A New Strategy for Humidity Independent Oxide Chemiresistors:...

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A New Strategy for Humidity Independent Oxide Chemiresistors: Dynamic Self-Refreshing of In 2 O 3 Sensing Surface Assisted by Layer-by-Layer Coated CeO 2 Nanoclusters

Ji-Wook Yoon , Jun-Sik Kim , Tae-Hyung Kim , Young Jun Hong , Yun Chan Kang , and Jong-Heun Lee *

1. Introduction

The chemiresistive gas sensing property of metal oxide semi-

conductors arises from the change in the carrier density due

to the interaction between the gas and negatively charged

oxygen ions adsorbed on the oxide surfaces. [ 1–4 ] Irreplaceable

DOI: 10.1002/smll.201601507

The humidity dependence of the gas sensing characteristics of metal oxide semiconductors has been one of the greatest obstacles for gas sensor applications during the last fi ve decades because ambient humidity dynamically changes with the environmental conditions. Herein, a new and novel strategy is reported to eliminate the humidity dependence of the gas sensing characteristics of oxide chemiresistors via dynamic self-refreshing of the sensing surface affected by water vapor chemisorption. The sensor resistance and gas response of pure In 2 O 3 hollow spheres signifi cantly change and deteriorate in humid atmospheres. In contrast, the humidity dependence becomes negligible when an optimal concentration of CeO 2 nanoclusters is uniformly loaded onto In 2 O 3 hollow spheres via layer-by-layer (LBL) assembly. Moreover, In 2 O 3 sensors LBL-coated with CeO 2 nanoclusters show fast response/recovery, low detection limit (500 ppb), and high selectivity to acetone even in highly humid conditions (relative humidity 80%). The mechanism underlying the dynamic refreshing of the In 2 O 3 sensing surfaces regardless of humidity variation is investigated in relation to the role of CeO 2 and the chemical interaction among CeO 2 , In 2 O 3 , and water vapor. This strategy can be widely used to design high performance gas sensors including disease diagnosis via breath analysis and pollutant monitoring.

Gas Sensors

J.-W. Yoon, J.-S. Kim, T.-H. Kim, Y. J. Hong, Prof. Y. C. Kang, Prof. J.-H. Lee Department of Materials Science and Engineering Korea UniversityAnam-dong Seongbuk-gu, Seoul 136-713 , Republic of Korea E-mail: [email protected]

advantages of oxide chemiresistors, such as high sensitivity

and a simple sensing mechanism, allow the detection of trace

concentrations of analyte gases, the facile integration of gas

sensors or sensor arrays into a small device, and the realiza-

tion of cost-effective artifi cial olfaction. [ 5–11 ] However, atmos-

pheric water vapor (H 2 O) reacts with oxygen ions on the

surface and forms less reactive hydroxyl groups (OH) prior

to the gas sensing reaction, [ 12,13 ] which signifi cantly changes

the sensor resistance and deteriorates gas response.

The water vapor concentration in ambient air (1 atm,

25 °C) is 6280 ppm at relative humidity (r.h.) of 20% and is

as high as 25 740 ppm at r.h. of 80%. In addition, r.h. levels

dynamically change depending on the climate (e.g., wind,

clouds, and rainfall), region, and season. Accordingly, the

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humidity dependence of oxide semiconductor gas sensors has

been the greatest obstacle for widespread applications since

their discovery in the 1960s. [ 14 ]

Recently, various oxide chemiresistors have been inves-

tigated in order to design sensitive exhaled breath gas sen-

sors for use in highly humid atmospheres (e.g., r.h. > 80%),

including Pt-SnO 2 , [ 15 ] Pt-WO 3 ,

[ 16 ] Si-WO 3 , [ 17 ] and Au-

In 2 O 3 . [ 18,19 ] However, these studies did not investigate the

humidity dependence of the gas sensing characteristics. To

date, the control of sensor temperature [ 20 ] or the change

of adsorbed oxygen species [ 21 ] in Pd-loaded SnO 2 and the

loading or doping of NiO, [ 22 ] and CuO [ 23 ] with high affi nity

to water molecules to SnO 2 have been explored to suppress

the humidity dependence of the gas sensing characteristics.

However, the design of humidity independent gas sensors

remains in a nascent stage and needs further breakthroughs

based on a novel strategy that is valid in high and continu-

ously changing humidity.

In this contribution, we suggest a new strategy for

designing humidity independent gas sensors by self-refreshing

of the In 2 O 3 sensor surface assisted by CeO 2 nanoclusters

deposited by layer-by-layer (LBL) coating. The key idea is to

maintain the same concentration/confi guration of adsorbed

oxygen ions regardless of the humidity via dynamic regen-

erative interaction between CeO 2 nanoclusters and the In 2 O 3

sensing surfaces. To demonstrate this novel concept, the gas

sensing characteristics of In 2 O 3 hollow spheres coated with

different concentrations of CeO 2 nanoclusters were system-

atically investigated. In 2 O 3 hollow spheres uniformly loaded

with an optimal concentration of CeO 2 produced sensors

with unprecedented humidity independence. The focus of this

work was to understand the mechanism of the self-refreshing

sensor surface and suggest a new oxide semiconductor gas

sensor without humidity dependence.

2. Results and Discussion

The concentrations of Ce in the CeO 2 -loaded In 2 O 3 hollow

sphere samples were determined to be 1.04, 2.33, 4.97, 11.7,

22.4, 39.9, 45.6, and 55.0 wt% (Ce:In) by inductively coupled

plasma analysis. Hereinafter, the specimens will be referred

to as “ M Ce-In 2 O 3 ” ( M = Ce concentration in wt% = 1.04,

2.33, 4.97, 11.7, 22.4, 39.9, 45.6, and 55.0). The scanning elec-

tron microscopy (SEM) images of In 2 O 3 and Ce-In 2 O 3

hollow spheres are shown in Figure 1 . The In 2 O 3 hollow

spheres prepared by ultrasonic spray pyrolysis showed clean

surfaces and their diameters ranged from 500 to 1000 nm

(Figure 1 a). Overall, the morphologies of all the Ce-In 2 O 3

hollow spheres were similar (Figure 1 b–g). In heavily Ce

loaded specimens (45.6 and 55.0 Ce-In 2 O 3 ), nanosheets and

nanocubes were frequently observed in addition to hollow

spheres (Figure 1 h,i).

Hollow morphologies of In 2 O 3 ( Figure 2 a–c) and

11.7 Ce-In 2 O 3 spheres (Figure 2 d–f) were confi rmed by

transmission electron microscopy (TEM). The surfaces of

the In 2 O 3 hollow spheres were clean (Figure 2 a) with shells

≈15 nm thick (Figure 2 b). In lattice-resolved TEM images

(Figure 2 c), two lattice planes were observed with the same

interplanar distance of 2.92 Å and an angle of 70° which

corresponded to the (222) and (222) fringes of the cubic

In 2 O 3 structures (Figure 2 c). After LBL coating of CeO 2

nanoparticles onto the In 2 O 3 hollow spheres and subsequent

heat treatment at 500 °C for 3 h (11.7 Ce-In 2 O 3 specimen),

the shell thickness remained similar but CeO 2 nanoclus-

ters (≈5 nm) were observed on the surface of the hollow

spheres (Figure 2 d,e). The compositional profi le of In and

Ce from energy dispersive X-ray spectroscopy (EDS) anal-

ysis revealed a Ce-rich composition at the outer surface of

the In 2 O 3 hollow spheres (line plot in Figure 2 e). For more

precise analyses, EDS line scanning and elemental map-

ping were performed on the sectioned spheres prepared by

focused-ion-beam (FIB) treatment of a single hollow sphere

(shown in Figure S1, Supporting Information). The results

clearly showed that most of the Ce was present on the outer

part of the shell. The CeO 2 phase of the nanoparticles on the

surface of the 11.7 Ce-In 2 O 3 hollow spheres was confi rmed

by observing high resolution lattice fringes and fast Fourier

transformation patterns (Figure 2 f). The scanning TEM ele-

mental mapping showed that CeO 2 nanoparticles were uni-

formly coated onto the In 2 O 3 hollow spheres (Figure 2 g).

The morphologies of 22.4, 39.9, 45.6, and 55.0 Ce-In 2 O 3

hollow spheres were observed by TEM (shown in Figures S2

and S3, Supporting Information). The CeO 2 nanoclusters on

the surface of 22.4 and 39.9 Ce-In 2 O 3 hollow spheres were

larger (15–30 nm) than the samples with lower Ce concen-

trations (shown in Figure S2a–c,e–g, Supporting Informa-

tion). The number density and the sizes of nanoclusters

further increased as the Ce concentration increased (red

arrows in Figure S2a,e, Supporting Information). In 45.6 and

55.0 Ce-In 2 O 3 hollow spheres, many nanosheets and nano-

cubes of CeO 2 (shown in Figure S3c,g, Supporting Informa-

tion) were observed in addition to the hollow spheres (shown

in Figure S3a,b,e,f, Supporting Information). Because a uni-

form distribution of CeO 2 nanoparticles was observed on

the In 2 O 3 hollow spheres regardless of the Ce concentration

(Figure 2 g and Figures S2d,h and S3d,h (Supporting Infor-

mation)), the maximum Ce loading concentration, without

coarsening of the CeO 2 nanoclusters or formation of CeO 2

nanosheets and nanocubes can be set as ≤11.7 wt%.

The confi gurations of the CeO 2 nanoparticle coatings

were also analyzed by measuring the pore size distributions

and specifi c surface areas determined from N 2 adsorption/

desorption (shown in Figure S4, Supporting Information).

Both the pure In 2 O 3 and 1.03 and 2.33 Ce-In 2 O 3 hollow

spheres showed micropores (≈2 nm), as shown by the

green arrows in Figure S4a2–c2 (Supporting Information),

respectively. In stark contrast, the micropores disappeared

abruptly when the Ce concentration reached 4.97 and were

also not observed for the 11.7 Ce-In 2 O 3 sample (green

arrows in Figure S4d2,e2, Supporting Information). This sug-

gests that the micropores were blocked by the CeO 2 nano-

particle coating. In 22.4 and 39.9 Ce-In 2 O 3 , a large volume

of mesopores (≈5 nm, blue arrows in Figure S4f2–i2, Sup-

porting Information) were formed, which can be attributed

to the interparticular pores within coarsened nanoclusters,

nanosheets, and nanocubes of CeO 2 . The specifi c surface area

of pure and Ce-In 2 O 3 hollow spheres gradually decreased

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from 18.5 to 15.4 m 2 g −1 as a function of Ce concentration

until 4.97 wt% (shown in Figure S4a3–d3, Supporting Infor-

mation) and then started to increase again as the Ce loading

concentration increased (shown in Figure S4e3–h3, Sup-

porting Information), consistent with the variation in pore

volume.

The phase and crystallinity were examined by X-ray dif-

fraction ( Figure 3 ). All specimens showed the cubic In 2 O 3

(JCPDS #06-0416) and CeO 2 (JCPDS #34-0394) phases and

no second phase was found (Figure 3 a1–i1). No obvious

peak shifts were observed, even with high Ce loading con-

centrations, indicating that Ce ions were not incorporated

into the In 2 O 3 lattice after heat treatment at 500 °C for 3 h

(Figure 3 a2–i2,a3–i3). The ionic radius of In 3+ at a coordina-

tion number (CN) of 6 is 0.80 Å, which is comparable to that

of Ce 4+ (0.87 Å at CN = 6) and signifi cantly smaller than that

of Ce 3+ (1.01 Å at CN = 6). Hence, the substitution of Ce 4+

for In 3+ is possible but was not observed in our experiments,

probably because the heat-treatment temperature (500 °C)

was too low for a solid state reaction to occur. [ 24 ] Gerasimov

et al. [ 25 ] suggested that up to 40 wt% of CeO 2 is not dissolved

in In 2 O 3 after heat treatment at 550 °C from the gradual

increase of electrical resistance with the addition of CeO 2

to In 2 O 3 . And this is consistent with the present result. The

CeO 2 peaks were observed when the loading concentration

of Ce was higher than 22.4 wt% (Figure 3 a4–i4). The absence

of CeO 2 peaks in samples with a Ce content ≤11.7 wt% means

that the small size of CeO 2 nanoparticles with relatively low

crystallinity are uniformly distributed without agglomeration

or particle coarsening.

The dynamic sensing transients of In 2 O 3 and Ce-In 2 O 3

sensors exposed to 20 ppm acetone at 450 °C in dry and

humid (r.h. = 20%, 50%, and 80%) atmospheres are shown

in Figure 4 . All the sensors showed n-type semiconducting

behavior, in which the sensor resistance decreased upon

exposure to acetone (reducing gas) and recovered the initial

sensor resistance in air (oxidizing atmosphere). The sen-

sors showed less than 10% fl uctuation of both response

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Figure 1. Scanning electron microscopy (SEM) images of a) pure In 2 O 3 hollow spheres and b–i) Ce-In 2 O 3 hollow spheres; b) 1.04, c) 2.33, d) 4.97, e) 11.7, f) 22.4, g) 39.9, h) 45.6, and i) 55.0 Ce-In 2 O 3 .



( S = R a R g −1 , R a : resistance in air, R g : resistance in gas)

and R a values. The gas sensing transients of the 45.6 and

55.0 Ce-In 2 O 3 sensors are not shown in Figure 4 because nei-

ther sensor showed reproducible gas sensing characteristics

( S and R a ) probably due to the interaction between the water

vapor and large CeO 2 secondary particles, such as nanosheets

and nanocubes. The pure In 2 O 3 sensors showed a high

response to acetone ( S = 22.2) in a dry atmosphere. However,

the response signifi cantly decreased in humid atmospheres

(e.g., S = 4.76 at r.h. = 80%) (Figure 4 a) and the R a value also

decreased signifi cantly with increasing humidity (e.g., 16.0 kΩ

in dry, 4.9 kΩ at r.h. 80%). These are typical characteristics

of humidity dependence for n-type oxide semiconductor gas

sensors such as SnO 2 [ 22 ] and ZnO. [ 26 ]

The humidity dependence of the gas sensing character-

istics steadily decreased (Figure 4 b–d) with increasing Ce

loading concentration and became nearly independent when

Ce loading concentration reached 11.7 wt% (Figure 4 e).

The 11.7 Ce-In 2 O 3 sensor showed relatively high responses

( S = 4.69 in dry, 4.46 in r.h. 80%) and short 90% response

times ( τ res = 5 s in dry, 4 s in r.h. 80%). It is noteworthy

that the 90% recovery times of the 11.7 Ce-In 2 O 3 sensor

( τ recov = 117 s in dry; 16 s at r.h. 80%) are signifi cantly shorter

than those of the pure In 2 O 3 sensor ( τ recov = 1012 s in dry,

34 s at r.h. 80%). Further increases in the Ce concentration

to 22.4 and 39.9 wt% slightly increased the humidity depend-

ence of the gas sensing behavior (Figure 4 f,g). The overall gas

sensing characteristics such as R a R g −1 , τ res , τ recov , and R a of the

sensors in dry and humid atmospheres (r.h. = 20%, 50%, and

80%) are summarized in Figure S5 (Supporting Information).

Generally, gas response as a function of sensor temperature

shows bell-shaped curve. It has been reported that the tem-

perature for maximum gas responses ( T M ) decreases with

adding CeO 2 to In 2 O 3 . [ 25 ] Thus, the decrease of gas response

with increasing Ce concentration to 2.33 wt% can be

explained by the shift of T M to lower temperature although

the subsequent increase of gas response by further increasing

Ce concentration to 11.7 wt% needs more investigation.

To quantify the humidity dependence of the gas sensing

characteristics, we calculated the ratios of the gas responses

( S wet / S dry ) and the resistances in air ( R a-wet / R a-dry ) ( Figure 5 ).

That is, S wet / S dry = 1 and R a-wet / R a-dry = 1 mean that the meas-

ured response and sensor resistance values, respectively, were

the same in wet or dry atmospheres and hence there was no

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Figure 2. Transmission electron microscopy (TEM) images of a–c) pure and d–f) 11.7 Ce-In 2 O 3 hollow spheres. g) Elemental mapping of 11.7 Ce-In 2 O 3 hollow spheres.



observed humidity dependence. Pure In 2 O 3 hollow sphere

sensors showed very low S wet / S dry values (0.37, 0.25, and 0.21

in r.h. = 20%, 50%, and 80%, respectively) (Figure 5 a). On

the contrary, the S wet / S dry values of the 1.04 Ce-In 2 O 3 sensor

were higher than 1.2 regardless of the humidity conditions,

indicating that gas responses in humid conditions were rather

higher than those in a dry atmosphere (Figure 5 a). The

S wet / S dry values gradually decreased as the Ce concentration

increased further, reaching around 1.0 at a Ce concentration

of 11.7 wt% (0.97 at r.h. 20%, 0.96 at r.h. 50%, and 0.95 at

r.h. 80%). On the other hand, the R a-wet / R a-dry values gradu-

ally increased as the Ce concentration increased, and fi nally

showed a value ≈1.0 at the Ce concentration of 11.7 wt%

(0.95 at r.h. 20%, 0.96 at r.h. 50%, and 0.97 at r.h. 80%). How-

ever, both the S wet / S dry and R a-wet / R a-dry values decreased to

0.85 for the 22.4 and 39.9 Ce-In 2 O 3 sensors (Figure 5 a,b).

These results clearly show that the 11.7 Ce-In 2 O 3 sensor can

detect acetone with a high response, rapid response/recovery

speed, and negligible humidity interference.

The humidity dependence of the acetone sensing char-

acteristics of pure and 11.7 Ce-In 2 O 3 sensors were system-

atically investigated further ( Figure 6 ). The measurements

were performed using the following method. After the sensor

resistance became constant in a dry atmosphere, the atmos-

phere was rapidly changed to 20 ppm acetone for 300 s, and

then changed back to the dry atmosphere for 1100 s to inves-

tigate the acetone sensing characteristics of the sensors in a

dry atmosphere (cycle #1). After changing the atmosphere to

humid conditions (r.h. = 20%, 50%, or 80%) for 300 s, the

four sensing transients to 20 ppm acetone were measured

by switching between humid air (for 300 s) and humid ace-

tone (for 300 s) (cycles #2–5). Both pure and 11.7 Ce-In 2 O 3

sensors showed abrupt drops in the resistance immediately

after exposure to humidity (green arrows in Figure 6 a1,b1).

However, the sensor resistances showed different behav-

iors before reaching steady state values. For the pure In 2 O 3

sensor, the R a value in a humid atmosphere remained

decreased compared to the initial R a level under dry condi-

tion, even after 2700 s. Note that the gas response deterio-

rated signifi cantly in a humid atmosphere (Figure 6 a1). The

S wet / S dry (Figure 6 a2–a4) and R a-wet / R a-dry values (Figure 6 a5–

a7) of the pure In 2 O 3 sensor were ≈0.3 and 0.4, respectively,

indicating a strong humidity dependence. In stark contrast,

the R a value for the 11.7 Ce-In 2 O 3 sensor in a humid atmos-

phere readily recovered to ≈0.9 of the R a in a dry atmosphere

within <300 s (Figure 6 b1). Both S wet / S dry (Figure 6 b2–b4)

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Figure 3. X-ray diffraction (XRD) patterns of a) pure, b) 1.04, c) 2.33, d) 4.97, e) 11.7, f) 22.4, g) 39.9, h) 45.6, and i) 55.0 Ce-In 2 O 3 hollow spheres.



and R a-wet / R a-dry (Figure 6 b5–b7) were ≈0.9. Humidity-

independent gas responses and sensor resistances as well as

rapid response times upon exposure to humidity can provide

highly reliable and precise gas sensing platform regardless of

humidity variation.

The gas sensing characteristics ( S and R a ) of oxide semi-

conductor chemiresistors are determined by the concentra-

tion and reactivity of negatively charged oxygen adsorbed

on the surface. In general, when an n-type oxide semicon-

ductor is exposed to reducing gas under humid atmosphere,

both the water vapor and the reducing gas react with the

negatively charged adsorbed oxygen and release electrons to

the semiconductor, as shown by the following equation. [ 12,13 ]

Accordingly, the introduction of water vapor generally

decreases the sensor resistance and the gas response

O H O( ) 2OH e

water vapor chemisorption

(M) 2 (M)v+ → + −

( 1)

The concentration of water vapor is extremely high in

humid atmosphere. Accordingly, it is very challenging to

design highly sensitive oxide semiconductor gas sensors

that can detect sub-ppm and/or several-ppm levels of a spe-

cifi c gas without interference from the humidity. In addition,

the water vapor chemisorption (reaction ( 1) ) mainly occurs

at typical sensor operation temperatures (200–450 °C), and

the complete removal of adsorbed sur-

face hydroxyl groups only occurs above

500 °C. [ 12,27 ] From these viewpoints, it is

not only challenging but also essential,

to avoid water adsorption and the con-

sequent deterioration of the gas sensing

characteristics at 200–450 °C without peri-

odic refreshing of the sensor by heat treat-

ments at >500 °C.

A humidity independent gas response

and sensor resistance was achieved by

loading 11.7 wt% CeO 2 onto an In 2 O 3

sensor (Figure 6 b), indicating that the

CeO 2 nanoparticles play the key role in

eliminating the interaction between the

In 2 O 3 surface and water vapor. Recently,

Prabhakaran et al. [ 28 ] and Wang et al. [ 29 ]

have reported that CeO 2 nanoparticles

can minimize the degradation of proton

exchange membranes (PEM) induced by

free radical (OH and OOH) generation

small 2016, 12, No. 31, 4229–4240

0.0 0.5

Sw

et/S

dry

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Ce concentration (wt%)

1 10 100 0.0 0.5

Ra-

wet

/Ra-

dry

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Ce concentration (wt%)

1 10 100

a b

r.h. 20 %

r.h. 50 %

r.h. 80 %

~ 1 ~ 1

Linearscale

Logscale

Linearscale

Logscale

x axis x axis

Figure 5. a) S wet / S dry and b) R a-wet / R a-dry of pure and Ce-In 2 O 3 hollow spheres exposed to 20 ppm of acetone at 450 °C.

Figure 4. Gas sensing transients of a) pure, b) 1.04, c) 2.33, d) 4.97, e) 11.7, f), 22.4, and g) 39.9 Ce-In 2 O 3 hollow spheres to 20 ppm acetone at 450 °C in dry and humid conditions (r.h. = 20%, 50%, and 80%).



during long-term operation of PEM fuel cells (PEMFC). The

CeO 2 nanoparticles can scavenge free radicals via the fol-

lowing reactions (reactions ( 2) and ( 3) ) induced by regenera-

tive oxidation/reduction of Ce 4+ and Ce 3+ .

OH Ce H Ce H O

free radical scavenging

3 4+2+ + → ++ +

( 2)

4Ce 2H O 4Ce 4H O Ce

regeneration

42

32

3+ → + ++ + + +

( 3)

In PEMFC, OH radicals can be effectively removed by

the reaction shown in reaction ( 2) because hydrogen ions

(H + ) are continuously supplied through the anode. Similarly,

in the present study, the hydrogen ions can be provided to

the In 2 O 3 surface when Ce-In 2 O 3 is exposed to water vapor

(H 2 O), according to the reaction shown in reaction 4, [ 30 ]

and the hydroxyl groups (OH) on the In 2 O 3 surface can be

effectively scavenged according to the following reactions

(reactions ( 4) , ( 5) , ( 6) , Scheme 1 ). When Ce-In 2 O 3 is exposed

to water vapor (Scheme 1 a), Ce 4+ ions are reduced to Ce 3+ and

hydrogen ions (H + ) are generated as shown in reaction ( 4)

(Scheme 1 b). The generated Ce 3+ and H + scavenge hydroxyl

groups (reaction ( 6) ) formed by the reaction between In 2 O 3

and water vapor (reaction ( 5) , Scheme 1 c). The rapid sensing/

recovery kinetics to steady-state signal (Figure 4 e) and the

R a-wet / R a-dry values close to unity (Figure 5 b) indicate that

scavenging reaction ( 6) is not rate-limited by the kinetics of

proton generation reaction ( 4)

4Ce 2H O( ) 4Ce 4H O

proton generation on CeO

4+2

3+2

2

v+ → + ++

( 4)

O H O( ) 2OH e

hydroxyl radical generation onIn O

(In) 2 (In) In

2 3

v+ → +− −

( 5)

OH Ce H Ce H O( )

hydroxyl radical scavenging

(In)3+ 4+

2 v+ + → ++

( 6)

The CeO 2 nanoparticles not only play the role of hydroxyl

group scavenger, but also supply oxygen to the In 2 O 3 sur-

face. The CeO 2 nanoparticles can store the generated oxygen

(reaction ( 4) ); CeO 2 is known to be an excellent oxygen res-

ervoir. [ 31 ] The oxygen can diffuse mainly along the surface

because of the highly defective and basic properties of CeO 2

and bulk oxygen diffusion is also feasible at 300–450 °C. [ 32 ]

The stored oxygen in CeO 2 nanoparticles can thus migrate

via both surface and bulk diffusion to the In 2 O 3 surface,

and readily reionize by capturing electrons on In 2 O 3 surface

(reaction ( 7) , Scheme 1 d)

12

O e O

oxygen readsorption on In O

2 (In) (In)

2 3

+ →− −

( 7)

The fast migration and readsorption of oxygen was con-

fi rmed by the improvement in the recovery characteristics

by loading CeO 2 onto In 2 O 3 sensors (shown in Figure S5d,

Supporting Information). No generation of excess electron

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Figure 6. Dynamic gas sensing transients of a1) pure and b1) 11.7 Ce-In 2 O 3 hollow spheres to 20 ppm acetone at 450 °C in dry and humid conditions (r.h. 20%, 50%, and 80%). The S wet / S dry and R a-wet / R a-dry values of a2–a7) pure and b2–b7) 11.7 Ce-In 2 O 3 hollow spheres exposed to 20 ppm of acetone at 450 °C in dry and humid conditions (r.h. 20%, 50%, and 80%).



through the reactions ( 4) , ( 5) , ( 6) , ( 7) is supported by the

humidity independent sensor resistance in the present study

(Figure 6 b5–b7). Scheme 1 a–d (involving reactions ( 4) , ( 5) ,

( 6) , ( 7) ) can occur in a repetitive manner, providing a novel

mechanism for the regenerative refresh of the In 2 O 3 surfaces

and Ce 4+ /Ce 3+ by OH scavenging and oxygen readsorption.

Therefore, the CeO 2 nanoparticles on the In 2 O 3 surface play

a key role in (1) scavenging of hydroxyl groups (OH) on the

In 2 O 3 surface, and (2) maintaining a constant concentration of

negatively charged oxygen ions by assisting oxygen ion read-

sorption. This mechanism results in the observed humidity

independent gas response and sensor resistance. Note that the

resistance in air ( R a ) of the 11.7 Ce-In 2 O 3 sensor decreased

immediately after exposure to humidity and rapidly recovered

to the R a value in a dry atmosphere (green

arrow in Figure 6 b1), indicating that a

series of above reactions ( 4) , ( 5) , ( 6) , ( 7)

occur at high speed at 450 °C.

The 11.7 Ce-In 2 O 3 sensor exhibited

approximately the same acetone sensing

characteristics ( S and R a ) regardless of the

level of humidity (r.h. = 20%, 50%, and 80%)

(Figure 6 b2–b7). The CeO 2 nanoparticles

can be further reduced (Ce 4+ to Ce 3+ ) at

higher humidity values (e.g., r.h. 80%)

(reaction ( 4) ), which enhance the scav-

enging of free radicals and oxygen trans-

port from CeO 2 to In 2 O 3 . This is supported

by the decrease of τ recov values with

increasing humidity (shown in Figure S5d,

Supporting Information). Accordingly, the

regenerative refresh of the sensor surface

by the processes shown in reactions ( 4) , ( 5) ,

( 6) , ( 7) can be attributed to the high reac-

tivity of CeO 2 nanoparticles toward water

vapor and the highly reversible redox reac-

tion of Ce ions.

The substantial amounts of Ce 3+ and

lattice defects such as oxygen vacan-

cies are advantageous for regenerative

refreshing of In 2 O 3 sensing surface. Thus,

the Ce 3+ /Ce 4+ ratios of the specimens

were analyzed by measuring the Ce 3d

core electron levels by X-ray photo-

electron spectroscopy (XPS), (shown in

Figure S6a2–g2, Supporting Information).

Shyu et al. [ 33 ] suggested that the relative percent area of the

u′″ peak in the total 3d region can be used as a measure of

the relative amount of Ce 4+ , since the peaks for Ce 3+ are

not related to the u′″ peak (shown in Figure S6a2–g2, Sup-

porting Information). The calculated u′″ areas in the present

study gradually increased from 7.25 to 12.3 u′″% as the Ce

loading concentration increased ( Table 1 ). However, these

values are slightly lower than that for pure CeO 2 (13.7 u′″%)

reported in the literature. [ 33 ] This might be due to the addi-

tional reduction of Ce 4+ to Ce 3+ induced by electron dona-

tion from In 2 O 3 to CeO 2 , which agrees with the In 3d peaks

shifting toward higher binder energies (0.03–0.32 eV) when

CeO 2 is loaded onto In 2 O 3 (shown in Figure S6a1–g1, Sup-

porting Information and Table 1 ). This strongly suggests that

small 2016, 12, No. 31, 4229–4240

Table 1. Corresponding binding energies (eV) of In 3d and the calculated u′″ percent area (%) for different characteristic peaks of Ce 3d in pure and Ce-In 2 O 3 hollow spheres.

Ce concentration [wt%]

Sample 0 1.04 2.33 4.97 11.7 22.4 39.9

In 3d Binding energy [eV]

In 3d 5/2 444.3 444.3 444.4 444.6 444.4 444.4 444.3

In 3d 3/2 451.8 451.8 451.9 452.1 452.0 451.9 451.9

Ce 3d Area percent [%]

u′″ 0 7.25 7.52 7.59 9.97 12.6 12.3

Scheme 1. Illustrations of the self-refreshing of the In 2 O 3 sensing surface by the CeO 2 nanoclusters; a) water vapor infl ow, b) chemisorption, c) desorption, and d) oxygen ion regeneration.


4237© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.small-journal.comsmall 2016, 12, No. 31, 4229–4240

the free radical scavenging and oxygen transport properties

of CeO 2 nanoparticles can be considerably enhanced when

they interact intimately with In 2 O 3 hollow spheres.

The Ce 3+ /Ce 4+ ratio decreased as Ce loading concentra-

tion increased (Table 1 ), which means that the 1.04 Ce-In 2 O 3

sensor is a reasonable candidate for use as a humidity-inde-

pendent gas sensor. However, the humidity-dependence of

the sensor steadily decreased as Ce loading concentration

increased (Figure 4 a–d), and became negligible when the

In 2 O 3 hollow spheres were predominantly covered with CeO 2

nanoparticles (11.7 Ce-In 2 O 3 ) (Figure 4 e). In principle, a high

Ce 3+ /Ce 4+ ratio is advantageous for the refresh of the sur-

face. However, 1.04 Ce-In 2 O 3 with the highest Ce 3+ /Ce 4+ ratio

showed a high humidity dependence, which can be attributed

to the small number of contacts between the In 2 O 3 and CeO 2

nanoparticles and the long inter-CeO 2 distances. This is fea-

sible considering that the H + and oxygen generated at the sur-

face of the CeO 2 nanoparticles should migrate to the surface

of In 2 O 3 to participate in OH scavenging and oxygen adsorp-

tion reactions. From this perspective, the gradual decrease

in humidity dependence with increasing Ce concentration

up to 11.7 can be explained by (a) the increase in the reac-

tion between the CeO 2 nanoparticles and water vapor, and

(b) the increased length of two-phase boundaries between

CeO 2 and In 2 O 3 for reactions ( 4) , ( 5) , ( 6) , ( 7) to occur. At the

same CeO 2 concentration, the smaller CeO 2 nanoparticles

are advantageous because they will provide the wider CeO 2

surface area for the reaction with water vapor and the longer

two-phase boundaries between CeO 2 and In 2 O 3 .

However, the sensor resistance and gas response slightly

decreased with further Ce additions to 22.4 and 39.9 wt%

(shown in Figure S2, Supporting Information). Note that

the CeO 2 itself is also an n-type semiconductor [ 34 ] which will

show a decrease in sensor resistance and gas response upon

exposure to water vapor. In this perspective, the increase in

the humidity dependence of the gas sensing characteristics of

heavily Ce-loaded sensors can be attributed to the formation

of additional conduction paths along the continuous CeO 2

layer. This clearly shows that CeO 2 nanoparticles should be

loaded in a uniform and discontinuous manner to achieve

humidity-independent gas sensing characteristics.

To investigate the effect of the uniform dispersion

of CeO 2 nanoparticles on the gas sensing characteristics,

CeO 2 -loaded In 2 O 3 hollow sphere samples were prepared

without applying surface cleaning and the LBL process

(shown in Figure S7a–d, Supporting Information) and with

surface cleaning but without the LBL process (shown in

Figure S7e–h, Supporting Information). The concentration

of CeO 2 nanoparticles was fi xed at 6.0 wt%. Even though

the surface charge of the In 2 O 3 hollow spheres was nega-

tive enough to electrostatically attach Ce cations regardless

of surface cleaning (shown in Figure S8, Supporting Infor-

mation), CeO 2 nanoparticles could not be uniformly coated

and showed signifi cant aggregation in both samples (shown

in Figure S7b,c,f,g, Supporting Information). Aggregation

can be attributed to the bridging of adjacent particles by

water via hydrogen bonding and the strong capillary forces

present during the drying process. [ 35 ] Both of these modifi ed

sensors showed enhanced humidity-independent gas sensing

characteristics compared to those of the pure In 2 O 3 sensor.

However, the humidity dependence could not be completely

removed (shown in Figure S9, Supporting Information).

These results clearly show that the uniform dispersion of

CeO 2 nanoparticles in a discrete manner (e.g., via the LBL

process) is essential for humidity independent gas sensing.

The humidity dependence of the gas selectivity of the pure

In 2 O 3 and 11.7 Ce-In 2 O 3 sensors was examined by measuring

the responses to 20 ppm of acetone, carbon monoxide,

ammonia, hydrogen, and toluene under dry and r.h. 80% con-

ditions at 450 °C ( Figure 7 ). The selectivity was defi ned as

the ratio of the gas response to acetone compared to that for

the other gas ( S acetone / S other gas ). Pure In 2 O 3 sensors showed

high gas response values ( S = 22.2) as well as good acetone

selectivity ( S acetone / S other gas = 3.52–4.13) in a dry atmosphere

(Figure 7 a1). The gas responses to all analyte gases were sig-

nifi cantly lower in a humid environment (r.h. = 80%) and

hence the acetone selectivity decreased to S acetone / S other gas =

1.78–2.04, probably because of strong water vapor chemisorp-

tion (Figure 7 a2). The gas response and acetone selectivity of

the 11.7 Ce-In 2 O 3 sensor were as high as 4.69 and 3.43–4.17,

respectively, in a dry atmosphere (Figure 7 b1). Under condi-

tions of r.h. = 80%, the gas response ( S = 4.46) and selectivity

( S acetone / S other gas = 3.54–4.13) were similar to those in a dry

atmosphere (Figure 7 b2). These results clearly show that the

sensing surface of 11.7 Ce-In 2 O 3 remained unchanged even

after exposure to water vapor (Figure 7 a3,b3), and that the

11.7 Ce-In 2 O 3 sensor can detect acetone in a highly sensitive

and selective manner even under highly humid conditions.

The humidity independent acetone sensing characteristics of

11.7 Ce-In 2 O 3 were confi rmed again by comparing the gas

responses to 20 ppm acetone and acetone-containing mixed

gases (20 ppm acetone + 20 ppm interference gas 1 + 20 ppm

interference gas 2) both in dry and r.h. = 20% atmospheres

(Figure S10, Supporting information).

The dynamic sensing transients of the 11.7 Ce-In 2 O 3 sensor

to 1–20 ppm acetone were measured at 450 °C in both dry and

r.h. 80% atmospheres ( Figure 8 a). The variation in humidity

did not change the sensing transients, sensor resistance in air,

or gas response. The detection limit of the sensor was ≈500 ppb

both in dry and r.h. 80% atmospheres when S > 1.2 was used

as the criterion for gas sensing (Figure 8 b). This clearly shows

that the 11.7 Ce-In 2 O 3 sensor can detect sub-ppm and several

ppm levels of acetone in a reliable manner.

Acetone is a colorless, volatile, and fl ammable gas that

can cause headache, fatigue, nausea, and even death with

exposure to high concentrations (<1000 ppm). [ 36 ] In addi-

tion, acetone gas is a representative biomarker, which can

be detected from the exhaled breath of diabetic patients

(>1.8 ppm). [ 37 ] The accurate detection of acetone from

human exhalation could be a crucial step for diagnosing

diabetes. Exhaled breath is highly humid (r.h. 95%) [ 38 ] and

the humidity varies signifi cantly with the sampling distance

from the patient’s mouth. Therefore, to diagnose the disease

directly from patient’s exhaled gas, the humidity dependence

of the gas sensing characteristics needs to be eliminated. The

11.7 Ce-In 2 O 3 sensor showed humidity independent sensing

performance to acetone with a low detection limit (500 ppb)

and excellent selectivity ( S acetone / S other gas = 3.54–4.13) even


4238 www.small-journal.com © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim small 2016, 12, No. 31, 4229–4240

under highly humid conditions (r.h. 80%) (Figure 8 ). Such

a sensor shows promising potential for real-time monitoring

of acetone in an ambient atmosphere for the application of

diagnosing diabetes.

In this study, we report a new and novel self-refreshing

surface concept to realize humidity independent gas

sensing characteristics. CeO 2 nanoclusters coated by the

LBL method onto the In 2 O 3 surface scavenge chemically

adsorbed hydroxyl groups and supply oxygen ions to the

In 2 O 3 surface, which eliminate the humidity dependence of

the sensor resistance and gas response of the In 2 O 3 sensor.

We confi rmed that this dynamic self-refreshing of the sensor

surface assisted by a uniform CeO 2 coating is also effective

in SnO 2 nanostructures (shown in Figure S11, Supporting

Information). This concept can pave a new way to design

humidity independent chemiresistors using various metal

oxide sensing materials.

3. Conclusion

Humidity independent oxide semiconductor gas sensors

were designed by dynamic self-refreshing of sensing surface

assisted by layer-by-layer coated CeO 2 nanoclusters. The

CeO 2 nanoclusters dynamically and effectively refreshed the

sensing surface of In 2 O 3 under humid atmosphere in a regen-

erative manner by scavenging chemically adsorbed hydroxyl

groups and supplying oxygen ions to In 2 O 3 surface. The

In 2 O 3 hollow spheres with LBL coated CeO 2 nanoclusters

exhibited humidity independent gas sensing characteristics

Figure 8. a) Dynamic sensing transients of 11.7 Ce-In 2 O 3 hollow spheres exposed to 0.5–20 ppm of acetone at 450 °C in dry (red) and r.h. 80% (blue). b) Gas responses as a function of acetone concentration.

Figure 7. Gas responses of a) pure In 2 O 3 and b) 11.7 Ce-In 2 O 3 hollow spheres exposed to 20 ppm acetone (Ace), CO (C), ammonia (A), H 2 (H), and toluene (T) at 450 °C in (a1,b1) dry and (a2,b2) r.h. 80%. Polar plots of gas responses of (a3) pure In 2 O 3 and (b3) 11.7 Ce-In 2 O 3 hollow spheres exposed to various reducing gases in dry (red) and r.h. 80% (blue).


4239© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.small-journal.comsmall 2016, 12, No. 31, 4229–4240

(gas response and sensor resistance) as well as a low detec-

tion limit, excellent acetone selectivity, and rapid response/

recovery speeds even under highly humid conditions (r.h.

80%). In contrast, pure In 2 O 3 hollow spheres showed sig-

nifi cant deterioration of the gas response, selectivity, and

sensor resistance in humid atmospheres (r.h. = 20%–80%).

The humidity independent acetone sensing performance

demonstrates a promising potential for real-time acetone

monitoring for diagnosing diabetes. In addition, the unique

self-refreshing mechanism described in the present study

based on the facile redox reaction (Ce 3+ /Ce 4+ ) of CeO 2 nano-

clusters can be widely used in most oxide semiconductor gas

sensors and provides a general solution for designing high

performance gas sensors without humidity dependence.

4. Experimental Section

Sample Preparation : In 2 O 3 hollow spheres were coated with CeO 2 nanoclusters by the direct reduction of Ce ions adsorbed onto polyelectrolyte-modifi ed surfaces of In 2 O 3 . The synthesis pro-cess consisted of three main steps: (1) preparation of In 2 O 3 hollow spheres using spray pyrolysis; (2) Ce ion coating via LBL assembly; (3) CeO 2 nanoparticle formation by NaBH 4 reduction and subse-quent heat-treatment at 500 °C for 3 h.

Preparation of In 2 O 3 Hollow Spheres : In 2 O 3 hollow spheres were prepared by one-pot spray pyrolysis of an aqueous solution containing indium(III) nitrate hydrate (In(NO 3 ) 3 · x H 2 O, 99.999%, Sigma-Aldrich, USA) and sucrose (C 12 H 22 O 11 , 99.5%, Sigma-Aldrich, USA). The concentration of indium nitrate and sucrose were 0.05 and 0.15 M , respectively. The spray pyrolysis system comprised a droplet generator, tubular reactor, and powder col-lecting chamber (shown in Figure S12, Supporting Information). Large quantities of droplets were generated by a 1.7 MHz ultra-sonic transducer with fi ve vibrators and directly carried into a high temperature (900 °C) tubular reactor (1200 mm in length and 50 mm in diameter) by O 2 gas at a fl ow rate of 5 L min −1 . The droplets were converted into In 2 O 3 hollow spheres during this heat treatment and collected with a Tefl on bag fi lter in a particle collec-tion chamber.

Ce Ion Coating : Prior to LBL assembly, the surfaces of In 2 O 3 hollow spheres (40 mg) were cleansed by boiling in 70 mL of a solu-tion containing distilled water, hydrogen peroxide, and ammonium hydroxide (5:1:1 by vol%, respectively) at 75 °C under magnetic stirring (300 rpm) for 30 min. After purifying the suspension by centrifuging it fi ve times (3000 rpm, 10 min), 20 mL of polyethylen-immine (PEI, H(NHCH 2 CH 2 )· n NH 2 , M w ≈25 000, Sigma-Aldrich, USA) solution (0.5 mg per 1 mL) was mixed with the slurry and stirred for 3 h to graft PEI onto the surfaces of the In 2 O 3 hollow spheres. It was noted that the zeta-potential of the In 2 O 3 in distilled water was suf-fi ciently negative (−56.2 mV) to allow electrostatic self-assembly of PEI onto the surfaces (shown in Figure S8, Supporting Information). The PEI-coated In 2 O 3 powders were centrifuged and washed in dis-tilled water fi ve times. Then 20 mL of poly(acrylic acid) ((C 3 H 4 O 2 ) n , M v ≈450 000, Sigma-Aldrich, USA) solution (0.5 mg per 1 mL) was added and stirred for 2 h to reverse the net charge of the surface to negative. The product was then centrifuged and washed fi ve times and then mixed with 40 mL of solution containing 0.3, 1.2, 3.0, 6.0, 12, 24, 48, or 96 wt% Ce nitrate hexahydrate (Ce(NO 3 ) 3 ·6H 2 O,

99.99%, Sigma-Aldrich, USA) and stirred for 3 h. All these pro-cesses were carried out at room temperature.

CeO 2 Nanoclusters Formation : Fresh NaBH 4 solution was attained by dissolving 0.2 g of NaBH 4 (99.9%, Sigma-Aldrich, USA) in distilled water (10 mL). Then, the solution was injected drop wise into the previously described suspensions and stirred at room temperature for 3 h. After washing and centrifuging fi ve times with distilled water, the fi nal slurry was dried at 70 °C for 1 d and sub-sequently converted into CeO 2 nanoclusters coated In 2 O 3 hollow spheres by crystallization via heat treatment at 500 °C for 3 h in air.

Characterization : The morphologies of the prepared materials were observed by SEM (S-4800, Hitachi) and high-resolution TEM (Titan 80-300, FEI). A FIB was applied for cross-sectional sample preparation with both TEM and line scanning. Elemental mapping was conducted using energy dispersive X-ray spectroscopy. The crystal structures were investigated using X-ray diffraction (XRD, Rigaku D/MAX-2500) with CuKα radiation ( λ = 1.5418 Å). The chem-ical state of the CeO 2 nanoclusters-loaded In 2 O 3 hollow spheres was analyzed using XPS (XTOOL, ULVAC-PHI). The specifi c surface areas of the powders were calculated from Brunauer–Emmett–Teller analysis of nitrogen adsorption measurements (TriStar 3000). Inductively coupled plasma atomic emission spectroscopy analyses were carried out to determine the accurate concentration of Ce on the In 2 O 3 hollow spheres. The zeta potential of the In 2 O 3 suspension was measured using an ELSZ-1000 instrument (Photal Otsuka Electronics, Japan). The humidity level (relative humidity) was measured using a HD100 hygrometer (KIMO, France).

Gas Sensing Characteristics : Prepared powders were dispersed in distilled water and the slurry was drop-coated onto an alumina substrate (size: 1.5 × 1.5 mm 2 ) with two Au electrodes on the top surface and a microheater on the bottom surface. Prior to the measurements, the sensor was heated to 500 °C using a power of 571 mW for 2 h to remove hydroxyl contents and stabilize the sensor. The sensor was contained within a specially designed quartz tube (1.5 cm 3 ). The atmosphere was controlled using a four-way valve to ensure a constant fl ow rate of 100 cm 3 min −1 of dry and/or humid air and introduce the analytic gas. The concentra-tions of gases and relative humidity (0%, 20%, 50%, and 80%) were independently controlled by mixing the gases (20 ppm of either acetone, ammonia, carbon monoxide, or toluene) and dry or humid synthetic air. The humidity of atmosphere was measured at 25 °C just before fl owing to the locally heated sensor within sensing chamber. Two-probe DC resistance of the sensor was measured using an electrometer interfaced with a computer.

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2016R1A2A1A05005331).


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Received: May 4, 2016 Revised: May 27, 2016Published online: June 30, 2016

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