influence of polymerization temperature on nh3 response of pani tio2 thin film gas sensor

Sensors and Actuators B 129 2008 319–326Xuan Chen, Zhihua Ying State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Optoelectronic Information, University of E

Sensors and Actuators B 129 (2008) 319–326

Xuan Chen, Zhihua Ying

State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Optoelectronic Information, University of Electronic Science and Technology of China (UESTC), Chengdu 610054, PR China

Received 23 May 2007; received in revised form 5 August 2007; accepted 7 August 2007

Available online 10 August 2007

Abstract

Polyaniline/titanium dioxide (PANI/TiO2) nanocomposite thin films were processed on a silicon substrate with gold interdigital electrodes by an in-situ self-assembly approach for NH3gas-sensing application, and the effect of polymerization temperature on the gas response of the PANI/TiO2

thin film gas sensor was investigated The results showed that the PANI/TiO2thin film prepared at 10◦C was superior to those prepared at other temperatures in terms of response properties, which also exhibited good reproducibility, selectivity and long-term stability UV–vis absorption and surface morphology characterization of the nanocomposite thin films were performed to explain these different gas-sensing properties The sensing mechanism was also discussed

© 2007 Elsevier B.V All rights reserved

Keywords: PANI/TiO2 ; Nanocomposite thin film; NH 3 ; Polymerization temperature; Gas sensor

1 Introduction

The importance of qualitative and quantitative analysis

of chemical substances has been well understood for the

sustainable development of human being and its habitation

envi-ronment, and a large volume of research has been focused on

the development and fabrication of gas sensors for detection of

gaseous chemicals Choice of suitable sensing materials along

with efficient microelectronics for the detection system is the

key step in such efforts [1] At present, the nanocomposite

of conducting polymer/metal oxide for gas-sensing application

has attracted a lot of attention, as it has been proved that the

hybridization could synergize or complement the sensitive

prop-erties of pure organic or inorganic gas-sensing material[2–4]

Among various conducting polymers, polyaniline (PANI)

was found to be a better choice for gas-sensing material due to

its good environmental and chemical stability, ease of synthesis,

∗Corresponding authors Tel.: +86 28 83207157; fax: +86 28 83206123.

E-mail addresses:jiangyd@uestc.edu.cn (Y Jiang), jsyu@uestc.edu.cn

(J Yu).

inexpensive monomer as well as higher sensitivity, reversible response and shorter response time compared with polypyr-role [1,5] Therefore, recent different gas sensors based on PANI nanocomposites combined with various metal oxides have been the subject of considerable interest, i.e., PANI/SnO2

[2,6], PANI/TiO2 [6,7], PANI/MoO3 [8], PANI/WO3[9], and PANI/In2O3[3], etc On the other hand, as a kind of inorganic gas sensitive material, TiO2is a typical n-type semiconductor and has received particular attention due to its good stability and environmental-friendliness [4] Therefore, it is hopeful to obtain novel materials with complementary behaviors between PANI and TiO2 Various PANI/TiO2nanocomposites have been prepared by using chemical polymerization or electrochemical polymerization of aniline in the presence of nano-colloidal TiO2 Moreover, our group also reported a PANI/TiO2 nanocompos-ite thin film gas sensor prepared at room temperature [10] It has been revealed that the PANI/TiO2nanocomposite thin film gas sensor exhibited higher response values, faster response and recover rates to NH3than those of a pure PANI thin film sen-sor fabricated under the identical conditions However, it was also found that the response time and long-term stability of the prepared PANI/TiO2gas-sensor was not good enough for prac-0925-4005/$ – see front matter © 2007 Elsevier B.V All rights reserved.

doi: 10.1016/j.snb.2007.08.013

tical application In order to improve the performance of the

gas sensor based on the PANI/TiO2 nanocomposite thin film

and develop a usable NH3gas sensor, the polymerization

tem-perature was systematically optimized for film processing in

this work, and the nanocomposites and films were characterized

by FTIR, UV–vis spectroscopy and scanning electron

micro-copy (SEM) The concentration and temperature characteristics

of the PANI/TiO2 thin films as well as the reproducibility,

selectivity and long-term stability of gas sensors were also

investigated

2 Experimental

2.1 Materials

Polydialyldimethyldiammonium chloride (PDDA),

poly-(sodium-p-styrenesulfonate) (PSS), aniline and colloidal TiO2

(particle size < 40 nm) were all obtained from Sigma–Aldrich

Co Ammonium persulfate ((NH4)2S2O8, APS) and

hydrochlo-ric acid (HCl) were purchased from Chengdu Kelong Chemical

Reagent Co All the chemical and reagents were used as received

without further purification

Fig 1 FTIR spectra of pure PANI and PANI/TiO 2 composite.

Fig 2 UV–vis absorption spectra of a PANI thin film prepared at 20 ◦C and

PANI/TiO 2 nanocomposite thin films prepared at different polymerization

tem-peratures.

2.2 Fabrication of PANI/TiO 2 nanocomposite thin films

PANI/TiO2nanocomposite thin films were synthesized by in-situ self-assembly technique in the presence of colloidal TiO2 with a dip-coater (KSV Co., Finland) A typical fabrication pro-cess for PANI/TiO2 nanocomposite thin film was as follows: 0.1 g PSS was dissolved in 50 mL of deionized (DI) water, and adjusted pH to 1 using a hydrochloric acid solution A positively charged surface was created via the deposition of a 1.0% PDDA aqueous solution for 15 min, and then the positively charged substrate was dipped into the PSS solution for 15 min to obtain

a negatively charged surface Later, the PSS-coated substrate was washed with DI water and dried by a nitrogen blow The PSS-coated substrate was suitable to fabricate PANI/TiO2thin films, where simultaneous polymerization of aniline monomer and oxidation of PANI molecules occurred The active solu-tion for PANI/TiO2nanocomposite material contained colloidal TiO2, aniline monomer and HCl followed by the addition of an oxidizing agent (APS) The optimum film of PANI/TiO2was achieved with a solution containing 0.1 mL of aniline, 10 mL

of the sonicated colloidal TiO2(0.1 wt.%), 20 mL of 2.0 M HCl and 10 mL of a hydrochloride solution of APS with an equal molar ratio to aniline The resulting mixture was kept still for

5 min after the addition of APS and then filtered Such a fil-tered solution was used for the deposition of PANI/TiO2film The PDDA/PSS substrate was removed from the reaction mix-ture after 20 min and then immersed into a 1.0 M HCl solution for 5 min and dried in air The polymerization of PANI/TiO2 nanocomposite was processed under various temperatures, i.e.,

−10, 0, 10, 20 and 30◦C A PANI thin film was also fabricated without TiO2at 20◦C.

Table 1 Maximum absorbance wavelengths of PANI/TiO 2 thin films prepared at different polymerization temperature

Polymerization temperature ( ◦C) λ1 (nm) λ2 (nm) λ3 (nm) λ4 (nm)

Table 2

Response time (T1) and recovery time (T2 ) of sensors based on PANI/TiO 2 thin films prepared at 0, 10 and 20 ◦C when exposed to NH

3 of various concentrations

at room temperature Concentration

of NH 3 (ppm)

2.3 Characterization of films

FTIR spectra of pure PANI and PANI/TiO2nanocomposite

samples palletized with KBr were performed by a NICOLET

MX-1E Fourier transformed spectrometer The UV–vis

absorp-tion spectra of pure PANI and PANI/TiO2nanocomposite thin

films were recorded on a Shimadzu UV1700 spectrometer

using an uncoated glass as the reference The surface

mor-phology was assessed with a JSM-5900LV scanning electronic

microscope

3 Results and discussion

3.1 FTIR spectra of pure PANI and PANI/TiO 2 nanocomposites

The resulting PANI and PANI/TiO2solutions described in Section2.2were left still at 20◦C for another 6 h The products were filtered and washed with 2.0 M HCl and DI water each for three times in turn The products were dried at 80◦C for 12 h under vacuum The FTIR spectra of pure PANI and PANI/TiO2

Fig 3 Dynamic responses of the sensors based on PANI/TiO 2 nanocomposite thin films prepared at (a) −10 ◦C, (b) 0◦C, (c) 10◦C, (d) 20◦C, and (e) 30◦C to NH

3

at room temperature.

Fig 4 Responses of the nanocomposite sensors prepared at different

polymer-ization temperature to NH 3 of different concentration.

nanocomposites are shown in Fig 1 The main characteristic

peaks of pure PANI are assigned as follows: the bands at 1565

and 1488 cm−1are attributed to C N and C C stretching mode

of vibration for the quinonoid and benzenoid units of PANI;

1291 and 1133 cm−1are the stretching peak of C N and C N,

respectively; the peak at 797 cm−1 is assigned to C H

bend-ing vibration out of the plane of the para-disubstituted benzene

rings[2,11–13] For PANI/TiO2 nanocomposite, the IR

spec-trum is almost identical to that of pure PANI, but all bands shift

slightly, indicating that some interaction exists between PANI

and nano-TiO2 In addition, the absorption band at 1408 cm−1

can be assigned to the in-plane bending vibration of O H on the

surface of TiO2[14]

3.2 UV–vis absorption spectra of PANI and PANI/TiO 2

nanocomposite thin films

Fig 2 depicts the UV–vis absorption spectra of PANI and

PANI/TiO2 thin films deposited on the PDDA/PSS glass

sub-strate under different polymerization temperatures It shows that

Fig 5 Schematic energy-band diagram for PANI/TiO nanocomposite.

three characteristic bands of the doped PANI thin film prepared

at 20◦C appear at about 361, 402 and 859 nm, which can be attributed to the␲–␲*, polaron–␲* and ␲–polaron transitions, respectively[6,7,11] It can be noted that the characteristic peaks

of the doped PANI all appear in the PANI/TiO2 nanocompos-ite thin films, but there are some shifts compared with pure PANI, and some new peaks are observed in the PANI/TiO2 nanocomposite thin films shown in Table 1, which indicates that encapsulation of nano-TiO2particles has the effect on the doping of conducting PANI, while this effect should owe to an interaction at the interface of PANI and nano-TiO2particles[11] Also it could be obviously observed that the absorption inten-sity increases with decreasing the temperature, i.e., the thickness

of the in-situ self-assembly deposited PANI/TiO2 thin films increases with decreasing the temperature, which is consistent with the previously reported results[15]

3.3 Gas-sensing properties measurements

NH3gas sensors were designed and fabricated to operate as

a resistive element Gold sputtered interdigital electrodes were fabricated on a 5 mm× 8 mm silicon substrate to form a trans-ducer, which were directly used to measure the resistance change

of the sensitive PANI/TiO2nanocomposite thin film layer when exposed to NH3gas of different concentrations

The device was put into a test box (320 mL), and a certain amount of NH3gas was injected into the test chamber after the resistance reached a steady value in clean air Gas exposure time was ca 150 s for each pulse of NH3gas and the chamber was purged with clean air for ca 200 s after each pulse to allow the surface of the sensitive film to regain atmospheric condition

A Keithley 2700 data acquisition system was used to measure the resistance variation of the sensors The measurement was processed at room temperature

Dynamic responses of the sensors based on PANI/TiO2thin films fabricated under different polymerization temperatures to NH3are shown inFig 3a–e It can be seen that the resistance of all sensors increases dramatically after exposed to NH3gas It is also observed that fast resistance increases were followed by a decrease of resistance, which is most obvious for the thin films

Fig 6 Gas responses of the nanocomposite sensor prepared via polymerization

at 10 ◦C to 23 ppm NH, 23 ppm CO and 500 ppm H.

Fig 7 Sensing reproducibility of the nanocomposite sensor prepared via

poly-merization at 10 ◦C to 23 ppm NH

3 at room temperature.

prepared at−10 and 30◦C, indicating that either more than one

type of reaction sites are available or that a number of different

reactions are possible[16] It is believed that the porous structure

of thin films leads to the predominance of surface phenomena

over the bulk material phenomena, and therefore the resistance

increases significantly with time initially when PANI

nanocom-posite thin films contact with NH3by gas injection, which may

be due to the surface adsorption effect, and the chemisorptions

leads to the formation of ammonium However, the interaction

process between the thin film and the adsorbed gas is a

dynami-cal process Thus, when the thin film is exposed to NH3gas, the

adsorption and desorption processes will simultaneously occur,

and the thinner the films, the quicker the gas desorption Then,

the resistance attains a stable value when dynamic equilibrium

is attained However, further interaction mechanism study needs

to be carried out to verify this speculation Additionally, the

sen-sors based on PANI/TiO2thin films prepared at−10 and 30◦C

exhibit incomplete reproducibility, which is especially serious

for the thin films fabricated at −10◦C, as shown inFig 3a,

indicating a low reproducibility to NH3

The gas response is defined as (Rgas − Rair)/Rair, where Rair

is the resistance of sensor in air and Rgasis the steady resistance

of sensor in the presence of a tested gas The response values of

all the samples are plotted as a function of NH3concentration in

Fig 4, indicating a highly linear characteristic and the highest

Fig 8 Long-term variation in resistance of the nanocomposite sensors prepared

via polymerization at room temperature (25 ◦C) and 10◦C.

Fig 9 Response of the sensor prepared via polymerization at 10 ◦C to NH3of

different concentrations after 0, 6, and 30 days.

response value for the sensor composed of the PANI/TiO2thin film prepared at 10◦C.

We define the response and recovery time as the time required

to reach 90% total resistance change As the sensors based on PANI/TiO2thin films prepared at−10 and 30◦C exhibited slow reaction equilibrium and low reproducibility to NH3, only the characteristic times of other three sensors in the case of various NH3concentrations are presented inTable 2 It shows that the

response time (T1) is almost independent on the gas

concentra-tion, and a fast response time of 2 s can be observed to different NH3 concentrations for the PANI/TiO2 thin film prepared at

10◦C; the recovery time (T2), however, decreases with increas-ing the gas concentration The reason to cause this phenomenon

is that an increase in concentration leads to an increased amount

of chemisorbed NH3, which in turn enhances the desorption rate and sensing site renewal[1]

Kukla et al [17] proposed that the mechanism to explain the sensitivity and reversibility of PANI layers to NH3 was a deprotonation–reprotonation process, and the resistance showed

an exponential growth with an increase in NH3concentration However, it can be seen fromFig 3a–e that the resistance of the

Fig 10 Response to 23 ppm NH 3 of the sensor prepared via polymerization at

10 ◦C as a function of temperature.

PANI/TiO2nanocomposite thin films grew instantaneously and

did not follow the resistance change trend observed with PANI

sensors We have postulated that PANI and TiO2 form a p–n

junction and the inter-particle electron transition from TiO2to

PANI causes the reduction of the activation energy and enthalpy

of physisorption for NH3gas[10].Fig 5exhibits a schematic

energy diagram to further illuminate the NH3gas sensing

mech-anism of PANI/TiO2nanocomposite thin films[14,18], where

HOMO presents the highest occupied molecular orbital, and

LUMO is the lowest unoccupied molecular orbital Li et al.[14]

showed that charge separation was enhanced due to well energy

bandgap matching between the conduction band of TiO2 and

the LUMO level of PANI for charge transfer Therefore, it is

also believed that such enhancement promotes NH3gas-sensing ability of the PANI/TiO2nanocomposite

Generally, a single chemical sensor has cross-sensitivity, which hinders it from practical application Herein we measured the responses of the sensor composed of the thin film prepared at

10◦C to 23 ppm CO and 500 ppm H2, and the results are shown

inFig 6 It can be seen that there is a distinct difference in gas responses to the tested gases, and the sensor showed very weak responses to CO and H2 Based on this observation, it is believed that the sensor exhibits high selectivity to NH3 In addition, the response of this sensor was monitored for the repeated expo-sure and removal of 23 ppm NH3up to three cycles, as shown

inFig 7, indicating high reproducibility and reversibility

Fig 11 SEM images of PANI/TiO nanocomposite thin films prepared at (a) −10 ◦C, (b) 0◦C, (c) 10◦C, (d) 20◦C, (e) 30◦C.

Our earlier work[10]showed that the resistance of the

sen-sor prepared via polymerization at room temperature (25◦C)

increased greatly when the sample was stored in air, whereas

the response of the sensor to NH3decreased by a factor of 2–3

during 30 days Therefore, the lack of long-term stability of

the fabricated sensor was of concern The resistance changes

of the sensors prepared via polymerization at room

tempera-ture and 10◦C are shown inFig 8 It can be observed that the

latter exhibits higher long-term stability in resistance than the

former The excellent stability of the latter was also confirmed

by testing its response after a 6- and 30-day period, as shown in

Fig 9, indicating that the response values do not exhibit

signifi-cant change Meanwhile, it was found that the response/recovery

time also kept stable

3.4 Temperature characteristic

It is well known that humidity and temperature have a

sig-nificant effect on the operation of gas sensors The effect of

humidity on the performance of PANI/TiO2nanocomposite thin

film sensors was investigated in our earlier work[10], and then

the temperature dependence of response was studied here The

response to 23 ppm NH3 of the sensor based on the thin film

fabricated at 10◦C is shown as a function of temperature in

Fig 10 It is observed that the response decreases with

increas-ing temperature, which is in good agreement with the results of

the reported NH3gas sensor based on PANI[17,19] This

indi-cates that the adsorption–desorption equilibrium shifts in the

desorption direction with increasing temperature[17]

Accord-ingly, the prepared gas sensor is preferred to be operated at room

temperature

3.5 Surface morphology of PANI/TiO 2 thin films

The effect of morphology of sensitive films such as grain size,

structural formation, surface-to-volume ratio and film thickness

on the gas sensitivity, was well recognized[3] Therefore, the

SEM images of all the PANI/TiO2 thin films prepared under

different polymerization temperatures are shown inFig 11 It

can be seen that all the films have a very porous structure,

inter-connected network of fibers and high surface area It has been

pointed out that such structure contributes to a rapid diffusion

of dopants into the film[10] However, as shown in Fig 11,

the PANI/TiO2 thin film prepared at −10◦C exhibits a

two-dimensional slab surface including some disorderly fibers, and

the PANI/TiO2 thin film prepared at 0◦C has a rough surface

and consists of coral-like particulates It is considered that such

kind of structure affects the diffusion of gas into and out of

the entire bulk of the fibrous film In contrast, there are almost

no granular particulates in the nanocomposite films prepared

at 10, 20 and 30◦C The difference is that the film fabricated

at 10◦C has many uniform cylinders and presents a compact

three-dimensional surface, whereas the films fabricated at 20

and 30◦C exhibit a relative sparse network structure All the

above factors are in accordance with the observation that the

sensor consisted of the thin film prepared at 10◦C has a faster

and higher response to NH3gas

Li et al [20] proposed that during the synthesis of PANI its aggregation was triggered by heterogeneous nucleation, and the nucleation behavior of PANI is strongly dependent on the polymerization rate determined by reaction temperature As the formation of new embryonic nuclei is much faster at high tem-perature, it is more likely that these embryonic nuclei evolve

to create homogeneous nuclei before they can diffuse into heterogeneous nucleation sites to nucleate Thus, more PANI molecules will precipitate via homogeneous nucleation at higher temperature, and the possibility of heterogeneous nucleation will

be decreased This is also in agreement with the observation that the thickness of in-situ self-assembly films increases with decreasing temperature, as shown inFig 2 Therefore, the films prepared at high temperatures have better uniformity than those prepared at−10 and 0◦C.

4 Conclusions

The NH3sensing capability of PANI/TiO2thin film sensors was investigated and the polymerization temperature was opti-mized The PANI/TiO2 thin film prepared at 10◦C exhibited stable, reproducible and reversible resistance change in the pres-ence of NH3in the range of 23–141 ppm The response time was kept 2 s, and the recovery rate was also very fast in the range

of 20–60 s depending on the NH3 concentration The sensor also had high selectivity and long-term stability The difference

of gas-sensing property among sensors prepared under differ-ent polymerization temperatures was characterized with UV–vis spectra and SEM, and a simple schematic energy diagram was presented to illuminate the gas sensing mechanism

The in-situ self-assembly approach proposed in this work is easy-processing and feasible, which is also prone to produce PANI/TiO2 thin film in large scale In addition, the prepared PANI/TiO2gas sensor is preferred to be operated at room tem-perature, which has potential to develop the practical NH3gas sensor at low cost

Acknowledgements

This work was supported by National Science Foundation of China via grant no 60425101 and Program for New Century Excellent Talents in University via grant no NCET-06-0812

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Biographies

Huiling Tai received her BS degree in the field of electronic materials and

components from UESTC in 2003 Currently she is a PhD candidate of School

of Optoelectronic Information at UESTC Her major field of scientific interests

is conducting polymer and its composite for gas sensor application.

Yadong Jiang graduated from Department of Material Science & Engineering

at UESTC with a BS degree in 1986 Then he got his MS degree and PhD degree in 1989 and 2001, respectively, from that department at UESTC He is Professor and Dean of School of Optoelectronic Information at UESTC His major research interests include optoelectronic material and devices, sensitive materials and sensors.

Guangzhong Xie got his MS degree in Physics from Sichuan University He

is the Associate Professor of School of Optoelectronic Information at UESTC since 2001 His research interests are sensitive material and sensors.

Junsheng Yu got his PhD degree from Graduate School of Bio-Applications &

System Engineering at Tokyo University of Agriculture and Technology in 2001 Currently he is Professor of School of Optoelectronic Information at UESTC His research field is organic optoelectronic materials and devices.

Xuan Chen received her BS degree in chemistry from UESTC in 2005 Currently

she is a MS student of School of Optoelectronic Information at UESTC Her major is polymer science for gas sensors.

Zhihua Ying received her BS degree in 2002 and MS degree in 2005 from

UESTC She is in study for her PhD degree Her research interests are sensitive materials and sensors.