Sensors and Actuators B 115 (2006) 198–204 Composite of TiO 2 nanowires and Nafion as humidity sensor material Ren-Jang Wu a,∗ , Yi-Lu Sun b , Chu-Chieh Lin b,∗∗ , Hui-Wen Chen c , Murthy Chavali c a Department of Applied Chemistry, Providence University, 200 Chungchi Road, Shalu, Taichung, Hsien 433, Taiwan, ROC b Department of Chemistry, National Chung-Hsing University, Taichung 402, Taiwan, ROC c Center 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 TiO 2 nanowires 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 TiO 2 nanowires, 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 quality 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.:+8864 26328001x15212;fax: +886 426327554. ∗∗ 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 sensorsusually show better chemical resis- tance and mechanical strength than polymer sensors. TiO 2 sens- ing materials are commonly used in research for the reason of easy fabrication. Gusmano and co-workers [21,22] used the TiO 2 modified 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 ZrO 2 –TiO 2 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–TiO 2 –Cr 2 O 3 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 TiO 2 doped with 1–10% K + and Li + in the RH range of 4–87% RH. TiO 2 nanowires 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 TiO 2 nanowires are very intriguing as a humidity-sensing material. In the present study, therefore, com- posite films ofTiO 2 nanowires 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 R J. Wu et al. / Sensors and Actuators B 115 (2006) 198–204 199 2. Experiment 2.1. Fabrication of TiO 2 nanowires TiO 2 nanowires were prepared by using hydrothermal method in our laboratory. One gram of anatase TiO 2 powder (Sigma–Aldrich Co., Inc., USA) was placed into a Teflon-lined autoclave, and40 ml of10 M aqueous NaOHsolution was added. Heating was maintained at 200 ◦ C for24 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 TiO 2 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 TiO 2 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:C 2 H 5 OH:H 2 O = 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 (see Fig. 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, MgCl 2 , NaBr, NaCl and K 2 SO 4 were 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% K 2 SO 4 salt 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= M sat M sat + M dry f × 100% Fig. 2. Humidity measurement system. 200 R J. Wu et al. / Sensors and Actuators B 115 (2006) 198–204 where M sat and M dry 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 TiO 2 –TEOS–Nafion were sub- jected to FTIR analysisusinga 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 −1 resolution 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 TiO 2 nanowires 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 TiO 2 nanowires: (a) magnification of ×40,000; (b) magnification of ×10,000. Fig. 4. Humidity characteristic curves of various TiO 2 materials: ()TiO 2 nanowires/Nafion, ()TiO 2 powder/Nafion, (᭹) Nafion, ()TiO 2 nanowires, ()TiO 2 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 TiO 2 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 TiO 2 powder and nanowires combined with TEOS in the humidity ranging from 10 to75%. The figure also reveals that mixingthe inorganic TiO 2 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, –SO 3 H −/+ [29]. The composite of TiO 2 nanowires/Nafion exhibited the higher sensitivity 2–3-folds than those of the composite TiO 2 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 TiO 2 nanowires/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 SiO 2 /Nafion composite can add to the stability and good linearity of the sensor [31]. A SEM picture of the composite TiO 2 nanowires/Nafion sensing film is shown in Fig. 5. Most of the TiO 2 nanowires 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 TiO 2 nanowires with homogeneous morphol- ogy and high specific surface area can adsorb moisture easily and R J. Wu et al. / Sensors and Actuators B 115 (2006) 198–204 201 Fig. 5. SEM photograph of TiO 2 nanowires/Nafion. uniformly. The high sensitivity of the TiO 2 nanowires/Nafion is therefore attributed to theenhanced water adsorption on theTiO 2 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 (see Fig. 6). Most of the obtained vibrational bands were similar to those of the published data. TiO 2 -anatase formnanowires having characteristic peaks at638, 513 and 397 cm −1 were observed in the FTIR spectrum. The peak at 397 cm −1 is 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 TiO 2 -anatase, is obviously visible from 404 cm −1 . A broad peak between 800 and 465 cm −1 is assigned to the Ti–O–Ti stretching vibrations [13–15] with a valley centered at 517 cm −1 . The peak at 638 cm −1 is 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 −1 are 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-TiO 2 ] shows a large broad band between 3250 and 3490 cm −1 and narrow bands at 1641 and 1454 cm −1 . The broad band at 3250–3490 cm −1 is assigned to the stretching mode of hydroxyl, δ OH , while those at 1641 and 1454 cm −1 are assigned to bending modes of hydroxyl δ OH , 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 TiO 2 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 –SO 3 − , respectively. However, the anti-symmetric stretching vibrations of the–SO 3 are 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 SO 3 H − group at 1410 and 910 cm −1 and the changes in relative peak area ratios at 1057 and 1300 cm −1 . Fig. 6. FTIR spectrum of a TiO 2 [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 TiO 2 nanowires/Nafion. 3.4. Response signal and hysteresis A response curve of the TiO 2 nanowires/Nafion film is shown in Fig. 7. The response and recovery time T 90 (T 90 is defined as the time requiredto 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 thehumidity of97%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 TiO 2 nanowires/ Nafion film. Hysteresis was calculated as: [log 10 (impedance descending) log 10 (impedance ascending)/log 10 (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 TiO 2 nanowires/Nafion film as shown in Fig. 9. R  = R 0 exp(−E a /RT) was used to calculate the activation energy (E a ) of water adsorption on the sensing film. R  represents the Fig. 8. Hysteresis effect of TiO 2 nanowires/Nafion film: () descending humid- ity, () ascending humidity. Fig. 9. Temperature effect of TiO 2 nanowires/Nafion film: ()35 ◦ C, ()25 ◦ C, ()17 ◦ C. Fig. 10. Long time stability test of TiO 2 nanowires/Nafion film: () 12% RH, (᭹) 33% RH, () 50% RH, () 76% RH, () 97% RH. resistance of the humidity sensor, R is the gas constant and T the absolute temperature. E a of 12.9 kcal mol −1 was 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 in Fig. 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 TiO 2 nanowires/Nafion material. 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Chen, S.J. Lee, L.H. Lee, J.L. Lin, Synthesis and characteriza- tion of trialkoxysilane-capped poly(methyl methacrylate)-titania hybrid optical thin films, J. Mater. Chem. 9 (1999) 2999. 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. . Sensors and Actuators B 115 (2006) 198–204 Composite of TiO 2 nanowires and Nafion as humidity sensor material Ren-Jang Wu a,∗ ,. curve was defined by using the materials of TiO 2 powder and nanowires combined with TEOS in Fig. 4. Impedance (Z/Ω) was the parameter of the humidity measurement.