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Sensors and Actuators B 144 (2010) 67–72

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

journa l homepage: www.e lsev ier .com/ locate /snb

ensor response formula for sensor based on ZnO nanostructures

iyom Hongsitha, Ekasiddh Wongrata, Teerakiat Kerdcharoenb, Supab Choopuna,∗

Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai 50200 and ThEP Center,HE, Bangkok 10400, ThailandDepartment of Physics, Faculty of Science, Mahidol University, Bangkok 10400, Thailand

r t i c l e i n f o

rticle history:eceived 4 August 2009eceived in revised form5 September 2009

a b s t r a c t

In this paper, we propose a new and general formula to describe ethanol adsorption mechanism underly-ing the response enhancement of ZnO nanostructure sensors. The derivation of sensor response formulabased on basic chemical reaction at the sensor surface is presented. The formula can be used to explainresponse enhancement due to effect of metal doping, surface-to-volume ratio, and surface depletion

ccepted 10 October 2009vailable online 20 October 2009

eywords:as sensornO

layer. Thus, it can be regarded as a general formula to describe the sensor response characteristics of ZnOsensors. This general formula is a powerful tool for designing ZnO sensor at any desired sensor response.Furthermore, it is reasonable to expand this formula to explain other sensing materials and also to explainfor different active gases.

© 2009 Elsevier B.V. All rights reserved.

ensor response formulaanostructure

. Introduction

Metal-oxide semiconductor sensors based on materials suchs SnO2, TiO2, WO3 or ZnO are widely used for ethanol sensors1–14]. Especially, ZnO is a promising material for gas sensor appli-ations due to feasibility for ultrahigh sensitive sensors or ppb-levelensors. Recently, various morphologies of ZnO such as belt-like,ire-like, rod-like or tetrapod for ethanol sensor applications have

een widely investigated [15–22]. It was suggested that the sen-or response characteristics of these sensors strongly depends onhe morphology of ZnO. Ethanol sensors based on ZnO nanostruc-ure such as nanobelts, nanorods or nanowires usually exhibit highensor response and sometime up to a few hundred folds over con-entional metal-oxide sensors at moderate concentration [9]. Onhe other hand, a sensor based on a larger size of ZnO such as thinlms or microtetrapods show lower sensor response [14,15]. Manyodels have been proposed to explain sensor response character-

stic of ZnO sensors and still be a subject of discussion.Basically, the ZnO ethanol gas sensing was simply observed

n the resistance change under ethanol atmosphere. It can bexplained by the chemical reaction between the active gas and oxy-

en ion adsorption on the surface of ZnO. In air atmosphere and atigh operating temperature, oxygen molecules are adsorbed ontohe surface of the ZnO sensor to form O− or O2− ions by attract-ng electrons from the conduction band of the ZnO. Under ethanol

∗ Corresponding author. Tel.: +66 53 943375; fax: +66 53 357511.E-mail address: supab99@gmail.com (S. Choopun).

925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2009.10.037

atmosphere, the ethanol gas reacts with oxygen ion molecule on thesurface and gives back electrons into the conduction band, therebylowering the resistance of ZnO sensors. The ratio between tworesistances is then calculated as the sensor response. Typically, thesensor response of the oxide semiconductor gas sensor can usuallybe empirically represented as [1,5]: S = aCb, where C is the target gaspartial pressure, which is direct proportion to its concentration, andthe sensor response is characterized by the parameter a and expo-nent b. The value of b may have some rational fraction value usually∼1 or 0.5 depending on the charge of the surface species [1,5].

Wang and co-workers have proposed a contact controlled model[7] and surface-depletion controlled model [8] that can explainthe improvement of sensor response based on nanostructure. Chenet al. [9] have used space-charge model to explain ultrahigh sen-sor response of ethanol sensor based on flower-like ZnO nanorodswith diameters less than 15 nm. Recently, Zhao and co-workers [10]have used density functional theory (DFT) to reveal the gas-sensingmechanism of ZnO and provided an exponentially formula of sen-sor response. However, there is still no general model or formulato explain all circumstances of ethanol sensor based on ZnO. Thus,it is interesting to generalize a simple model or formula in order todescribe the sensor response characteristics of ZnO sensor.

In this work, the sensor response formula based on ethanoladsorption mechanism has been developed and obtained from

basic chemical reaction between ethanol molecule and oxygen ionsby including surface depletion layer. The sensor response formulacan be used to explain response enhancement due to effect of metaldoping, surface-to-volume ratio, and surface depletion layer. Thissensor response formula can be regarded as a general formula to

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8 N. Hongsith et al. / Sensors a

escribe the sensor response characteristics of ZnO sensors or anyetal-oxide sensors.

. Theory

.1. Ethanol adsorption mechanism

Normally, the ethanol vapor gas sensor based on ZnO semicon-uctor has an operating temperature at high temperature (about50–350 ◦C). When the ZnO semiconductor is heated at lower tem-erature about 100–200 ◦C, oxygen molecules in the atmospherere adsorbed on the surface of ZnO and form oxygen ion moleculess shown in Eq. (1).

2(gas) + e− ⇔ O−2 (adsorb) (low temperature) (1)

At higher temperature about of 250–350 ◦C, the oxygen ionolecules are dissociated into oxygen ion atoms with singly or

oubly negative electric charges by attracting an electron from theonduction band of ZnO as shown in Eqs. (2) and (3).

12 O2 + e−kOxy⇔ O−

ads (2)

12 O2 + 2e−kOxy⇔ O2−

ads (3)

The oxygen ions on the surface of ZnO are active with the ethanololecule and give up the electrons from the surface back to the

onduction band of ZnO semiconductor. The chemical reactionetween ethanol molecule and oxygen ions is shown in Eqs. (4)nd (5) for O− and O2−, respectively.

H3CH2OHads + O−ads

kEth→ C2H4O + H2O + 1e− (4)

r

H3CH2OHads + O2−ads

kEth→ C2H4O + H2O + 2e− (5)

hese cause an increasing in the conductivity and thus the decreas-ng resistance of the sensor.

.2. Sensor response of ZnO sensor

From Eqs. (4) and (5), rate equation of electron density can beritten as Eqs. (6) and (7), respectively, which can then be com-

ined to Eq. (8).

dn

dt= kEth(T)[O−

ads]1[CH3CH2OH]1 (6)

r

dn

dt= kEth(T)[O2−

ads]1/2

[CH3CH2OH]1/2 (7)

dn

dt= kEth(T)[Oion

ads]b[CH3CH2OH]b (8)

n Eq. (8), n is the electron density or electron concentration underhe ethanol atmosphere, b is a charge parameter having value of 1or O− and 0.5 for O2− and kEth(T) is the reaction rate constant oreaction rate coefficient described as

Eth(T) = A exp(

− Ea

kBT

)(9)

here Ea is the activation energy of reaction, kB is the Boltzmannonstant and T is absolute temperature. Integrating Eq. (8) leads tohe solution as

= kEth(T)[Oionads]

b[CH3CH2OH]bt + n0 (10)

here n0 is the electron concentration of sensor at an operatingemperature in the air atmosphere. At equilibrium under ethanol

tuators B 144 (2010) 67–72

and air atmosphere, carrier concentration n and n0 can be con-sidered as a constant with time. Thus, Eq. (10) can be rewrittenas

n = �tkEth(T)[Oionads]

b[CH3CH2OH]b + n0 (11)

where � t is a time constant. The carrier concentration is definedas n = ˛/R, where R is a resistance and ˛ is a proportional constant,and can be substituted in Eq. (11) as

1Rg

= �tkEth(T)[Oionads]

b[CH3CH2OH]b

˛+ 1

Ra(12)

The sensor response, Sg, of the sensor is defined as Ra/Rg whereRa is the electrical resistance of the sensor in air, and Rg is the elec-trical resistance of the sensor in ethanol–air mixed gas. Therefore,the sensor response relation can be obtained as

Sg = Ra

Rg= �tkEth(T)[Oion

ads]b[CH3CH2OH]b

n0+ 1. (13)

Usually, temperature dependence of sensor response is con-trolled by two parameters; reaction rate coefficient kEth(T) betweenadsorbed oxygen ions with ethanol molecules, and electron densityof the sensor n0. The reaction rate coefficient and electron densityincreases exponentially with rising temperature. However, sensorresponse is proportional to reaction rate coefficient but inverselyproportional to electron density. These two parameters competewith each other and result in maximum sensor response at opti-mum operating temperature. Ethanol gas sensor based on ZnOmaterial has optimum operating temperature around 300 ◦C [5–9].

Sometimes, a compact form of the sensor response relation onethanol concentration (Cg) can be rewritten as

Sg = aCbg + 1. (14)

where a is a controllable parameter. At the optimum operatingtemperature condition, Eq. (14) can be rewritten as:

log(Sg − 1) = log a + b log Cg. (15)

It can be seen that log(Sg − 1) has a linear relation with log Cg havinga slope of b value. Thus, b value which represents oxygen ion specieson the surface of ZnO sensors can be obtained from a slope of a plotbetween log(Sg − 1) and log Cg.

In the nanostructure regime, the surface-to-volume ratio is animportant parameter and should be included in the sensor responserelation. Generally, this surface-to-volume ratio can be related tothe density of adsorbed oxygen ions. Thus, we propose that thedensity of adsorbed oxygen ions can be written in term of surface-to-volume ratio as

[Oionads] = �0˚Vm

Vs(16)

where �0 is a number of oxygen ion per unit area, ˚ is a ratio of sur-face area per volume of material (Vm), and Vs is the system volume.Thus, substituting Eq. (16) onto Eq. (13) give

S˚ = �tkEth(T)(�0˚(Vm/Vs))b

n0Cb

g + 1. (17)

2.3. Surface depletion layer models

According to the depletion layer or the space-charge model, Ld(a Debye length), can be expressed by [9]

Ld =(

εkBT

q2n

)1/2

(18)

where ε is the static dielectric constant, q is the electrical chargeof the carrier, and n is the carrier concentration. It can be seen that

N. Hongsith et al. / Sensors and Actuators B 144 (2010) 67–72 69

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ao

mcaelad

rZicl

n

wsob(

S

(d

(

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ig. 1. Schematic diagram of the depletion layer on the surface of cylinder with ahickness of Ld , under condition of D ∼ 2Ld and D � 2Ld (not in scale).

t steady operating temperature, the depletion layer is dependentnly on the carrier concentration.

In this model, a cylinder, which is one of the most basic geo-etric shapes in one-dimension, is considered and a conductive

hannel is assumed to be along the axis of the cylinder. At an oper-ting temperature, the oxygen ions were adsorbed by attaching anlectron on the surface of the cylinder. Therefore, the depletionayer is formed on the surface of cylinder with a thickness of Ld,nd then a size of conductive channel is reduced along the radialirection as shown in Fig. 1.

When exposed to the ethanol atmosphere, the ethanol gaseacts with oxygen ions on the surface and gives back electrons tonO sensors resulting in increasing conductive channel (decreas-ng depletion layer). The conductive channel can be related to thearrier concentration, and can be written in term of the depletionayer thickness (Ld) as

′ = n0�(D − 2Ld)2

�D2(19)

here n0 is the carrier concentration of intrinsic material, n′ repre-ents carrier concentration of the Debye length, and D is diameterf the cylinder. Thus, the depletion layer effects on sensor responseased on cylindrical ZnO nanostructure are given by inserting Eq.19) in Eq. (17) and obtained

Ld=

(�tkEth(T)(�0˚(Vm/Vs))

b

n0

)D2Cg

b

(D − 2Ld)2+ 1. (20)

Let consider a diameter of cylinder D compared to Debye lengthLd). Since Debye length is in the order of nanometer, it can be

ivided into three conditions.

1) Under condition D � 2Ld, Eq. (20) turns in to Eq. (17)In this condition, a diameter of cylinder is much larger than

micrometer which is the case of microstructure or bulk materi-

able 1ensor response of ZnO sensor with different morphologies.

Sensor response Ethanol concentration (ppm)

Morphology D (nm) 5 10 50 100

Nanowires [5] 25 ± 5 ∼8 ∼15 – 32.5Nanorods [7] 150 4.4 5.8 11.4 14.6Nanorods [8] 15 – 10 18 30Nanorods [9] <15 – 20.5 104.9 176.8Au-doped nanorods [23] 15 ± 5 16.4 20.1 41.8 89.5Nanobelts [4] a 50–150 – – 7.3 11.8Nanowires [11] a 60–180 – – – 5.07

a Our previous work.

Fig. 2. Plot of sensor response (Sg − 1) and ethanol concentration in log scale forethanol sensor based on different ZnO morphologies. The linear line in the graphhas a slope of 0.5.

als. The depletion layer thickness is very small compared withthe cylindrical diameter (D � 2Ld) and Eq. (20) can be approx-imated to Eq. (17) which is an equation that has no depletionlayer effect.

(2) D > 2Ld, Eq. (20) can be approximated to Eq. (17)When a diameter of cylinder is in the order of nanometer

but still larger than Debye length (D > Ld), Eq. (20) again can beapproximated to Eq. (17) with no depletion layer effect. How-ever, sensor response strongly depends on oxygen ion densitydue to the surface-to-volume ratio, ˚, parameter.

(3) D ∼ 2Ld, sensor response strongly depends on D.When a cylindrical diameter decreases down to the order of

nanometer and is comparable to Debye length (D ∼ 2Ld), thedepletion layer has strong effect and the sensor response isstrongly dependent on a cylindrical diameter. Thus, Eq. (20) canbe used to explain sensor response of all structural sizes fromnanometer to bulk and can be considered as a general form ofsensor response.

3. Results and discussion

3.1. Oxygen ion species on the surface of ZnO sensors

Recently, the ethanol gas sensing based on different ZnO mor-phologies and various sizes have been widely investigated, forexamples, nanorods [7–9], nanobelts [4] and nanowires [5,11] and

the sensor response of sensors were listed in Table 1. As aforemen-tioned, a slope value of a plot between log (Sg − 1) and log Cg canrepresent the oxygen ion species. The plots between log (Sg − 1)and log Cg of some previous works as listed in Table 1 are displayedin Fig. 2.

b value

200 300 500 1000 2000

47 – – – – 0.504– – 25.2 30.1 – 0.496– 55 72 100 – 0.512

224.2 258 267.7 – – 0.677– 193.6 ∼250 – – 0.630– – 21.1 23.2 46.5 0.500– – 9.79 14.14 14.19 0.504

7 nd Actuators B 144 (2010) 67–72

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It was found that the value b of all sensors is close to 0.5 suggest-ng that the adsorbed surface oxygen species on ZnO sensor is O2−

3]. In addition, this suggests that the oxygen adsorption speciesn the surface is independent on the grain size of ZnO regardless ofulk, microstructure or nanostructure.

However, the value b of ZnO sensor which has diameter close toebye length (2Ld) was not that close to 0.5, for example, the valueof nanorods [9,24] with diameter size <15 nm is equal 0.677 and.630. Such deviation arises because the surface depletion layeras some small effect on the oxygen adsorption species at ZnOurface.

.2. The effect of metal doping

From previous report, doping on semiconductor can modifyesistance [20] that also affect the sensor response. There haveeen several reports on sensor response enhancement due to metaloping effect such as Au-doped [11,23,24], Pd-doped [13,25] andt-doped [26,27] sensors. However, the reason for sensor responsenhancement is still not clear. The metal doping effect can be simplyxplained in our model by using Eq. (13).

From Eq. (13), it can be seen that the sensor response is pro-ortional to the reaction rate constant, kEth(T) and kOxy(T), throughxygen density. Basically, the reaction rate constant can be affectedy noble metals (such as Au, Pd, and Pt) in oxide semiconductorensor due to the catalytic effect. Thus, metal doping causes anncreasing of reaction rate constant and resulting in enhancementf sensor response. The increase of reaction rate constant (or oxy-en density) can be confirmed by observation of an increasing ofesistance in air due to metal doping [24]. The increasing resistancen air suggests that the gold metal catalytically activates the disso-iation of molecular oxygen due to higher reaction rate that resultsn increasing quantity of oxygen adsorption. Therefore, an electronn ZnO nanostructure was captured by oxygen adsorption to formxygen ion and hence ZnO nanostructure loses more electrons andaused the larger depletion layer at ZnO surface resulting in higheresistance.

.3. The effect of surface-to-volume ratio

From earlier reports on conductometric ZnO sensor, it is quitemazed that ZnO nanostructures which are not aligned in the con-uctive direction exhibited sensor response higher than the bulk16,19]. This can be simply explained, based on our model, by theffect of the surface-to-volume ratio as shown in Eq. (17). Underondition of ZnO sensor (D > 2Ld and D � 2Ld for nanostructure,icrostructure and bulk) with no depletion layer effect, the sen-

or response strongly depends on the surface-to-volume ratio ˚s discussed earlier. For example, let consider a thin film with anrea of 1 cm2 as shown in Fig. 3(a). Then, given cylindrical nanos-

ructures of 10 �m in length with various diameters are grown onhis area, as shown in Fig. 3(b). The surface-to-volume ratio cane calculated and put in Eq. (21) for the sensor response ratio. Theensor response ratio as a function of diameter can be plotted ashown in Fig. 3(c). It can be seen that sensor response is enhanced

able 2ist of sensor response formula for ethanol sensor based on ZnO.

Sensor response formula Oxygen ion species Met

Sg = aCbg + 1

√

Sg = �t kEth(T)[Oionads

]b

n0Cb

g + 1√ √

S˚ = �t kEth(T)(�0˚(Vm /Vs))b

n0Cb

g + 1√ √

SLd=

(�t kEth(T)(�0˚(Vm /Vs))b

n0

)D2Cg

b

(D−2Ld )2 + 1√ √

Fig. 3. (a) Ethanol sensor based on ZnO thin films with an area of 1 cm2, (b) ethanolsensor based on vertical alignment ZnO nanorods with diameter D and length L and(c) sensor response ratio as a function of diameter where L = 10 �m.

by decreasing the diameter due to increasing of surface-to-volumeratio.

S˚(B) − 1S˚(A) − 1

=(

˚B

˚A

)b

(21)

3.4. The effect of depletion layer for ultrahigh sensor response

Recently, several works has been reported on ultrahigh sen-sor response of ZnO sensors [8,9] and explained qualitatively bysurface depletion layer. In our model, we shall consider the twocases of ethanol sensors based on microstructure (D � 2Ld andlimitD→∞SLd

= Sg) and nanostructure (D > 2Ld and D ∼ 2Ld). Sinceonly the effect of depletion layer is considered, we neglect the effectof surface-to-volume ratio by taking into account that the surfacearea of sensor is the same in both cases. The sensor response inEq. (20) can be rewritten as the ratio between the two cases ofmicrostructure and nanostructure as

SLd− 1

Sg − 1= D2

(D − 2Ld)2. (22)

The value of Ld can be obtained in Eq. (18) by using the elec-tron density of about 1017–1018 cm−3 [21,22] T = 573 K (optimumtemperature), ε = 7.9 × 8.85 × 10−12 F m−1 and then, give Ld of about

5 nm (for n = 8 × 1017 cm−3).

Eq. (22) is plotted as shown in Fig. 4 by varying the rod’s diam-eter. It can be seen that the sensor response of nano-sensor isincreasing when the diameter steps down below 50 nm, whereinthe sensor response dramatically surging when the diameter is

al doping Nanostructure (D > 2Ld) Nanostructure (D ∼ 2Ld)

√

√ √

N. Hongsith et al. / Sensors and Act

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in 2003 and the M.S. degree in applied physics in 2006from Chiang Mai University, Chiang Mai, Thailand. He iscurrently a Ph.D. candidate in physics at Chiang Mai Uni-versity. His current research interests are in the field ofmetal-oxide semiconductor nanostructures such as ZnOfor gas sensor application.

ig. 4. Sensor response ratio (SLd− 1/Sg − 1) as a function of rod diameter. The

arked points on the graph are the sensor response of ZnO sensors in Table 1.

lose to 15 nm. The experimental results (SLd) from Table 1 are also

lotted in Fig. 4 by taking diameter of 150 nm as a microstructureSg) in order to neglect the surface-to-volume ratio effect. Surpris-ngly, the experimental results are in good agreement with Eq. (22).

oreover, the sensors having size very close to the value of Debyeength (2Ld) exhibited ultrahigh sensor response and have potentialo detect for a ppb level of ethanol concentration. This suggests thathe sensor response enhancement is prominent when the nanos-ructure is close to 15 nm [8,9].

. Conclusions

In summary, the sensor response formulas for ethanol sensorased on ZnO nanostructures are listed in Table 2. It can be seenhat the formula of SLd

can be used to explain for all circumstancesf ethanol sensors based on ZnO. Thus, it can be regarded as a gen-ral formula to describe the sensor response characteristics of ZnOensor. This formula is a powerful tool for designing ZnO sensor atny desired sensor response, especially in ppb level of concentra-ion, and for further application such as designing electronic nose.urthermore, it is reasonable to expand this formula to explainther sensing materials such as SnO2, TiO2, MoO3 or WO3 and also,o explain for different active gases such as CO2, CO, NOx, NH3 or2S.

cknowledgment

This work was supported by Thailand Research Fund (TRF).iyom Hongsith would like to acknowledge the financial supportia the DPST scholarship, and the Graduate School, Chiang Mai Uni-ersity.

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Biographies

Niyom Hongsith received the B.Sc. degree in physics in2003 and the M.Sc. degree in applied physics from Chi-ang Mai University, Chiang Mai, Thailand, in 2006. Heis currently a Ph.D. candidate in Physics at Chiang MaiUniversity. His current research interests are in the fieldof metal-oxide semiconductor nanostructures includingsynthesis, fabrication and application such as ZnO nanos-tructures for gas sensor and dye-sensitized solar cell.

Ekasiddh Wongrat received the B.S. degree in physics

7 nd Ac

University of Maryland College Park in 2002. Currently,he is an assistant professor at Department of Physics andMaterials Science, Faculty of Science, Chiang Mai Univer-

2 N. Hongsith et al. / Sensors a

Teerakiat Kerdcharoen received B.Sc. and M.Sc. inchemistry and physical chemistry from ChulalongkornUniversity in 1990 and 1992, respectively. As an exchangestudent, he received his PhD in physical chemistry from

University of Innsbruck in 1995. Presently, he is a fac-ulty member of Mahidol University. His research interestscover the topics of organic electronics ranging from the-oretical modeling of materials to fabrication of devices,such as tactile and chemical sensors, and to applications,such as electronic skin and electronic nose.

tuators B 144 (2010) 67–72

Supab Choopun received a Ph.D. in chemical physics from

sity, Thailand. His current research interests are in the fieldof metal-oxide semiconductor nanostructures such as ZnOfor gas sensor and dye-sensitized solar cell applications.