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Title

NO2 Sensing Properties of Porous In2O3-Based Powders Prepared byUtilizing Ultrasonic-Spray Pyrolysis Employing PMMA MicrosphereTemplates: Effects of the Size of the PMMA Microspheres on Their Gas-Sensing Properties

Author(s) Fujii, Eriko; Hyodo, Takeo; Matsuo, Katsuhide; Shimizu, Yasuhiro

Citation ECS Transactions, 50(12), 273-278; 2012

Issue Date 2012

URL http://hdl.handle.net/10069/33985

Right

© The Electrochemical Society, Inc. 2012. All rights reserved. Except asprovided under U.S. copyright law, this work may not be reproduced,resold, distributed, or modified without the express permission of TheElectrochemical Society (ECS). The archival version of this work waspublished in ECS Transactions, 50(12), pp.273-278; 2012.

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ECS T":OJl，"dioJls， 50 (12) 27.1-278 (20lZ)

相 TheElcctrochcmical Socicl-，'

NO， Sensing Properties of Porous 1lI，03-bllscd I'owdcrs Prcaprcd by Utilizillg

Ultrasonic-spray Pyrolysis EllIllloyillg PMMA 肘licrosphcr-cTempllltcs

: Effccts ofthc Size ofthe I'MMA Microsphcrcs 011 their Gas-sclIsillg Propcrties

E. Fujii， T 卜Iyodo，K. Matsuo， al1d Y. ShimizlI

Graduate School of Engineering， Nagasaki University 1-14 BUl1kyo-machi， Nagasaki 852-8521， Japal1

PMMA microspheres were sYl1lhesized il1 distilled water by

1Iltrasol1ic-assisted emlllsiol1 polyrnerizatiol1. The average particle

size of PMMA microspheres was depel1del1t rnarkedly 011 the kind

of sllrfactant used. Porolls (pr-) In，03 powders ¥Vere prepared by

ultrasonic叩 raypyrolysis of In(N03)3 aqlleolls solution contail1ing

the PM加lAmicrosph町 田 synthesized.The NO， response of pr-

111203 was much larger than that of conventional In20J powdel

prepared by the similar techl1ique employil1g PMMA一自白 [n(N03)3

aqlleous solution. The il1trodllctiol1 of controlled macroporous strtlcture into the powder of the SenSQr material was founc1 to be

efたctivefor improving N02 response properties

Introdllctioll

As nitTogen oxide (N02) is one of the 11105t hannful gases to the human body and also a C3l1se of air pollutiOl1， highly sensitive anc1 selective N02 SenSQrs at 10w cost， 5mall size and good reliability are indispensable for the detection 10 low concel1tration ofNO， in the

atmosphere. Therefo悶， I1UmerOlls efforts have been directed to optim日川呂 the

microstructuralmorphology of various gas-sensing materials with different sizes of well-

developed pores to improve their gas-sensil1g properties. because optimizatiol1 of the size

and the amount of pores in the gas sensor materials a悶 effectivein controlling gas reactivity (ト6).We also have prepared mesoflorolls al1d macroporous oxides by lItilizing

the assembly of sllrfactal1t (several l1al1ometers 川 size) and commercial

polymethylmethacrylate (PMMA) microspheres C主150 nm in size) as a templatc， respectively (7-13). However， we have 110t yet established a preparation method to

prepare gas-sensing materials having well-developecl middle-sized pores ¥:vith a diametet of several I1m-150 11m. Such tecbl1 iqlle is absolutely essential 日oroptimizing the

microstructural morphology of various gas-sensing materials. In this study， we have attempted to sYl1lhesize smaller PM MA microspheres with controlled particle size (several 11111.....150 11m il1 diameter) by microwave-assisted emulsiol1 polymerization

employil1g different surfactal1ts. 111 additiol1， porous (pr-) In，03 powders were prepared by lI1trasol1ic-spray pyrolysis employil1g tbe sYl1thesized PMMA microspheres， and their

N02-sensing propelties have been investigated

ECS Transactions， 50 (12) 273・278(2012)

Experimental

Svnthesis of PMMA microsohere disoersion bv ultrasonic-assisted emulsion Dolvmerization

PMMA microspheres were synthesized in deionized water (100 cm3) by ultrasonic (19.5 kHz)-assisted emulsion polymerization (50 min) employing methylmethaclγlate monomer (8 g) as a polymer source. ammonillm perslllfate (0.3 g) as an initiator and a surfactant (0.1 g) and sllbsequent stirring (400 rpm) at 600C tor 6 h. The surfactant used was sodillm lauryl sulfate (SLS， CH3(CHUIIOS03Na)， Triton X・100 (Triton， (C2H40) lOC 14H220) 01' P 123 (E020P070E020ヲ EO:polyethylene oxide， PO: polypropylene oxide). The PMMA microspheres in the resultant dispersions were denoted as PMMA(M) (M: surfactant employed tor the synthesis of PMMA microspheres (SLS. Triton or P123)).

Preoaration of OOrollS In'201・basedoowders bv utilizing llltrasonic-sorav ovrolvsis emolovine: PMMA microsoheres

The PMMA microsphere dispersion (37.5 cm3) was mixed with 0.05 mol dm・3ln(N03)3 aqlleous solution (62.5 cnr') to make a precllrsor sollltion. Atter the precursor solution was set in a plastic container. the mist of the precllrsor solution was generated by ultrasonic irradiation (2.4 MHz) and then it was ted to an electric furnace at 11000C under air t10wing (1.5 dm3 min

o1) by using a feeding system as shown in Fig. 1 and then

was directly heat-treated in the furnace. The pr-11l203 powder obtained was denoted as pr・In203(M). COllventional In203 powder (c・In203)was also prepared by the similar technique employing PMMA-free precursor solution.

‘一---ミ¥¥Air(1500 cm3 minぺ)

Plastic conlainer

‘一一

Fiqure 1. Schematic drawing of a teeding system of mist of precursor solution atomized by ultrasonication (2.4 MHz).

Characterization of PMMA microsoheres and In'201 oowders

The particle size distriblltion of the PMMA microspheres obtained was measured by dynamic light scattering (DLS‘ Malvern lnstrllment Ltd.‘HPPS). The microstructure of PMMA microspheres and 1n203 powders was observed with scanning electron microscope (SEM: JEOL Ltd.. JSM-7500F). The specitic sllrnlce area (SSA) and pore size distribution of pr・In203(M)and c-ln203 powders were measllred by the srllnauer-Emmett-Teller (BET) and Barret-Johner・Halenda(BJH) methods. respectively. using a N2 adsorption isotherm (Micromeritics Instrument Co叩..Tristar3000).

ECS Transnclions. 50 (12) 273・278(2012)

Gas resoonse measlIrement

Thick日1msensors were fabricated by screen printing employing the paste of each 111，03 powder prepared with orgal1ic lacquer (Goo Chemical Co.， Ltd.， OS-4530) 011 al1 alumil1a substrate equipped 、vitha pair of il1terdigitated Pt electrodes and subsequel1t heat-汀ealmel1tat 5500C for 5 h. Gas response properities ofthese sel1sors were measllred to 10 ppm NO， il1 air at 150-5000C. The magl1itude ofNO， response was defil1ed as the ratio (RglIし)of sel1sor resistance aRer 10 111il1 exposllre to NO， balal1ced with air (Rg) to thal il1 air (1し)

Rcsll1ts ，md disclIssions

Figures 2 and 3 show particle size distriblltiol1s al1d SEM photographs of all PMMA microspheres synthesized by the microwave-assisted emulsion polymerization， respectively. The average particle size of PMMA microsphe問 swas depel1del1t markedly on the kind of the surfactal1t used. The particle size distributiol1 of PMMA(SLS) was narrow; and lhe meal】 particlesize of PMMA(SLS) was ca. 55.5 nm. As showl1 il1 SEM photographs， the partic1es of PMMA(SLS) are relatively IIniform al1d the size was slight1y smaller thal1 that estimated by the DLS (Fig. 2). 011 the other hal1d， the particle sizes of PMMA(Triton) al1d PMMA(P 123) were relatively large (ca. 106 11m PMMA(Tritol1) al1d ca. 159 11m: PMMA(P 123)) al1d their particle size distriblltions were broader thal1 that of PMMA(SLS). Their SEM photographs showed that particle sizes of PMMA(Trilon) and PMMA(PI23) were lIniform and larger than that ofPMMA(SLS)

ポ 35、

i 30 ~ 25 O 20 』

s 15 E e 10 ~ 5 2 。υ区 、 10 102

Oiameter I nm

Figllre 2. Particle size distriblllions of PMMA(M) microspheres synthesized by microwave-assisted emulsion po[ymerization， togelher with their average diameter in parentheses

Figure 3. SEM photographs of PMMA(M) microspheres sYl1thesized by microwave-assisted emulsioI1 polymerization.

ECS Tr:lJIS~II: li l)fl ~ ， SO (12) 27.1-278 (20"12)

Figllres 4 and 5 show pore size distriblltions together with specific surface area (SSA) and SEM photographs of pr-In，O，(M) and c-In，O， powders， respectively. The c-[n，03

powder showed slllall SSA (3.3 m' g-') and pore volllme， and its morphology was almost

spherical (100-600 nm in diameter) and relatively dense. On the other hand， SSA and pore volume 01" the pr-fn，03(M) powders were larger than those 01" the c-fn，O， powder， and the SEM. photographs revealed that well-developed pores were forl11ed in the pl

[n，03(M) powders. However， the morphology of pores and the pore size distriblltions were dependent on the kind of PMMA l11icrospheres used. Nal11ely， the pr-fn，O，(SLS) powder had mediull1 pores with a centered diameter 01"' ca. 30 nll1， which was probably

originated frol11 the morphology of PMMA(SLS)， as shown in Fig. 4， and small pores

with a diameter less than ca. 10 11m hardly existed in the pr-ln，O)(SLS) powder. On the

other hand， pl・In，03(Triton)and pr-In，03(PI23) powders had well-developed small pores

(iess than 10 nm in dial11eter) as well as medillm pores (ca. 50 nm in centered diameter)

which were confirmed from Fig. 4， along with large pores (more than 100 nm in

diameter) which wel宅 confirmedfrom Fig. 5 (c) al1d (d)

Figure 4. Pore size distriblltion ofpr-ln，03(M) al1d C-II1，03 powders

Figure 5. SEM photographs ofpr-ln，03(M)削ldc-1 1120] powders

ECS T"3IlS:II:liOIlS) 50 (12) 273-278 (201.2)

Figure 6 shows response lransients of aJl sensors 10 10 ppl1l NO， in air al 400oC. The

l1lagnitllde ofN02 responses of aJl pr-lnl03(M) sensors was 11l1lch larger than that of a c-1nlOJ sensor， while the sensor resistance of aJl the pト1n，OJ(M)sensors in air was 11l1lch smaller than that of the c-ln20j sensor. Among 川Ithe pr-1n，03(M) sensors， the pト

1n，03(SLS) selisor sbowed the 1argest response to 10 ppl1l N01 (Rg/R， = 36.0) and the fastesl response and recovery speeds

4.0 Gas in

iEEE 30 E 25

JH 2.0

15O L 5 10 15 20 25 30

Time I min

Figllre 6. Response transients ofaJl sensors to 10 ppl1l N01 in air al 4000C.

Figure 7 show operating tel1lperature dependence of the magnitllde of response (a) and 90% response til1le (b) of pr-ln103(M) and c・In10J sel1sors to 10 ppl1l NO， il1 ai， The magl1itude of NO， responses of aJl the pr-1I1，OJ(M) sensors was l1luch 1arger than that of c-1n，03 sensor il1 the tel1lperature l'ange of 250-5000C. AJI the pl・1n103(M)

sensors showed the largest response at 3000C， and the response at lower temperatures (三

200oC) was relatively smal!. 011 the other hand， aJl sensors showed extrel1le1y slow

respol1se speeds (over 8 mil1) at lower temperatllres (壬 300oC).However， the respol1se

speed of pr-1n，03(SLS) sensors was faster thal1 oth引 sensorsat higher temperatures (~

300oC)

200 250 300 350 400 450 500 1日emperatureI ~C

SR g 2

150 200 250 300 350 400 450 500 Temperature I ~C

Figllre 7. Operating tel1lperature dependence of the magnitude 01' respol1se (a) and 90% respol1se time (b) ofpト In103(M)and c-1n10J sel1sors to 10 ppm NO， in air

Conclusions

PMMA l1licrospheres with relatively-controJled particle dial1leter (PMMA(M)) were easi1y synthesized by microwave-assisted ell1l1lsion po1ymerization emp10ying a typica1

ECS T，・IIlSfl叶仙"，，50(12) 273-278 (2012)

sllrfaClal1t (M: SLS， Tritol1 Or 1'123). The PMMA(SLS) was the most ul1ifolln among them. pr-111203 powde目 wereprepared by 1I1lrasol1ic-spray pyrolysis employil1g the synthesized PMMA microspheres. The morphology of pores and the pore size distribl1tiol1s were depel1del1t 011 the kind of PMMA microspheres l1sed. The magl1itl1de of O2 respol1ses of all the pr-II1，03(M) sel1sors was larger thal1 that of C-l1120J sel1Sor. The

introduction of controlled macroporous stl'l1cture into the powder of the sensor material was口ol1ndto be effective for illlproving NO， response properties

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