing no d i s in he p o ec i e a mosphe e.
Keywo ds:
Indium in oxide; nanopa icle; inkje ink; ma e ial p in ing; dimensionless numbe ;
gas senso ; oom empe a u e
1. In oduc ion
A g ea deal o e o has been spen on desc ibing he mechanism esponsible o he gas sensing
o me al oxide nanoma e ials as ac i e componen s o sensing de ices [
1
–
3
]. In o de o boos he
sensi i i y and achie e be e selec i i y and s abili y o hese de ices, sophis ica ed hie a chical and
hyb id nanos uc u ed ma e ials ha e ecen ly been p epa ed and es ed, e.g., [
4
,
5
]. On he o he hand,
esea ch pape s ocused on he ab ica ion and imp o emen o sensing de ices a e less equen [
6
,
7
].
Ne e heless, se e al well es ablished gene al me hods a e known o p epa ing hin and hick
me al oxide nanoma e ial ilms which can be used in he ab ica ion o gas moni o ing de ices.
The mos commonly used echniques can be di ided in o wo main g oups. The i s is a ple ho a o
gas phase ab ica ion echniques (PVD, CVD o he mal apou anspo me hods). The second g oup
o me hods elies on liquid phase employing ab ica ion echniques such as sol-gel, sp ay py olysis,
dip-coa ing, spin-coa ing and ma e ial p in ing me hods [8–10].
The abo e-men ioned con en ional deposi ion echniques, gas phase and in-si u liquid phase
g owing mechanism echniques a e s ic ly cons ained by a ious physical and chemical pa ame e s
Senso s 2018,18, 3246; doi:10.3390/s18103246 www.mdpi.com/jou nal/senso s
Senso s 2018,18, 3246 2 o 17
dic a ing he u iliza ion o special equipmen and he exposi ion o he subs a e o eac ion
en i onmen . The disad an ages o hese echniques include, in mos cases, he need o using
acuum echnology, s encils and masks o subsequen sepa a ing, emo ing o e ching p ocesses.
Such p oblems can be easily o e come by using ma e ial p in ing echnology i he ma e ial can be
p epa ed in o m o a p in able ink, pas e, and powde o o he s in sui able o mula ion. Ma e ial
p in ing echnologies (including sc een-p in ing, oll- o- oll p in ing, g a u e p in ing and ink-je
p in ing) a e widely used especially in elec onics ( ab ica ion o conduc i e ci cui s and pa hs,
sola cells, ligh -emi ing and sensing de ices, an ennas, memb anes e c.). Thei applica ions esul in
simpli ied and accele a ed ab ica ion p ocesses. These low-cos echnologies a e based on deposi ing
unc ional ma e ials on o a used subs a e such as glass o anspa en and lexible oils, ex ile
ma e ials, ce amics o me al wa e s. Unlike o he p in ing me hods, ink-je p in ing does no
equi e any mas e o m, s encils o masks; he e o e, i allows ins an aneous and apid designs
and p o o yping wi h no delay be ween digi al mo i e gene a ion and ma e ial deposi ion. The ink-je
p in ing is an in e es ing and e sa ile me hod o make con olled and localized deposi ion o unc ional
ma e ials wi h sui able geome y on a ious subs a es a low p ocessing empe a u es [
11
–
14
].
The p in ed pa e ns a e designed by common compu e p og ams and can be sa ed as simple
bi map images o p in ing. I is possible o use a wide ange o ino ganic and o ganic ma e ials,
including inks based on me al o me al oxide nanopa icles and polyme solu ions [
15
–
17
]. The size
o he oxide nanopa icles, hei dispe sion in a p ope liquid medium and a sui able dispe sion
s abiliza ion, iscosi y and he su ace ension o he ink composi ion o je ing a e i ally impo an
and challenging pa ame e s o be de eloped o he equi ed low o he ink h ough he nozzles o a
p in ing head yielding he p ope gene a ion and ejec ion o d ople s, which is he limi ing ac o o
his ma e ial deposi ion p in ing p ocess.
T anspa en conduc i e oxides (among hem indium in oxide—ITO being he mos widely
used) a e indispensable ma e ials in ab ica ion o op oelec onic de ices and can be used as a
semi-conduc i e ma e ial o gas de ec ing senso s based on he esis ance a ia ion due o i s exposu e
o he a ge gas. Films made om ITO (In
2
O
3
:10 w % SnO
2
) ha e been ex ensi ely s udied in
ecen yea s because hey exhibi a ela i ely a e combina ion o high isible ansmission and
signi ican elec ical conduc i i y, high subs a e adhe ence, good ha dness, and chemical ine ness [
18
].
The applicabili y o his degene a e n- ype semiconduc o as a hin ilm senso y de ice deposi ed
by a ious echniques (such as he he mal e apo a ion echnique, spu e ing, sc een-p in ing) has
been widely epo ed wi h espec o sensing educing and oxidizing gases and o ganic compounds
(NH
3
, CO, H
2
, NO
2
, oluene e c.) [
6
,
7
,
19
–
21
]. Fo example, oluene is a widely used o ganic sol en
and one o he signi ican con ibu o s o indoo ai pollu ion wi h se ious sa e y conce ns. A high
empe a u e ope a ing (a 230
◦
C) ITO hin ilm de ice o oluene apou s sensing was p epa ed by
spu e coa ing ecen ly [19].
Some applica ions o pa icula e ITO ma e ials ha e been epo ed [
15
,
22
], al hough only a ely o
sensing de ices. I has al eady been demons a ed ha he sc een p in ing o o ganome allic p ecu so
pas e ollowed by i ing a high empe a u es (600
◦
C) o a ela i ely long ime ( o 40 min a op imum
o ITO c ys alliza ion) can be u ilized o senso ab ica ion [
6
]. The la es e o s demons a ed he
sui abili y o sc een p in ing o pas es di ec ly composed om ITO nanoc ys als and binde s o
senso ab ica ion [23,24].
To he bes o ou knowledge, he e is no epo a ailable in li e a u e conce ning he p epa a ion
o an ITO based senso di ec ly p epa ed om ITO nanopowde wi h he aid o an ink-je ma e ial
p in ing. We a e con inced, ha i is possible o use his me hod ea ning all bene i s i can o e and he
p in ing p ocess is in ocus o ou s udy. Mo eo e , a second challenge is p epa ing a de ice ope a ing a
oom empe a u e, as his may be o p ime impo ance o applica ions in bio- ela ed ields, wea able
elec onics, ene gy sa ing app oaches and hea ing minimiza ion in al eady exis ing echnologies,
he mal camou lage e c. In his wo k, we desc ibe he de elopmen o an ITO nanopa icle based
wa e bo ne dispe sion o he deposi ion o ilms by ink-je p in ing echnology and a demons a ion
Senso s 2018,18, 3246 3 o 17
o he low empe a u e esponse o he p epa ed senso s upon exposu e o sa u a ed oluene apou s.
The senso was es ed in ambien labo a o y condi ions as well as unde a pu e ni ogen a mosphe e.
2. Ma e ials and Me hods
2.1. Fo mula ion and Cha ac e iza ion o Indium Tin Oxide Inks
ITO nanopa icle based aqueous inks we e p epa ed in he o m o dispe sions. The p ope
amoun (weighed) o indium in oxide nanopowde <50 nm pa icle size (Sigma-Ald ich spol. s .o.,
P ague, Czech Republic) was mixed wi h he op imum a io o polyme ic dispe sing agen and silicon
su ac an p o ided by BYK-Chemie, ALTANA (Dispe byk
®
-190 and Byk
®
-348). The concen a ion
o bo h hese addi i es was se o be sligh ly abo e hei CMCs. E hylene glycol was chosen o
modi ying he densi y and iscosi y o he dispe sion (Sigma-Ald ich p oduc ). E hylene glycol (EG)
also p o ides a high boiling and humec an componen o he sol en sys em. The dispe sions we e
mixed o se e al hou s in a sealed lask and we e sonica ed o 30 min by UZ Sonopuls HD 2070
homogenize and il e ed h ough a 0.22
µ
m PTFE il e o emo e agg ega es and agglome a ed
pa icles be o e illing he ca idge (n.b.). The concen a ions o he ITO nanopa icles in he p epa ed
inks we e se o 10, 15, 20 and 25 w % o compa e he o e all pe o mance o each.
The iscosi y o he inks was de e mined a he low-shea a e by olling ball Mic o iscome e
Lo is 2000 ME (1.59 mm capila y) and he densi y was measu ed by Densi y me e DMA 5000 M
(An on Paa s. .o., P ague, Czech Republic). The su ace ension o he ITO ink was measu ed by
Tensionme e K100MK3 (K üss GmbH, Hambu g, Ge many) based on he Wilhelmy pla e me hod.
The he mog a ime ic analysis was pe o med using a TA Q500 he mal analyze wo king in
TGA mode wi h a p og ammed cons an empe a u e g ow h a e o 10
◦
C pe minu e and a ni ogen
a mosphe e wi h a o al low o 100 sccm.
2.2. Senso Fab ica ion and P in ing Condi ions
The pa e ns we e designed in a basic Windows p og am in a scale co esponding o
15 mm ×15 mm
, hen sa ed as a bi map image and loaded in D op Manage So wa e. The senso
ilms we e ab ica ed by a ma e ial piezoelec ic inkje p in e Fuji ilm Dima ix DMP 2831 Se ies
(nozzle diame e 21.5
µ
m and 10 pL nominal d op olume ca idge, p in e and consumables
p o ided by FUJIFILM Dima ix Inc., Lebanon, NH, USA) on mic oscope glass slides, which we e
cleaned in de-ionized wa e , ace one and isop opanol and d ied by lowing ai and in a acuum o en.
The op imum p in ing condi ions we e ound and used as ollows: he d i ing ol age a he nozzles
du ing he p in ing p ocess was in he ange o 25–27 V wi h a je ing equency o 1 kHz, a nozzle
empe a u e o 32
◦
C and a pla e empe a u e o 50
◦
C, he spacing o d ople s was de ined o 30
µ
m
which is equi alen o a esolu ion o 847 dpi. The ilms we e made by one p in ing un (one laye ).
The p in ed ilms we e d ied in an o en a 60
◦
C o 30 min and hen annealed in a mu le u nace.
The annealing empe a u es we e chosen o cla i y he e ec o annealing o 400, 500 and 600
◦
C in
an ambien ai a mosphe e. The sample was always inse ed in o he cool o en and he empe a u e
p og am was se o maximum a e inc ease. The desi ed empe a u e was achie ed wi hin a ew
minu es and hen he empe a u e was kep cons an o 30 min. The o en was hen le o na u ally
cool down o he labo a o y empe a u e be o e he samples we e aken ou . The elec odes we e
connec ed o he coppe wi es by he highly conduc i e sil e pas e COATES XZ250 which was d ied
a 120 ◦C o 20 min. The design o he sensing de ice is schema ically depic ed in Figu e 1.
Senso s 2018,18, 3246 4 o 17
Senso s 2018, 18, x 4 o 17
Figu e 1. Schema ic design o he sensing de ice.
2.3. Films Cha ac e iza ion
The gene al quali y, con igui y and compac ness o he p in ed ilms we e p elimina y obse ed
by he mic oscope LEICA DVM2500 Digi al Came a. The hickness o he p in ed ilms was measu ed
in con ac mode by he s ylus p o ile Dek ak XT (B uke , Bille ica, MA, USA) p o ided by a diamond
s ylus (25 μm adius). The elec ical esis i i y o he p in ed pa e ns was measu ed using a ou -
poin p obe (Van de Pauw me hod). The su ace mo phology was examined by he scanning elec on
mic oscope No a NanoSEM 450 in acuum and by he a omic o ce mic oscope Dimension ICON
(B uke ) unde an ambien condi ion a h ee di e en a eas o 25 μm2 in PeakFo ce mode by he
ScanAsys –Ai p obe whose ip has a nominal adius o 2 nm. The su ace oughness was calcula ed
and a e aged om ob ained da a using NanoScope Analysis 9.1. The cu en mapping was
in es iga ed h ough he PeakFo ce unneling module wo king in semi-con ac mode wi h he
PFTUNA p obe whose ip has a nominal adius o 25 nm coa ed by conduc i e pla inum/i idium and
was used o obse e he cu en signal dis ibu ion on he su aces o 25 μm2 o annealed hin ilms.
The applied DC bias be ween he elec ically conduc i e ip and he sample was se o 100 mV. Bo h
a scan a e o 0.5 Hz and a esolu ion o 512 lines we e used in all cases.
2.4. Senso Pe o mance
The esponse o he senso s was measu ed using he mul ime e UNI-T HC-UT71D wi h
in e ace so wa e ha eco ds obse ed esis ance. The changes be ween he “On” and “O ” senso
s a es upon exposu e o he apou s o oluene and he emo al om he apou s we e measu ed.
The expe imen s we e ca ied ou a an ambien a mosphe e in he labo a o y as well as in a ni ogen
a mosphe e (less han 1 ppm o O2 and H2O) in he glo e box GP Campus (Jacomex).
Whe e e indica ed in his a icle, he oom o labo a o y empe a u e means (24 ± 2) °C unless
s a ed mo e p ecisely. The labo a o y is ai condi ioned.
3. Resul s and Discussion
3.1. Ink Composi ion De elopmen
The mos challenging s ep h oughou he ab ica ion p ocess o such a de ice as desc ibed in
he in oduc o y pa is he o mula ion o he ink. I is necessa y o conside ha he pa ame e s o
he inal p in ed laye ha e o mee all necessa y chemical and physicochemical c i e ia o assu e he
compa ibili y o he ink wi h he subs a e, he je ing pe o mance and he s o age s abili y o he
suspension while p epa ing he ink composi ion. I is a mul idimensional pa ame e space ha mus
be esea ched and in which an op imum be ween all (and some imes con adic o y) equi emen s
imposed on he ma e ial and i s p ocessing wi h he gi en equipmen mus be ound. The c ucial
pa ame e s enabling op imal p in abili y a e he e o e ink densi y, iscosi y, su ace ension, he
s abili y o he suspension and he size o he nanopa icles, which is pa ially ela ed o he inal
esis ance and he esponse o he sensing laye [16,25]. The key p ope ies de e mining good
p ocessabili y by he used ink-je p in e Dima ix DMP-2800 Se ies a e lis ed by he p oduce o
Figu e 1. Schema ic design o he sensing de ice.
2.3. Films Cha ac e iza ion
The gene al quali y, con igui y and compac ness o he p in ed ilms we e p elimina y obse ed
by he mic oscope LEICA DVM2500 Digi al Came a. The hickness o he p in ed ilms was measu ed
in con ac mode by he s ylus p o ile Dek ak XT (B uke , Bille ica, MA, USA) p o ided by a diamond
s ylus (25
µ
m adius). The elec ical esis i i y o he p in ed pa e ns was measu ed using a ou -poin
p obe (Van de Pauw me hod). The su ace mo phology was examined by he scanning elec on
mic oscope No a NanoSEM 450 in acuum and by he a omic o ce mic oscope Dimension ICON
(B uke ) unde an ambien condi ion a h ee di e en a eas o 25
µ
m
2
in PeakFo ce mode by
he ScanAsys –Ai p obe whose ip has a nominal adius o 2 nm. The su ace oughness was
calcula ed and a e aged om ob ained da a using NanoScope Analysis 9.1. The cu en mapping
was in es iga ed h ough he PeakFo ce unneling module wo king in semi-con ac mode wi h he
PFTUNA p obe whose ip has a nominal adius o 25 nm coa ed by conduc i e pla inum/i idium and
was used o obse e he cu en signal dis ibu ion on he su aces o 25
µ
m
2
o annealed hin ilms.
The applied DC bias be ween he elec ically conduc i e ip and he sample was se o 100 mV. Bo h a
scan a e o 0.5 Hz and a esolu ion o 512 lines we e used in all cases.
2.4. Senso Pe o mance
The esponse o he senso s was measu ed using he mul ime e UNI-T HC-UT71D wi h in e ace
so wa e ha eco ds obse ed esis ance. The changes be ween he “On” and “O ” senso s a es upon
exposu e o he apou s o oluene and he emo al om he apou s we e measu ed. The expe imen s
we e ca ied ou a an ambien a mosphe e in he labo a o y as well as in a ni ogen a mosphe e (less
han 1 ppm o O2and H2O) in he glo e box GP Campus (Jacomex).
Whe e e indica ed in his a icle, he oom o labo a o y empe a u e means (24
±
2)
◦
C unless
s a ed mo e p ecisely. The labo a o y is ai condi ioned.
3. Resul s and Discussion
3.1. Ink Composi ion De elopmen
The mos challenging s ep h oughou he ab ica ion p ocess o such a de ice as desc ibed in
he in oduc o y pa is he o mula ion o he ink. I is necessa y o conside ha he pa ame e s o
he inal p in ed laye ha e o mee all necessa y chemical and physicochemical c i e ia o assu e he
compa ibili y o he ink wi h he subs a e, he je ing pe o mance and he s o age s abili y o he
suspension while p epa ing he ink composi ion. I is a mul idimensional pa ame e space ha mus be
esea ched and in which an op imum be ween all (and some imes con adic o y) equi emen s imposed
on he ma e ial and i s p ocessing wi h he gi en equipmen mus be ound. The c ucial pa ame e s
enabling op imal p in abili y a e he e o e ink densi y, iscosi y, su ace ension, he s abili y o he
suspension and he size o he nanopa icles, which is pa ially ela ed o he inal esis ance and
he esponse o he sensing laye [
16
,
25
]. The key p ope ies de e mining good p ocessabili y by he
used ink-je p in e Dima ix DMP-2800 Se ies a e lis ed by he p oduce o disposable ca idges
Senso s 2018,18, 3246 5 o 17
wi h p in ing heads as ollows. The ink luid iscosi y shall be kep in a ange o 10–12 mPa
·
s and i s
su ace ension in a ange o 28–42 mN
·
m
−1
. Fu he mo e, i is sugges ed o use a d op eloci y in
a ange o 7–9 m
·
s
−1
as he i s guess [
26
], howe e , acco ding o li e a u e and expe ience in his
s udy, a luid ejec ion eloci y o abou 6 m
·
s
−1
is commonly used. I is also ecommended o il e all
luids o 0.2
µ
m, because pa icles bigge han 1/100 o he nozzle diame e may cause nozzle clogging.
The i s wo pa ame e s may be subs an ially changed by he ink composi ion and he size o he
nanopa icles mus be chosen o be smalle han he c i ical le el. The p ocessing pa ame e s, namely
he luid ejec ion eloci y and he d ople o ma ion, a e con olled by using wa e o m; howe e ,
hey can be only sligh ly a ied in compa ison wi h he ela i ely ee choice o he ink iscosi y and
he su ace ene gy. Acco ding o he p oduce , he iscosi y ange may be ex ended om 1 mPa
·
s
o wa e -like luids up o 30 mPa
·
s which is decla ed as he highes iable o he p in e . Simila ly,
a su ace ension o abou 70 mN
·
m
−1
ep esen s he uppe limi o p in ing wi h a gi en machine
while he lowe limi is 20 mN
·
m
−1
. The o iginal ink composi ion was expe imen ally de eloped
wi hin his space o pa ame e s by he ial-e o me hod o changing and al e na ing always only one
a iable un il a sa is ac o y pe o mance o he p ocess was achie ed. The p ope ies o de eloped
inks wi h a iable nanopa icle loading a e lis ed in Table 1. The su ace ension o he used ink
composi ions is compa ible wi h he su ace cha ac e is ics o he chosen subs a e. The iscosi y and
he su ace ension unde gi en dynamic condi ions did no esul in he op imum o ma ion o single
d ople s only, bu he sys em wo ked in he egime o he one sa elli e d ople o ma ion ha me ges
wi h he main d ople du ing i s ligh be o e hi ing he su ace. Al hough his is a subop imum
p ocess, i yielded good p in ing quali y while o he pa ame e s we e ela i ely easily kep a su icien
le els oo.
Table 1. Main p ope ies o ITO inks.
Nanopa icles Loading/w % 10 15 20 25
Su ace ension/mN·m−121.6 ±0.1 21.6 ±0.1 21.6 ±0.1 21.7 ±0.1
Densi y/kg·m−31127.6 1186.7 1241.9 1306.6
Viscosi y/mPa·s 3.078 3.518 4.128 4.703
The s abili y o he p epa ed nanodispe sions ep esen s ano he ex emely impo an issue o
hei applica ion as inks. Su icien shel li e is a g ea ad an age and a leas easy edispe seabili y
mus be assu ed o any p ac ical u iliza ion o inks. Due o he abo e-men ioned equi emen s
he op imal amoun and a io o he wo addi i es was de e mined based on he c i ical micelle
concen a ion in an aqueous medium. The su ace ension was con olled by using a we ing agen
which p oduces a signi ican dec ease in he su ace ension o he aqueous sys em and he e o e
pa icula ly imp o es subs a e we ing and le elling. The achie ed s abili y o he ink composi ions
gua an eed hei sa e use wi hin he imescale o se e al days and is displayed in he example in
Figu e 2. The dissol ed mac omolecules o non-ionic su ac an in he dispe sion medium a oid he
loccula ion o nanopa icles by means o s e ic s abiliza ion h ough he unc ional g oups ha ing
an a ini y o he pa icle su ace. The hyd ophilic segmen s o m a epulsi e laye supp essing he
coagula ion o he nanopa icles p esen and s abilize hem. On he ole and e ec s o he dispe san
agen s in ink-je inks, he eade is e e ed o Soleimani-Go gani [
27
] and Young-Sang Cho [
28
] o
u he de ails.
The composi ion and p ocessing p ope ies o used inks wi h he inkje p in e DMP Dima ix 2800
Se ies can be analyzed a ionally wi h he use o dimensionless c i e ia. U iliza ion o his analysis
g ea ly enhanced he o iginal ial-e o de elopmen o he ink and accele a ed op imiza ion o he
ink p in ing condi ions. The eade is e e ed o he comp ehensi e wo ks o Shlomo Magdassi [
16
],
Zhidong Pan [
29
] and E. Kim and J. Baek [
30
]. Acco ding o he li e a u e, he ollowing c i e ia a e he
mos impo an . Gene ally, he p in abili y ange used o be de e mined by a Znumbe , which is he
in e se o he Ohneso ge numbe (Oh). The ecommenda ions o he op imum ange o he Z numbe

Senso s 2018,18, 3246 6 o 17
a y om 1–10 up o 4–14. Howe e , i was ound ha he Znumbe (o ob iously Oh numbe oo)
alone is insu icien o desc ibing he d ople o ma ion dynamics because all he e ms desc ibing
dynamic e ec s a e cancelled in i s o mula
Z=Re
√We =Oh−1(1)
and only he ma e ial cons an s and cha ac e is ic leng h emain [
30
]. Ac ually, he cha ac e is ic leng h
(A) is a ool- ela ed p ope y since i is a p in ing nozzle squa e shaped o i ice cha ac e is ic diame e ,
i.e., he side size o he squa e (A= 21.5
µ
m). The e o e, o he impo an non-dimensional pa ame e s
such as he Reynolds numbe (Re), he Webe numbe (We), and he capilla y numbe (Ca) should also
be aken in o conside a ion o comple e he ma e ial-p ocess- ool pa ame e - ela ed iad. Re ep esen s
he a io be ween he iscous and he ine ial o ces in mo ing luid, We is dependen on he a io
be ween he ine ia and he su ace ension, Oh e lec s he physical p ope ies o he liquid ( he iscous
o ces, he ine ial and he su ace ension o ces) and he size scale o he nozzle, bu is independen o
he d i ing condi ions, and Ca is he a io o he iscous o ce o he capilla y o ce. Table 2summa ises
he calcula ed dimensionless c i e ia o he p epa ed inks wi h he use o ink ejec ion luid eloci y
inally es ima ed wi h he help o analysis discussed below. The in luence o g a i y is conside ed as
being o e y low ele ance on he in es iga ed ange o pa ame e s (including he s and-o being
1 mm) and he e o e is no conside ed in his analysis. The o mulas and desc ip ion o used a iables
a e gi en in Appendix A—Used equa ions o dimensionless c i e ia.
Senso s 2018, 18, x 6 o 17
Figu e 2. Demons a ing o he achie ed s abili y and he sedimen a ion p og ess o he ink
composi ion (25 w % loading) o e ime (a) = 0 h, (b) = 24 h, (c) = 72 h, (d) = 10 days.
The composi ion and p ocessing p ope ies o used inks wi h he inkje p in e DMP Dima ix
2800 Se ies can be analyzed a ionally wi h he use o dimensionless c i e ia. U iliza ion o his
analysis g ea ly enhanced he o iginal ial-e o de elopmen o he ink and accele a ed
op imiza ion o he ink p in ing condi ions. The eade is e e ed o he comp ehensi e wo ks o
Shlomo Magdassi [16], Zhidong Pan [29] and E. Kim and J. Baek [30]. Acco ding o he li e a u e, he
ollowing c i e ia a e he mos impo an . Gene ally, he p in abili y ange used o be de e mined by
a Z numbe , which is he in e se o he Ohneso ge numbe (Oh). The ecommenda ions o he
op imum ange o he Z numbe a y om 1–10 up o 4–14. Howe e , i was ound ha he Z numbe
(o ob iously Oh numbe oo) alone is insu icien o desc ibing he d ople o ma ion dynamics
because all he e ms desc ibing dynamic e ec s a e cancelled in i s o mula
𝑍=
R
e
√
W
e
=𝑂ℎ
 (1)
and only he ma e ial cons an s and cha ac e is ic leng h emain [30]. Ac ually, he cha ac e is ic
leng h (A) is a ool- ela ed p ope y since i is a p in ing nozzle squa e shaped o i ice cha ac e is ic
diame e , i.e., he side size o he squa e (A = 21.5 μm). The e o e, o he impo an non-dimensional
pa ame e s such as he Reynolds numbe (Re), he Webe numbe (We), and he capilla y numbe
(Ca) should also be aken in o conside a ion o comple e he ma e ial-p ocess- ool pa ame e - ela ed
iad. Re ep esen s he a io be ween he iscous and he ine ial o ces in mo ing luid, We is
dependen on he a io be ween he ine ia and he su ace ension, Oh e lec s he physical p ope ies
o he liquid ( he iscous o ces, he ine ial and he su ace ension o ces) and he size scale o he
nozzle, bu is independen o he d i ing condi ions, and Ca is he a io o he iscous o ce o he
capilla y o ce. Table 2 summa ises he calcula ed dimensionless c i e ia o he p epa ed inks wi h
he use o ink ejec ion luid eloci y inally es ima ed wi h he help o analysis discussed below. The
in luence o g a i y is conside ed as being o e y low ele ance on he in es iga ed ange o
pa ame e s (including he s and-o being 1 mm) and he e o e is no conside ed in his analysis. The
o mulas and desc ip ion o used a iables a e gi en in Appendix A—Used equa ions o
dimensionless c i e ia.
Table 2. Calcula ed dimensionless c i e ia o p epa ed inks.
Nanopa icles Loading/w % 10 15 20 25
Re 47.26 43.51 38.81 35.84
We 40.44 42.50 44.42 46.60
Oh 0.13 0.15 0.17 0.19
Z 7.43 6.67 5.82 5.25
Ca 0.86 0.98 1.14 1.30
Figu e 2.
Demons a ing o he achie ed s abili y and he sedimen a ion p og ess o he ink composi ion
(25 w % loading) o e ime (a) =0h,(b) =24h,(c) =72h,(d) = 10 days.
Table 2. Calcula ed dimensionless c i e ia o p epa ed inks.
Nanopa icles Loading/w % 10 15 20 25
Re 47.26 43.51 38.81 35.84
We 40.44 42.50 44.42 46.60
Oh 0.13 0.15 0.17 0.19
Z7.43 6.67 5.82 5.25
Ca 0.86 0.98 1.14 1.30
The p epa ed suspensions we e in en ionally heo e ically ea ed as New onian luids only,
neglec ing he e en ual weak iscoelas ici y. Such an app oach simpli ies he p oblem adically;
howe e , i can be applied un il i p oduces p edic ions compa able wi h expe imen . Acco ding o an
analysis o ink-je p in ing egimes by E. Kim and J. Baek [
30
], all p epa ed inks all in o he “ egime II”
which is cha ac e ized by ela i ely small Ca and la ge We numbe s. These condi ions a e mani es ed
by a sa elli e o ma ion and he me ging o he sa elli e wi h he main d ople du ing i s ligh , hus
ul illing he de ini ion o good p in abili y when “a single d op is o med ei he di ec ly wi hou a
second pinch-o o he sa elli e d op me ges wi h he main d op wi hin i s a el dis ance less han
20 imes A o ming hus a single d op”. Re e ing back o li e a u e sou ces, a simple and ea lie
Senso s 2018,18, 3246 7 o 17
solu ion in ol ing all ma e ial- ool-p ocess cha ac e is ics may be ound in he wo k o McKinley and
Rena dy [
31
] who ed ew he schema ic diag am o iginally cons uc ed by De by [
32
] o show he ield
o pa ame e s o s able ope a ions o d op on demand inkje p in ing by using loga i hmic coo dina e
sys em de ined by plo ing Ohneso ge agains Reynolds numbe . The g aph cons uc ed wi h he help
o hei de ini ion o p in abili y bounda ies is p esen ed in Figu e 3. The quad angle ABCD de ines
a egion, in which he pa icula luids a e p in able and single d op o ma ion may be achie ed o
me ging wi h he sa elli e can be expec ed. The diag am does no esol e be ween egime I and II as
de ined la e by E. Kim and J. Baek [27].
Senso s 2018, 18, x 7 o 17
The p epa ed suspensions we e in en ionally heo e ically ea ed as New onian luids only,
neglec ing he e en ual weak iscoelas ici y. Such an app oach simpli ies he p oblem adically;
howe e , i can be applied un il i p oduces p edic ions compa able wi h expe imen . Acco ding o
an analysis o ink-je p in ing egimes by E. Kim and J. Baek [30], all p epa ed inks all in o he
“ egime II” which is cha ac e ized by ela i ely small Ca and la ge We numbe s. These condi ions
a e mani es ed by a sa elli e o ma ion and he me ging o he sa elli e wi h he main d ople du ing
i s ligh , hus ul illing he de ini ion o good p in abili y when “a single d op is o med ei he
di ec ly wi hou a second pinch-o o he sa elli e d op me ges wi h he main d op wi hin i s a el
dis ance less han 20 imes A o ming hus a single d op”. Re e ing back o li e a u e sou ces, a
simple and ea lie solu ion in ol ing all ma e ial- ool-p ocess cha ac e is ics may be ound in he
wo k o McKinley and Rena dy [31] who ed ew he schema ic diag am o iginally cons uc ed by
De by [32] o show he ield o pa ame e s o s able ope a ions o d op on demand inkje p in ing
by using loga i hmic coo dina e sys em de ined by plo ing Ohneso ge agains Reynolds numbe .
The g aph cons uc ed wi h he help o hei de ini ion o p in abili y bounda ies is p esen ed in
Figu e 3. The quad angle ABCD de ines a egion, in which he pa icula luids a e p in able and
single d op o ma ion may be achie ed o me ging wi h he sa elli e can be expec ed. The diag am
does no esol e be ween egime I and II as de ined la e by E. Kim and J. Baek [27].
Figu e 3. Map o Oh and Re dimensionless co ela ions space o a p in ing p ocess wi h he
p in abili y a ea ABCD eplo ed acco ding o McKinley and Rena dy [31]. Fo a de ailed desc ip ion
please see ex .
The pu pose o p in ing p ocess op imiza ion is an achie emen o he good p in abili y
condi ions which is an absolu e p e equisi e o ob aining good-quali y hin ilm laye including i s
p ecise loca ion, esolu ion and unc ionali y. We ound i o be use ul o discuss he p in ing
pa ame e s acco ding o he sugges ions o he p in e p oduce and compa e hem wi h ou
expe imen al p ocedu e. The small black ull quad angle in he cen e o he ABCD a ea in Figu e 3
ep esen s he op imum p in abili y space o he Dima ix p in e (i.e., iscosi y 10–12 mPa·s and he
su ace ension in he ange o 28–42 mN·m−1) while he ypical alue o he densi y o ou suspensions
a ies sligh ly be ween 1.1 g·mL−1 and 1.3 g·mL−1, he e o e 1.2 g·mL−1 was chosen o he model. The
d ople eloci y can be a ied by he wa e o m p ocess con ol; none heless ou ypical inal
op imum alue 6 m·s−1 is aken in o accoun in his analysis. Finally, he cha ac e is ic leng h A is
s ic ly gi en by he nozzle geome y in he used p in ing heads and canno be changed a all. The
changes o he a iables in ol ed in he dimensionless c i e ia esul in ypical shi s o ex ensions o
his pa ame e space in he di ec ions indica ed by he six a ows ma ked om a o . Dec easing he
Figu e 3.
Map o Oh and Re dimensionless co ela ions space o a p in ing p ocess wi h he p in abili y
a ea ABCD eplo ed acco ding o McKinley and Rena dy [
31
]. Fo a de ailed desc ip ion please
see ex .
The pu pose o p in ing p ocess op imiza ion is an achie emen o he good p in abili y condi ions
which is an absolu e p e equisi e o ob aining good-quali y hin ilm laye including i s p ecise loca ion,
esolu ion and unc ionali y. We ound i o be use ul o discuss he p in ing pa ame e s acco ding o
he sugges ions o he p in e p oduce and compa e hem wi h ou expe imen al p ocedu e. The small
black ull quad angle in he cen e o he ABCD a ea in Figu e 3 ep esen s he op imum p in abili y
space o he Dima ix p in e (i.e., iscosi y 10–12 mPa
·
s and he su ace ension in he ange o
28–42 mN
·
m
−1
) while he ypical alue o he densi y o ou suspensions a ies sligh ly be ween
1.1 g
·
mL
−1
and 1.3 g
·
mL
−1
, he e o e 1.2 g
·
mL
−1
was chosen o he model. The d ople eloci y can
be a ied by he wa e o m p ocess con ol; none heless ou ypical inal op imum alue 6 m
·
s
−1
is
aken in o accoun in his analysis. Finally, he cha ac e is ic leng h Ais s ic ly gi en by he nozzle
geome y in he used p in ing heads and canno be changed a all. The changes o he a iables
in ol ed in he dimensionless c i e ia esul in ypical shi s o ex ensions o his pa ame e space in
he di ec ions indica ed by he six a ows ma ked om
a
o
. Dec easing he luid eloci y co esponds
o he di ec ion
a
while he use o highe eloci y shi s he a ea in he di ec ion
d
. Inc easing he
su ace ension shi s he pa ame e space bo de downwa ds in di ec ion
while dec easing he
su ace ension esul s in he shi upwa ds in di ec ion
c
. Inc easing he luid iscosi y esul s in he
shi along he a ow
b
and dec easing he iscosi y ex ends he a ea in he di ec ion o he a ow
e
.
Changes in he cha ac e is ic leng h o he densi y will esul in a diagonal shi oo, howe e , he slope
will be
−
1/2. The ull ex ension o he p in abili y span acco ding o he ex eme iscosi ies and he
su ace ensions decla ed by he p in e p oduce is indica ed by he quad angle wi h as e isks in i s
co ne s and he sho dash do sides. The posi ion o he p epa ed inks is ma ked by he ou ull
ci cle da a poin s labelled by ITO concen a ions. I mus be no ed ha he poin s a e aligned along a
i ual line, which has slope
−
1 and ep esen s a line wi h a cons an alue o We
1/2
o abou 6.6 which
co esponds o he a e age We numbe alue om Table 2o abou 43.5. Al hough a good p in abili y
Senso s 2018,18, 3246 8 o 17
was inally achie ed o all composi ions by a ying he pa ame e s, he poin closes o he cen e o
he p in abili y a ea co esponds o he ink composi ion wi h 25 w % o ITO.
Ac ually, he g aph in Figu e 3can be pa ame ised by a se o We
1/2
isolines based on he o mula
in Equa ion (1). These hype bolic isolines a e ep esen ed as s aigh lines using loga i hmic axes.
The simila i y o hese lines wi h he diagonal bo de s o he ABCD a ea in okes somewha he old
idea o he impo ance o a single dimensionless numbe again, We numbe his case. The line AC
co esponds exac ly o he We alue 9 (means We
1/2
= 3). The impo ance o he We c i e ion was
aised by De by himsel again in [
33
], whe e a co ec ed e sion o his o iginal p in abili y g aph is
published. Howe e , We alue 4 is used by De by acco ding o Duine eld e al. [
34
] as a minimum
Webe numbe o a d op gene a ion hus delinea ing he bo de o o e come he su ace ension a
he exposed nozzle. This alue co esponds o he do ed line A’C’ wi h We
1/2
= 2 in he g aph in
Figu e 3, p edic ing a la ge p in abili y a ea. On he o he hand, he splashing h eshold line BD is
no cha ac e ized by one alue o he Webe numbe only. Ac ually, We
1/2
linea ly a ies om 20 o
he poin B o 13 o he poin D. To gene alize his lesson, we belie e ha he exclusi eness o We
among all he o he dimensionless numbe s used o desc ibing he p in abili y o inks i is due o
he p ime impo ance o he su ace ension among all he discussed ink-je ma e ial cha ac e is ics.
The absolu e condi ion sine qua non o p in ing is he o ma ion o he ink d ople s and he e is only
one physical a iable ha is he sou ce o he o ces o ming he d ople sphe ical shape, and i is he
su ace ension. I shall be men ioned ha i he luid is iscoelas ic, he elas ici y will con ibu e o
he d op o ma ion also. Mo eo e , he Webe numbe also con ains he ool (A) and he p ocess (
ν
)
cha ac e is ics in con as o he Znumbe which is based on he ma e ial cha ac e is ics only. In ou
speci ic case, we expe ienced he good p in abili y when dec easing he We alue below 47.
3.2. Annealing Tempe a u e Op imiza ion
Op imizing he annealing empe a u e is he key s ep in pos -p in ing ea men o he
manu ac u ed de ice. Indeed, his impo an ac o (i) in luences g ain size, densi ica ion o po osi y o
me al oxide ilm, which is closely ela ed o he sensing e iciency [
8
,
35
,
36
] u he mo e i (ii) de e mines
he esis ance o he ilm and (iii) allows he emo al o addi i es, which could cause in e ac ions
wi h he gas being sensed and hus in luence he esponse o he senso y laye . Finally, he annealing
empe a u e in luences (i ) he choice o an app op ia e subs a e o he inal de ice. The e o e, a
he mog a ime ic analysis o he p epa ed ink composi ions has been accomplished o obse e he
empe a u e anges co esponding o he e apo a ion o sol en s and he apo iza ion o su ace
ac i e polyme addi i es. A decomposi ion o he ink p oceeded as shown in Figu e 4. In he i s
wo s eps low-molecula subs ances (wa e and e hylene glycol) we e ola ilized. Thei comple e
e apo a ion was achie ed below a empe a u e o 200
◦
C. The comple e decomposi ion s ep o he
p esen su ac an and he dispe san agen s was obse ed in a ange o 300
◦
C o 400
◦
C hence he
lowes annealing empe a u e o p in ed ilms was se o 400
◦
C, and o he samples we e made a
500 ◦C and 600 ◦C o compa ison oo.
The mo phology o o iginal ITO pa icles as well as annealed ilms is shown in Figu e 5a in he
igu e depic s he ITO nanopowde as ecei ed. P esence o a wide size dis ibu ion can be obse ed.
The bigges e agonal bipy amides a e o size below 100 nm bu he as majo i y o pa icles is
signi ican ly smalle and he powde ma e ials gene ally co esponds o he speci ica ion p o ided
by he supplie (<50 nm pa icle size). T ea ing deposi ed laye s a empe a u es o 400
◦
C, 500
◦
C
and 600
◦
C leads o he o ma ion o sepa a ed g ains, which a e in physical con ac , see Figu e 5b–d
espec i ely. Fo ma ion o sho necks be ween indi idual g ains was no con i med and could be
expec ed o highe empe a u es. Thus, he sensing mechanism can be desc ibed using he “g ains” o
“g ain bounda y” model, whe e he Scho ky in e ace be ween he g ains, he heigh o po en ial ba ie
and he e o e he dimension o he deple ion egion a ia ion depends on he ambien a mosphe e
composi ion. The conduc i i y o his ea ed ype o nanoc ys alline me al oxide ope a es by g ain
bounda y space cha ge (band bending) on in e -g ain con ac in e aces as shown in Figu e 6whe e
Senso s 2018,18, 3246 9 o 17
R
gi
ep esen s he a e age in e g ain esis ance, d— he g ain diame e , E
F
— he Fe mi le el, E
C
— he
conduc i e band [35,37].
Senso s 2018, 18, x 9 o 17
Figu e 4. The he mog a ime ic cu e and i s de i a i e eco ded o he ink composi ion wi h ITO
pa icle loading 25 w %.
The mo phology o o iginal ITO pa icles as well as annealed ilms is shown in Figu e 5a in he
igu e depic s he ITO nanopowde as ecei ed. P esence o a wide size dis ibu ion can be obse ed.
The bigges e agonal bipy amides a e o size below 100 nm bu he as majo i y o pa icles is
signi ican ly smalle and he powde ma e ials gene ally co esponds o he speci ica ion p o ided
by he supplie (<50 nm pa icle size). T ea ing deposi ed laye s a empe a u es o 400 °C, 500 °C and
600 °C leads o he o ma ion o sepa a ed g ains, which a e in physical con ac , see Figu e 5b–d
espec i ely. Fo ma ion o sho necks be ween indi idual g ains was no con i med and could be
expec ed o highe empe a u es. Thus, he sensing mechanism can be desc ibed using he “g ains”
o “g ain bounda y” model, whe e he Scho ky in e ace be ween he g ains, he heigh o po en ial
ba ie and he e o e he dimension o he deple ion egion a ia ion depends on he ambien
a mosphe e composi ion. The conduc i i y o his ea ed ype o nanoc ys alline me al oxide
ope a es by g ain bounda y space cha ge (band bending) on in e -g ain con ac in e aces as shown
in Figu e 6 whe e Rgi ep esen s he a e age in e g ain esis ance, d— he g ain diame e , EF— he
Fe mi le el, EC— he conduc i e band [35,37].
Figu e 5. SEM images o ITO pa icles as ecei ed (a) and ilms made om he ink wi h ITO pa icle
loading 25 w % annealed o 400 °C (b), 500 °C (c) and 600 °C (d).
Figu e 4.
The he mog a ime ic cu e and i s de i a i e eco ded o he ink composi ion wi h ITO
pa icle loading 25 w %.
Senso s 2018, 18, x 9 o 17
Figu e 4. The he mog a ime ic cu e and i s de i a i e eco ded o he ink composi ion wi h ITO
pa icle loading 25 w %.
The mo phology o o iginal ITO pa icles as well as annealed ilms is shown in Figu e 5a in he
igu e depic s he ITO nanopowde as ecei ed. P esence o a wide size dis ibu ion can be obse ed.
The bigges e agonal bipy amides a e o size below 100 nm bu he as majo i y o pa icles is
signi ican ly smalle and he powde ma e ials gene ally co esponds o he speci ica ion p o ided
by he supplie (<50 nm pa icle size). T ea ing deposi ed laye s a empe a u es o 400 °C, 500 °C and
600 °C leads o he o ma ion o sepa a ed g ains, which a e in physical con ac , see Figu e 5b–d
espec i ely. Fo ma ion o sho necks be ween indi idual g ains was no con i med and could be
expec ed o highe empe a u es. Thus, he sensing mechanism can be desc ibed using he “g ains”
o “g ain bounda y” model, whe e he Scho ky in e ace be ween he g ains, he heigh o po en ial
ba ie and he e o e he dimension o he deple ion egion a ia ion depends on he ambien
a mosphe e composi ion. The conduc i i y o his ea ed ype o nanoc ys alline me al oxide
ope a es by g ain bounda y space cha ge (band bending) on in e -g ain con ac in e aces as shown
in Figu e 6 whe e Rgi ep esen s he a e age in e g ain esis ance, d— he g ain diame e , EF— he
Fe mi le el, EC— he conduc i e band [35,37].
Figu e 5. SEM images o ITO pa icles as ecei ed (a) and ilms made om he ink wi h ITO pa icle
loading 25 w % annealed o 400 °C (b), 500 °C (c) and 600 °C (d).
Figu e 5.
SEM images o ITO pa icles as ecei ed (
a
) and ilms made om he ink wi h ITO pa icle
loading 25 w % annealed o 400 ◦C (b), 500 ◦C (c) and 600 ◦C (d).
Senso s 2018, 18, x 10 o 17
Figu e 6. A schema ic diag am wi h ene gy bands along he sensing laye .
Highe annealing empe a u es inc ease he con ac be ween he g ains and can lead o he
sin e ing o he g ains in he agglome a es (>500 °C) [38], which con ibu es o elec ic anspo and
inc eases conduc i i y. On he o he hand, lowe annealing empe a u es allow o o ming a mo e
po ous s uc u e and he olume o he laye is mo e accessible o he de ec ed gas, hus, he ac i e
su ace is kep [35,37]. As can be seen in Figu e 7, he o ma ion o a g anula and po ous s uc u e is
e iden . A highe concen a ion o he cha ge due o he densi ica ion o he s uc u e a a highe
annealing empe a u e o 600 °C, and on he con a y a mo e po ous s uc u e in dep h con aining
less neck connec ed g ains a a lowe annealing empe a u e o 400 °C was obse ed using scanning
unnelling mic oscopy combined wi h su ace mo phology and scanning elec on mic oscopy o
p in ed and hea ed ilms.
Figu e 7. The cu en mapping (le side) and he SEM images ( igh side) on he su ace o he p in ed
and annealed ilms a 400 °C ( op), 500 °C (middle) and 600 °C (bo om).
Figu e 6. A schema ic diag am wi h ene gy bands along he sensing laye .
Highe annealing empe a u es inc ease he con ac be ween he g ains and can lead o he
sin e ing o he g ains in he agglome a es (>500
◦
C) [
38
], which con ibu es o elec ic anspo and
inc eases conduc i i y. On he o he hand, lowe annealing empe a u es allow o o ming a mo e
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