composite of tio2 nanowires and nafion as humidity sensor material

Composite humidity sensing films were made by using TiO2nanowires, TEOS and Nafion.. The composite films coated on a pair of gold electrodes were tested for humidity sensors of resistanc

Sensors and Actuators B 115 (2006) 198–204

Ren-Jang Wua,∗, Yi-Lu Sunb, Chu-Chieh Linb,∗∗, Hui-Wen Chenc, Murthy Chavalic

aDepartment of Applied Chemistry, Providence University, 200 Chungchi Road, Shalu, Taichung, Hsien 433, Taiwan, ROC

bDepartment of Chemistry, National Chung-Hsing University, Taichung 402, Taiwan, ROC

cCenter for Measurement Standards, Industrial Technology Research Institute, Hsinchu 300, Taiwan, ROC

Received 21 January 2005; received in revised form 6 September 2005; accepted 6 September 2005

Available online 10 October 2005

Abstract

Homogeneous TiO2nanowires were fabricated by hydrothermal method SEM pictures proved the yield of nanowires to be more than 90% Composite humidity sensing films were made by using TiO2nanowires, TEOS and Nafion FTIR absorption spectroscopy was used as a semi-quantitative method to get information about the protonation The sensing films were prepared by a dip-coating method The composite films coated

on a pair of gold electrodes were tested for humidity sensors of resistance type The measurement was carried out at five fixed humidity points

in the range of 12–97% relative humidity, which were controlled by employing five different salt solutions Resistance changes were about three orders of magnitude The nanowires-based humidity sensors showed moderate sensitivity, short response and recovery time (<2 min) at relative humidity less than 76%, and good long-term stability

© 2005 Elsevier B.V All rights reserved

Keywords: Nanowires; Nafion; Humidity; Impedance

1 Introduction

Needs for humidity sensors are growing in industrial and

agricultural applications for monitoring and controlling the

surroundings is growing Different measuring techniques, like

impedance[1,2], capacity[3–5], field effect transistors (FET)

[6], surface-acoustic wave (SAW)[7], quartz crystal

microbal-ance (QCM)[8–11], fiber optic[12–15]and microwave sensors

[16], have been explored for humidity detection In recent years,

nanorod and nanowire films were fabricated and their

humid-ity sensitive characteristics have been investigated[17–19], and

these nanomaterial films were found to be efficient

humid-ity sensors In consideration of qualhumid-ity and cost, impedance

type humidity sensors are becoming more prevalent

Humidity-sensitive materials used in various fields are classified into three

groups: electrolytes, organic polymers and porous ceramics

[20]

∗Corresponding author Tel.: +886 4 26328001x15212; fax: +886 4 26327554.

∗∗Corresponding author Tel.: +886 4 22840411x718.

E-mail addresses: rjwu@pu.edu.tw (R.-J Wu), cchlin@mail.nchu.edu.tw

(C.-C Lin).

Ceramic humidity sensors usually show better chemical resis-tance and mechanical strength than polymer sensors TiO2 sens-ing materials are commonly used in research for the reason

of easy fabrication Gusmano and co-workers[21,22]used the TiO2modified with 1–10% K+and Li+through a sol–gel method

as a sensing material The electrical resistance of the material showed a variation of seven orders of magnitude with the change

in relative humidity (RH) from 4 to 90% A humidity sensing material ZrO2–TiO2 increased the sensitivity by doping with

Li+in the research of Jain et al.[23] Nitsch et al.[24]used an active thick film layer based on ZnO–TiO2–Cr2O3 as a sens-ing material Traversa and co-workers[25]used the technology

of electrochemical impedance spectroscopy to investigate the humidity-sensing electrical conduction mechanism of the films

of TiO2doped with 1–10% K+and Li+in the RH range of 4–87% RH

TiO2nanowires are a kind of nano-scale material and have been successfully synthesized by some research groups through hydrothermal treatment, chemical vapor deposition or other methods[26–28] The TiO2nanowires are very intriguing as a humidity-sensing material In the present study, therefore, com-posite films of TiO2nanowires and Nafion were made, since such composites of fine ceramic particles and polymers are often used

as humidity sensors[1,2,31] 0925-4005/$ – see front matter © 2005 Elsevier B.V All rights reserved.

doi:10.1016/j.snb.2005.09.001

2 Experiment

2.1 Fabrication of TiO 2 nanowires

TiO2 nanowires were prepared by using hydrothermal

method in our laboratory One gram of anatase TiO2 powder

(Sigma–Aldrich Co., Inc., USA) was placed into a Teflon-lined

autoclave, and 40 ml of 10 M aqueous NaOH solution was added

Heating was maintained at 200◦C for 24 h without stirring After

the autoclave was cooled to room temperature naturally, the

obtained sample was washed sequentially with a dilute aqueous

HCl solution, distilled deionized water and ethanol sequentially

several times A fibrous white crystalline product was obtained

after drying the sample at 70◦C for 6 h.

2.2 Sensing material fabrication

Humidity sensing materials were fabricated by mixing the

TiO2 material (powder or nanowires), a Nafion solution and a

tetraethyl orthosilicate (TEOS) solution by the weight ratio of

1:500:500, but at various ratios for subsequent studies, including

0% for each component The purity of the anatase TiO2

pow-der was >99% (Sigma–Aldrich Co., Inc., USA) The Nafion®

solution was obtained from Aldrich (USA) and the

concen-tration was 5 wt.% in a mixture of lower aliphatic alcohols

and water The TEOS (98%) as a binding material was

pur-chased from ACROS Organic Co., Inc., USA and was dissolved

into a mixed solution of methanol and water at a volume ratio

of TEOS:C2H5OH:H2O = 5:16:2 The sensing films were dip

coated on an alumina substrate of 10 mm× 5 mm on which pair

of comb-like gold electrodes had been made (seeFig 1),

fol-lowed by drying at 120–150◦C for 1 h.

2.3 Measurement systems setup

An LCZ meter (DU-6022, made from Delta United

Instru-ment Co., Ltd.) was used in measuring the impedance signals of

the humidity sensors

The standard humidity measurement system is shown

in Fig 2 Actually, five systems with different humidity’s

were setup The humidity in each setup was controlled by

employing five different saturated salt solutions, and was

calibrated with a standard fixed-point calibration with a

standard hygrometer (Rotronic M-131, UK) to the

humid-Fig 1 Structure of a humidity sensor element.

ity standard of National Measurement Laboratory, Taiwan, ROC The five humidity-controlling salt solutions of LiCl, MgCl2, NaBr, NaCl and K2SO4were kept at a constant tem-perature 25± 2◦C and the resulting humidity values were 12.0± 0.2, 33.2 ± 0.4, 50.0 ± 0.4, 75.8 ± 0.2 and 96.9 ± 0.6%

RH, respectively Each humidity system has a dimension of

150 mm× 120 mm × 100 mm Before first measurement of a sensor, aging of each sensor was performed for 2 weeks in a 97%

K2SO4salt solution system Long-term stability of the sensors was tested in the humidity measurement systems for about 8 months

Data on the temperature effect were obtained from experi-ments carried out under the divided flow humidity system[8] The divided flow humidity generator contained a dry-air flow and a saturated humidity-air flow The saturated humidity-air and dry air were mixed together and then fed into a bottle-like test chamber with a volume of 10 l to generate air of the required humidity at a total flow rate of 10 l min−1.

The apparatus for the divided flow system was Protec PC-540 from Sierra Instruments Inc., which was equipped with two mass flow controllers and flow display power-supply The humidity sensors prepared were tested and calibrated in the test chamber The relative humidity, RH, of the test chamber was approxi-mately given by

% RH= Msat

Msat+ Mdryf × 100%

Fig 2 Humidity measurement system.

200 R.-J Wu et al / Sensors and Actuators B 115 (2006) 198–204

where Msatand Mdry were the flow rates of saturated air and

dry air, and ‘f’ was the coefficient of this system which was

dependent on temperature and flow rate The homemade

sim-ple apparatus has been developed for producing air of a known

relative humidity at temperatures ranging from 15 to 35◦C.

2.4 FTIR experimental

The composite materials of TiO2–TEOS–Nafion were

sub-jected to FTIR analysis using a Horiba Fourier transform infrared

spectrometer [FT-720, Japan] equipped with a DTGS

detec-tor A NaCl crystal of 25 mm× 4 mm size (Spectral Systems

Inc., #955-3616, USA) was used to obtain the spectra Each

spectrum was collected at room temperature under atmospheric

pressure, at an average of 64 scans with a 2 cm−1resolution in

a transmission mode from 400 to 5000 cm−1 In all experiments

background spectra were measured

3 Results and discussion

3.1 SEM observations of TiO 2 nanowires

SEM pictures revealed that TiO2nanowires were successfully

fabricated by a hydrothermal method in our laboratory.Fig 3a

and b and other SEM pictures revealed a high yield of nanowires

Fig 3 SEM photographs of TiO2 nanowires: (a) magnification of ×40,000; (b)

magnification of ×10,000.

Fig 4 Humidity characteristic curves of various TiO2 materials: (
) TiO2 nanowires/Nafion, ( ) TiO2 powder/Nafion, ( 䊉) Nafion, () TiO2 nanowires, ( ) TiO2 powder.

over 90% The average length of the wires was about 5–10␮m, and the average diameter was 40–50 nm

3.2 Response due to addition of TiO 2 and Nafion

The original response calibration curve was defined by using the materials of TiO2 powder and nanowires combined with TEOS in Fig 4 Impedance (Z/ Ω) was the parameter of the

humidity measurement A good sensitive characteristic curve was not observed with the sensing materials of TiO2powder and nanowires combined with TEOS in the humidity ranging from

10 to 75% The figure also reveals that mixing the inorganic TiO2 and organic hydrophilic Nafion changes the sensing curve The addition of Nafion was found to result in a remarkable increase

in sensitivity and a decrease in the impedance of the humidity sensor Nafion has been reported to enhance the water adsorption

at the sites of hydrophilic ionic group, –SO3H−/+[29]. The composite of TiO2 nanowires/Nafion exhibited the higher sensitivity 2–3-folds than those of the composite TiO2 powder/Nafion or Nafion film alone The impedance change from the humidity range of 12–97% was more than three orders

of magnitude Especially in the humidity range of 10–40%, the TiO2nanowires/Nafion revealed a better sensitivity curve Some nanomaterials like carbon nanotubes (CNTs) had been used in our laboratory process to promote the adsorption of water[30] It has been reported that the SiO2/Nafion composite can add to the stability and good linearity of the sensor[31] A SEM picture of the composite TiO2nanowires/Nafion sensing film is shown in Fig 5 Most of the TiO2nanowires kept their original shape A part of nanowires penetrated into the surface of Nafion and some remained on the surface Nanowires have unusual electrical, optical, magnetic, mechanical, thermal and biological proper-ties due to their dimensions and high aspect ratios [length to width ratio] Thus TiO2nanowires with homogeneous morphol-ogy and high specific surface area can adsorb moisture easily and

Fig 5 SEM photograph of TiO2 nanowires/Nafion.

uniformly The high sensitivity of the TiO2nanowires/Nafion is

therefore attributed to the enhanced water adsorption on the TiO2

nanowires

3.3 FTIR experimental data

A typical infrared (IR) spectrum of the composite material

was recorded on a NaCl crystal Several bands were identified

from the obtained FTIR spectrum typical to the components

of the composite material (seeFig 6) Most of the obtained

vibrational bands were similar to those of the published data

TiO2-anatase form nanowires having characteristic peaks at 638,

513 and 397 cm−1were observed in the FTIR spectrum The

peak at 397 cm−1is not completely evident in the spectrum due

to the usage of NaCl crystal that has a cutoff nearly equal to

400 cm−1 Nevertheless, an initiation of the prominent peak,

that is specifically characteristic to TiO2-anatase, is obviously

visible from 404 cm−1.

A broad peak between 800 and 465 cm−1is assigned to the Ti–O–Ti stretching vibrations [13–15] with a valley centered

at 517 cm−1 The peak at 638 cm−1is superimposed with that

of a strong IR absorption peak of Si (640 cm−1) from TEOS

in the combined composite material spectrum The peaks at

800 and 969 cm−1are due to Si–O–Si and Si–OH, respectively, also from TEOS In addition there were also less significant

IR peaks at 509, 611, 755 and 805 cm−1 and a prominent peak at 669 cm−1 The band Ti–OH observed below 3500 cm−1 indicates the existence of hydrogen bonding This interaction between the organic and inorganic phase is favorable for the improvement of the thermal stability and optical transparency

of the composite films The IR spectrum of the hydrated titanium dioxide [h-TiO2] shows a large broad band between 3250 and

3490 cm−1and narrow bands at 1641 and 1454 cm−1 The broad band at 3250–3490 cm−1is assigned to the stretching mode of hydroxyl,δOH, while those at 1641 and 1454 cm−1are assigned

to bending modes of hydroxylδOH, respectively[32–34] Com-pared with the intensity of the characteristic absorption bands

of Ti–OH below 3500 cm−1, an increase in peak area ratio sug-gests increase in humidity sensing is due to the anatase TiO2 nanowires

Humidity sensing of Nafion is usually observed at 1300 and

1056 cm−1, which are anti-symmetric stretching and symmet-ric stretching vibrations of the –SO3 −, respectively However, the anti-symmetric stretching vibrations of the –SO3are unfortu-nately shrouded by the strong C–F stretching bands exactly over

1300 cm−1 Nevertheless humidity sensing can be evidenced with the transfer of proton and the shift in peak centered at

1057 cm−1 The presence of –SO

3 − also accounts indirectly for the proton transfer, supported by the increase in relative peak area ratio at 1300 cm−1, thus resulting in the disappear-ance of asymmetric bands of the SO3H− group at 1410 and

910 cm−1and the changes in relative peak area ratios at 1057 and 1300 cm−1.

Fig 6 FTIR spectrum of a TiO2 [anatase] nanowires–TEOS–Nafion composite.

202 R.-J Wu et al / Sensors and Actuators B 115 (2006) 198–204

Fig 7 Humidity response curve of TiO2 nanowires/Nafion.

3.4 Response signal and hysteresis

A response curve of the TiO2nanowires/Nafion film is shown

inFig 7 The response and recovery time T90(T90is defined as

the time required to reach the 90% of the final equilibrium signal)

was less than 1.5 min at the humidity range, of 12–75% The

response time at the humidity of 97% was much longer than those

for the humidity ranging from 12 to 76% This phenomenon was

also observed with the Nafion film

Fig 8 reveals the hysteresis data of the TiO2 nanowires/

Nafion film Hysteresis was calculated as: [log10(impedance

descending) log10(impedance ascending)/log10(impedance

descending at fixed point RH)] The hysteresis of the sensing

film was small (calculated <2%)

3.5 Temperature effect and long time stability test

Temperature has some interference on the resistance

sig-nal of the TiO2 nanowires/Nafion film as shown in Fig 9

R= R0exp(−Ea/RT) was used to calculate the activation energy

(Ea) of water adsorption on the sensing film Rrepresents the

Fig 8 Hysteresis effect of TiO2 nanowires/Nafion film: ( ) descending

humid-ity, ( ) ascending humidity.

Fig 9 Temperature effect of TiO2 nanowires/Nafion film: (
) 35 ◦C, () 25 ◦C, ( ) 17 ◦C.

Fig 10 Long time stability test of TiO2 nanowires/Nafion film: ( ) 12% RH, ( 䊉) 33% RH, (
) 50% RH, () 76% RH, () 97% RH.

resistance of the humidity sensor, R is the gas constant and T the absolute temperature Eaof 12.9 kcal mol−1was calculated

from the slope of the plot of ln(R) with ln(1/T) in 60% humidity

with a temperature interval of nearly 10◦C.

The test data of long-term stability is shown inFig 10 The impedance values of the sensor at five different testing points of

12, 33, 50, 76 and 97% RH did not show obvious deviation for

250 days

4 Conclusion

In this research, humidity sensing was investigated by using the TiO2 nanowires/Nafion material For humidity range of 12–97% the change in resistance of the TiO2nanowires/Nafion sensing film was observed to be more than 1000 The nanowires humidity sensor showed moderate sensitivity, short response and

recovery time (<2 min) for smaller than 76% humidity and a

good long-term stability for up to 250 days

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Biographies

Ren-Jang Wu is an assistant professor in Department of Applied Chemistry

at Providence University He received a BS in Chemistry from National Tsing Hua University in 1986, an MS in Chemistry from National Taiwan University

in 1988 and a PhD in Chemistry from National Tsing Hua University in 1995 His main areas of interest are chemical sensors, catalysis, nanoscience and chemical standard technology.

Yi-Lu Sun received a BS degree in Chemistry from Soochow University in

1995, and an MS degree in Chemistry from National Chung-Hsing University

in 1997 He entered the PhD course of Chemistry at National Chung-Hsing University in 2003 His main areas of interest are inorganic chemistry and chemical sensor technology.

Chu-Chieh Lin is a professor of Department of Chemistry at National

Chung-Hsing University He received a BS degree in Chemistry from Soo-chow University in 1981, an MS degree in Nuclear Science from National Tsing-Hua University in 1983 and a PhD degree in Chemistry from Texas Tech University in 1992 His research interests are in inorganic chemistry and chemical sensor technology.

Hui-Wen Chen received a BS in Chemistry from Chung Yuan Christian

University in 1998, and an MS in Chemistry from National Chung-Hsing University in 2000 Her main areas of interest are electroanalytical chemistry and chemical sensor technology.

Murthy Chavali received MSc (Tech.) in Chemistry from Jawaharlal Nehru

Technological University, India in 1994 and PhD Tech in 2000 from Technis-che Universit¨at Wien, Austria in Analytical Chemistry He was a postdoctoral

204 R.-J Wu et al / Sensors and Actuators B 115 (2006) 198–204

scientist at Center for Instrumental Analysis, Kobe University, Japan, on

Japanese National Fellowship (JSPS) worked for NIR combustion sensors.

He served as Researcher at NSC-Taiwan for a short period After that he

joined as a Researcher with sensors and standards group at

CMS/ITRI-Taiwan His research interests are optical waveguide technology, IR sensors,

LIF, chip based chemical and biochemical sensors ( ␮ & n), development and application of spectroscopic techniques for the study of nanomaterials His present work focuses on synthesis and fabrication of various organic and inorganic nanostructures, nanocomposite materials, broadly nanotechnology applications for gas and liquid sensors.