Properties of humidity sensing ZnO nanorods-base sensor ...

Sensors and Actuators B 133 (2008) 638–643
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Sensors and Actuators B: Chemical
journal homepage: www.elsevier.com/locate/snb
Properties of humidity sensing ZnO nanorods-base sensor fabricated by
screen-printing
Qi Qi a , Tong Zhang a,b,∗ , Qingjiang Yu c , Rui Wang a , Yi Zeng a ,LiLiu a , Haibin Yang c
a State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, PR China
b Key Laboratory of Low Dimensional Materials & Application Technology, Ministry of Education, Xiangtan 411105, PR China
c National Laboratory of Superhard Materials, Jilin University, Changchun 130012, PR China
article
info
abstract
Article history:
Received 29 December 2007
Received in revised form 25 March 2008
Accepted 31 March 2008
Available online 8 April 2008
Keywords:
Humidity sensitivity
Flower-like ZnO nanorods
Complex impedance plots
Sensor
The humidity sensitive characteristics of a sensor fabricated from flower-like ZnO nanorods by screenprinting
on a ceramic substrate with Ag–Pd interdigital electrodes have been investigated. The sensor
shows high humidity sensitivity, rapid response and recovery, small hysteresis, and good stability. It is
found that the impedance of the sensor decreases by about five orders of magnitude with increasing
relative humidity (RH) from 11 to 95%. The response and recovery time of the sensor is about 5 and
10 s, respectively. These results indicate that the flower-like ZnO nanorods can be used in fabricating
high-performance humidity sensors.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
Fabrication of sensitive chemical sensors has gained special
focus driven by their diverse applications in air-quality detection,
inflammable-gas inspection, environmental monitoring, healthcare,
defense and security, and so on [1–5]. Recently, inspired
by the advantages of high surface-to-volume ratios, fabrication
of nanomaterial electronic devices and exploration of their properties
are of current interest [6–18]. Hitherto, different types of
sensors based on three-dimensional (3D), two-dimensional (2D),
one-dimensional (1D), and zero-dimensional (0D) architectures
have been successfully obtained [10–18]. Among those nanostructures,
1D sensors, based on ceramic structures (SnO 2 [19],TiO 2 [20],
ZnO [14–18],In 2 O 3 [21], and WO 3 [22]), have attracted much focus
owing to their high surface area and low dimensionality, which
could facilitate fast mass transfer of the analyte molecules to and
from the interaction region as well as require charge carriers to
transverse the barriers introduced by molecular recognition along
the 1D nanostructures [23]. Although many successes have been
obtained, most of those papers focus on the gas sensors (e.g. CO,
O 2 , and C 2 H 5 OH) [10–18], and few papers on humidity sensors have
∗ Corresponding author at: State Key Laboratory on Integrated Optoelectronics,
College of Electronic Science and Engineering, Jilin University, Changchun 130012,
PR China. Tel.: +86 431 85168385; fax: +86 431 85168270.
E-mail address: zhangtong@jlu.edu.cn (T. Zhang).
been explored. Additionally, the fabrication of sensitive and stable
humidity sensors with rapid response and recovery is still in great
demand.
In this paper, we report a highly sensitive humidity sensor with
rapid response and recovery, which is based on the flower-like ZnO
nanorods. ZnO has been chosen in our experiment for its versatile
properties in optoelectronic devices, sensors, lasers, transducers,
and photovoltaic devices [24,25]. Additionally, ZnO nanostructures
are believed to be nontoxic, bio-safe, and possibly biocompatible,
and have been used in many applications in our daily life. We
believe that our method not only provides a new avenue for fabricating
highly effective humidity sensors, but also offers a powerful
platform to understand and design desirable humidity sensors.
2. Experimental
2.1. Preparation and characterization of materials
Flower-like ZnO nanorods were synthesized by a simple wet
chemical method [26]. All chemicals (analytical grade reagents)
were purchased from Beijing Chemicals Co. Ltd. and used as
received without further purification. Deionized water with a resistivity
of 18.0 M cm −1 was used in all experiments. In a typical
synthesis process, 100 mL of an aqueous solution of zinc nitrate and
100 mL of a hexamethylenetetramine (HMT) aqueous solution of
equal concentration (0.05 M) were mixed together and kept under
mild magnetic stirring for 5 min. Then the solution was transferred
0925-4005/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.snb.2008.03.035

Q. Qi et al. / Sensors and Actuators B 133 (2008) 638–643 639
Fig. 1. (a) Scheme of the sensor structure and (b) top-view optical micrograph of the sensor.
into a 500-mL flask and heated at 90 ◦ C for 3 h with refluxing. Subsequently,
the resulting white products were centrifuged, washed
with deionized water and ethanol and dried at 60 ◦ C in air for further
characterization.
X-ray diffraction (XRD) analysis was conducted on a
Rigaku D/max-2500 X-ray diffractometer with Cu K radiation
( = 1.5418 Å). Field-emission scanning electron microscopy
(FE-SEM) images were performed on a JEOL JEM-6700F microscope
operating at 3 and 5 kV. Transmission electron microscope (TEM)
images and selected area electron diffraction (SAED) patterns were
obtained on a JEOL JEM-2000EX microscope with an accelerating
voltage of 200 kV. Raman-scattering spectrum was measured by an
HR-800 LabRam confocal Raman microscope with a backscattering
configuration made by JY company in France, excited by the
514.5 nm line of an argon-ion laser at room temperature (25 ◦ C).
2.2. Fabrication and measurement of sensors
The flower-like ZnO nanorods powders were ground and mixed
with deionized water in a weight ratio of 100:25 to form a paste. The
paste was screen-printed on a ceramic substrate (6 mm × 3 mm,
0.5-mm thick) with five pairs of Ag–Pd interdigital electrodes (electrodes
width and distance: 0.15 mm) to form a film with a thickness
of about 10 m, and then the film was dried in air at 60 ◦ Cfor
5 h. In order to improve the sensor antipollution, we made a 0.1 g
of an ethyl cellulose solution in ethyl ester acetate (4 mL), which
was coated on the surface of the sensitive film as a protective
layer [27]. Finally, the humidity sensor was obtained after aging
at 95% relative humidity (RH) with a voltage of 1 V, 100 Hz for
24 h. Fig. 1 shows the structure and optical micrograph of the
sensor.
The characteristic curves of humidity sensitivity were measured
on a ZL-5 model LCR analyzer (Shanghai, China). The voltage applied
in our studies was ac 1 V, and the frequency varied from 40 Hz to
100 kHz. The sensor was successively put into several chambers
with different RH levels at a temperature of 25 ◦ C. The RH range of
11–95% was obtained using saturated salt solutions as the humidity
generation sources [27]. The six different saturated salt solutions
were LiCl, MgCl 2 , Mg(NO 3 ) 2 , NaCl, KCl, and KNO 3 , and their corresponding
RH values were 11, 33, 54, 75, 85, and 95% RH, respectively.
The structure of the flower-like ZnO nanorods has been characterized
by XRD as shown in Fig. 2. All the diffraction peaks can
be indexed as hexagonal ZnO with lattice constants a = 3.249 Å and
c = 5.206 Å, which are consistent with the values in the standard
card (JCPDS 36-1451). No diffraction peaks from any other impurities
are detected.
Fig. 3(a) and (b) shows the FE-SEM images of the as-prepared
products at different magnifications. Fig. 3(a) shows the lowresolution
image of the sample, indicating the flower structure
composed of closely packed nanorods with lengths of 1.5–3 m and
diameters of 200–400 nm. The high-resolution image in Fig. 3(b)
clearly reveals that the obtained ZnO exhibits well-defined flowerlike
morphology and each of the rods has one end outside and
another end binds to other rods. Further morphology characterization
of the ZnO sample was performed on a transmission electron
microscope (TEM) as shown in Fig. 3(c), which agrees with the
FE-SEM results. To further determine the accurate structure of the
product, the TEM–selected area electron diffraction pattern of the
product was recorded as shown in Fig. 3(d). From the SAED, all
the detectable dots are perfectly indexed to the same position as
those from hexagonal wurtzite ZnO structure, which grows along
the[0001]direction [28].
Raman spectroscopy is also carried out to study the vibrational
properties of the flower-like ZnO nanorods. Fig. 4 shows the roomtemperature
Raman spectrum of the ZnO nanorods. All observed
spectral peaks can be assigned to a wurtzite ZnO structure according
to the literature values [29]. The peak at 437 cm −1 is attributed
to the ZnO nonpolar optical phonon E 2 (high) mode. The peak at
409 cm −1 corresponds to the E 1 (TO) mode, but it is not obvious.
As the characteristic peak of hexagonal wurtzite ZnO, the E 2 (high)
at 437 cm −1 is very intense and has a full width at half-maximum
of 12 cm −1 . The asymmetrical and line-broadening characteristics
mask E 1 (TO) on the left-hand side of E 2 (high). The peak at 579 cm −1
is attributed to the E 1 (LO) mode, which is caused by the defects such
as oxygen vacancy, zinc interstitial, or their complexes [30]. In addition,
the peak at 378 cm −1 corresponds to the A 1 (TO) mode. Besides
these “classical” Raman modes, the Raman spectrum also shows
other modes with frequencies of 333, 541, 661, and 1147 cm −1 . These
3. Results and discussion
Fig. 2. XRD pattern of the flower-like ZnO nanorods.

640 Q. Qi et al. / Sensors and Actuators B 133 (2008) 638–643
Fig. 3. (a) Low-resolution FE-SEM image, (b) high-resolution FE-SEM image, (c) TEM image of flower-like ZnO nanorods, and (d) SAED pattern of a single nanorod.
additional peaks cannot be explained within the framework of the
bulk phonon modes, which are attributed to multiphonon scattering
processes [29].
The dependence of the impedance on RH is measured for the
flower-like ZnO nanorod sensor as shown in Fig. 5. From the curve,
it can be clearly seen that the impedance of the film decreases
remarkably with increasing the frequency at low RH, and the
impedance difference between adjacent two working frequencies
becomes progressively smaller with increasing RH. The reason
can be explained to be that at higher frequencies, the adsorbed
water cannot be polarized and the dielectric phenomenon does
not appear [31]. In order to gain high RH sensitivity and good linearity
over the entire RH range, low working frequency should be
applied. At a frequency of 100 Hz, the impedance change is found
to be about five orders of magnitude, which is more than those of
many humidity sensors reported in the literature.
It is well known that response and recovery behavior is an
important characteristic for evaluating the performance of humidity
sensors. Fig. 6 shows response and recovery characteristic
(corresponding to water molecules adsorption and desorption process)
curves for one cycle based on the ZnO sensor. The response
time (as the humidity changes from 11 to 95% RH) is about 5 s and
the recovery time (as the humidity changes from 95 to 11% RH) is
about 10 s for our sample, which is super-rapid than all the results
reported before (it is important to note that the time taken by a
sensor to achieve 90% of the total impedance change is defined as
the response time in the case of adsorption or the recovery time in
the case of desorption).
Fig. 4. Room-temperature Raman spectrum of the flower-like ZnO nanorods.
Fig. 5. Impedance vs. RH of flower-like ZnO nanorod sensor.

Q. Qi et al. / Sensors and Actuators B 133 (2008) 638–643 641
Fig. 6. Response of the flower-like ZnO nanorod sensor measured at 100 Hz.
Fig. 7 shows the properties of capacitance versus frequency at
different RH. It can be seen that the change of capacitance is inconspicuous
at high frequency (10 and 100 kHz), and the large change
of capacitance can be obtained at lower frequency (40 and 100 Hz).
This is because the electrical field direction changes slowly at low
frequency and there obviously appears the space–charge polarization
of adsorbed water. The higher the RH is and the more the water
molecules are adsorbed, the stronger the polarization is, and then
the larger the dielectric constant is. When the frequency is high, the
electrical field direction changes fast, the polarization of the water
cannot catch up with it, and hence the dielectric constant is small
and independent of RH [31,32].
Fig. 8 shows the humidity hysteresis characteristic of the humidity
sensor based on our products. The solid line in the figure is
measured from low RH to high RH, i.e. for the adsorption process,
and the dotted line is for desorption process (measured in the opposite
direction). A hysteresis of about 2% is observed under 80% RH.
This indicates a good reliability of the sensor. Moreover, the linear
dependence of the impedance on RH is observed in the range of
54–95% RH.
To test the stability, the sensor was exposed in air for 30 days, followed
by measuring impedances at various RH. As shown in Fig. 9,
there was almost no change in the impedances, which directly confirms
the good stability of the sensor. From the criteria as discussed
above, the sensor has prominent stability and is quite promising for
a practical humidity sensor.
The analysis of complex impedance plots is useful for studying
the sensing behavior of humidity sensors [33–35]. The complex
impedance plots of the flower-like ZnO nanorod sensor at differ-
Fig. 8. Hysteresis of the flower-like ZnO nanorod sensor measured at 100 Hz.
ent RH are shown in Fig. 10 in the frequency range from 40 Hz
to 100 kHz. At low RH (11, 33, and 54%), a semicircle due to the
film impedance is observed. The semicircle indicates a “non-Debye”
behavior, and many investigations have explained that it is due to
a kind of polarization and can be modeled by an equivalent circuit
of parallel resistor and capacitor [35–37]. With increasing the
RH (75, 85, and 95%), a line appears in the low-frequency range
and the semicircle becomes small. The higher the RH is, the longer
the line and smaller the semicircle is. The line represents Warburg
impedance, and is due to the diffusion of the electroactive
species at the electrodes [38–41]. The equivalent circuits of such
complex impedance plots are shown in Fig. 10(b). Here R f represents
the resistance of the flower-like ZnO nanorod film, which
decreases as RH increases. C f is the capacitance of the film and Z i
the impedance at the electrode/sensing film interface. According to
Fig. 10(a), R f ≪ Z i at low RH, and the impedance change of the sensor
is mostly determined by R f . At high RH (Fig. 10(b)), the magnitudes
of R f and Z i are the same and the impedance change of the sensor
is determined by both R f and Z i .
A possible qualitative mechanism to explain the humidity
sensing properties of the flower-like ZnO nanorods is proposed
hereafter. According to Kulwicki [41], water-related conduction in
ceramic and porous materials mainly occurs as a surface mechanism.
In our case, the large increase in conductivity with increasing
RH of the flower-like ZnO nanorods may also relate to the adsorption
of water molecules on the surface of the ZnO film. At low
humidity, only a few water molecules are adsorbed. Since the cov-
Fig. 7. Capacitance vs. RH of the flower-like ZnO nanorod sensor.
Fig. 9. Stability of the flower-like ZnO nanorod sensor measured at 100 Hz.

642 Q. Qi et al. / Sensors and Actuators B 133 (2008) 638–643
Fig. 10. (a) The complex impedance plots and (b) equivalent circuits of the flower-like ZnO nanorod sensor at different RH.
erage of water on the surface is not continuous, the electrolytic
conduction is difficult. Based on the mechanism of Schaub et al.
[42], the tips and defects of the ZnO nanorods present a high local
charge density and a strong electrostatic field, which promote water
dissociation. The dissociation provides protons as charge carriers of
the hopping transport. At high humidity, one or several serial water
layers are formed among ZnO nanorods, and electrolytic conduction
between nanorods takes place along with protonic transport,
and becomes dominating in the transport-process.
4. Conclusion
A flower-like ZnO nanorod sensor is successfully fabricated. The
sensing characteristics to RH are carefully studied. High sensitivity,
rapid response and recovery are found in the investigation. These
results demonstrate that flower-like ZnO nanorods can be used
as the humidity sensing material for fabricating highly sensitive
sensors.
Acknowledgements
This research was financially supported by Science and Technology
Office, Jilin Province, China (Grant No. 2006528) and the Open
Project of Key Laboratory of Low Dimensional Materials & Application
Technology (Xiangtan University), Ministry of Education, China
(Grant No. KF0706).
References
[1] V.V. Kovalenko, A.A. Zhukova, M.N. Rumyantseva, A.M. Gaskov, V.V.
Yushchenko, I.I. Ivanova, T. Pagnier, Surface chemistry of nanocrystalline SnO 2:
effect of thermal treatment and additives, Sens. Actuators B: Chem. 126 (2007)
52–55.
[2] S.M.A. Durrani, E.E. Khawaja, M.F. Al-Kuhaili, CO-sensing properties of undoped
and doped tin oxide thin films prepared by electron beam evaporation, Talanta
65 (2005) 1162–1167.
[3] M. Matsumiya, W. Shin, N. Izu, N. Murayama, Nano-structured thin-film Pt catalyst
for thermoelectric hydrogen gas sensor, Sens. Actuators B: Chem. 93 (2003)
309–315.
[4] S.P. Yawale, S.S. Yawale, G.T. Lamdhade, Tin oxide and zinc oxide based doped
humidity sensors, Sens. Actuators B: Chem. 135 (2007) 388–393.
[5] J. Xu, Q. Pan, Y. Shun, Z. Tian, Grain size control and gas sensing properties of
ZnO gas sensor, Sens. Actuators B: Chem. 66 (2000) 277–279.
[6] S. Dubois, C. Marchal, J.M. Beuken, L. Piraux, J.L. Duvail, A. Fert, J.M. George,
J.L. Maurice, Perpendicular giant magnetoresistance of NiFe/Cu multilayered
nanowires, Appl. Phys. Lett. 70 (1997) 396–398.
[7] N. Zabala, M.J. Puska, R.M. Nieminen, Spontaneous magnetization of simple
metal nanowires, Phys. Rev. Lett. 80 (1998) 3336–3339.
[8] Y.H. Gao, Y. Bando, T. Sato, Nanobelts of the dielectric material Ge 3N 4, Appl.
Phys. Lett. 79 (2001) 4565–4567.
[9] K.A. Dean, B.R. Chalamala, The environmental stability of field emission from
single-walled carbon nanotubes, Appl. Phys. Lett. 75 (1999) 3017–3019.
[10] M.E. Franke, T.J. Koplin, U. Simon, Metal and metal oxide nanoparticles in
chemiresistors: does the nanoscale matter Small 2 (2006) 36–50.
[11] N. Barsan, U. Weimar, Conduction model of metal oxide gas sensors, J. Electroceram.
7 (2001) 143–167.
[12] A. Rothschild, Y. Komem, The effect of grain size on the sensitivity of nanocrystalline
metal-oxide gas sensors, J. Appl. Phys. 95 (2004) 6374–6380.
[13] J. Janata, M. Josowicz, D.M. Devaney, Chemical sensors, Anal. Chem. 66 (1994)
207R–228R.
[14] Z.P. Sun, L. Liu, L. Zhang, D.Z. Jia, Rapid synthesis of ZnO nano-rods by onestep,
room-temperature, solid-state reaction and their gas-sensing properties,
Nanotechnology 17 (2006) 2266–2270.
[15] N. Wu, M. Zhao, J.G. Zheng, C. Jiang, B. Myers, S. Li, M. Chyu, S.X. Mao, Porous
CuO–ZnO nanocomposite for sensing electrode of high-temperature CO solidstate
electrochemical sensor, Nanotechnology 16 (2005) 2878–2881.
[16] Q. Wan, Q.H. Li, Y.J. Chen, T.H. Wang, X.L. He, J.P. Li, C.L. Lin, Fabrication and
ethanol sensing characteristics of ZnO nanowire gas sensors, Appl. Phys. Lett.
84 (2004) 3654–3656.
[17] Q.H. Li, Y.X. Liang, Q. Wan, T.H. Wang, Oxygen sensing characteristics of
individual ZnO nanowire transistors, Appl. Phys. Lett. 85 (2004) 6389–
6391.
[18] T.J. Hsueh, Y.W. Chen, S.J. Chang, S.F. Wang, C.L. Hsu, Y.R. Lin, T.S. Lin, I.C. Chen,
ZnO nanowire-based CO sensors prepared on patterned ZnO: Ga/SiO2/Si templates,
Sens. Actuators B: Chem. 125 (2007) 498–503.

Q. Qi et al. / Sensors and Actuators B 133 (2008) 638–643 643
[19] E. Comini, G. Faglia, G. Sberveglieri, Z. Pan, Z.L. Wang, Stable and highly sensitive
gas sensors based on semiconducting oxide nanobelts, Appl. Phys. Lett. 81
(2002) 1869–1871.
[20] R.J. Wu, Y.L. Sun, C.C. Lin, H.W. Chen, M. Chavali, Composite of TiO 2 nanowires
and Nafion as humidity sensor material, Sens. Actuators B: Chem. 115 (2006)
198–204.
[21] A. Vomiero, S. Bianchi, E. Comini, G. Faglia, M. Ferroni, N. Poli, G. Sberveglieri,
In 2O 3 nanowires for gas sensors: morphology and sensing characterisation,
Thin Solid Films 515 (2007) 8356–8359.
[22] K.M. Sawicka, A.K. Prasad, P.I. Gouma, Metal oxide nanowires for use in chemical
sensing applications, Sens. Lett. 3 (2005) 31–35.
[23] Y.N. Xia, P.D. Yang, Y.G. Sun, Y.Y. Wu, B. Mayers, B. Gates, Y.D. Yin, F. Kim, Y.Q.
Yan, One-dimensional nanostructures: synthesis, characterization, and applications,
Adv. Mater. 15 (2003) 353–389.
[24] Z.W. Pan, Z.R. Dai, Z.L. Wang, Nanobelts of semiconducting oxides, Science 291
(2001) 1947–1949.
[25] M.H. Huang, S. Mao, H. Feick, H.Q. Yan, Y.Y. Wu, H. Kind, E. Weber, R. Russo, P.D.
Yang, Room-temperature ultraviolet nanowire nanolasers, Science 292 (2001)
1897–1899.
[26] K. Govender, D.S. Boyle, P.B. Kenway, P. O’Brien, Understanding the factors that
govern the deposition and morphology of thin films of ZnO from aqueous solution,
J. Mater. Chem. 14 (2004) 2575–2591.
[27] J. Wang, F.Q. Wu, K.H. Shi, X.H. Wang, P.P. Sun, Humidity sensitivity of composite
material of lanthanum ferrite/polymer quaternary acrylic resin, Sens. Actuators
B: Chem. 99 (2004) 586–591.
[28] J. Yahiro, Y. Oaki, H. Imai, Biomimetic synthesis of wurtzite ZnO nanowires
possessing a mosaic structure, Small 2 (2006) 1183–1187.
[29] J.M. Calleja, M. Cardona, Resonant Raman scattering in ZnO, Phys. Rev. B 16
(1977) 3753–3761.
[30] A.K. Pradhan, K. Zhang, G.B. Loutts, U.N. Roy, Y. Cui, A. Burger, Structural and
spectroscopic characteristics of ZnO and ZnO:Er 3+ nanostructures, J. Phys.: Condens.
Matter 16 (2004) 7123–7129.
[31] V. Bondarenka, S. Grebinskij, S. Mickevičius, V. Volkov, G. Zacharova, Thin films
of poly-vanadium-molybdenum acid as starting materials for humidity sensors,
Sens. Actuators B: Chem. 28 (1995) 227–231.
[32] J. Wang, H. Wan, Q. Lin, Properties of a nanocrystalline barium titanate on silicon
humidity sensor, Meas. Sci. Technol. 14 (2003) 172–175.
[33] Y. Sadaoka, M. Matsuguchi, Y. Sakai, S. Mitsui, Electrical properties of -
zirconium phosphate and its alkali salts in a humid atmosphere, J. Mater. Sci.
23 (1988) 2666–2675.
[34] Y. Sadaoka, M. Matsuguchi, Y. Sakai, H. Aono, S. Nakayama, H. Kuroshima,
Humidity sensors using KH 2PO 4-doped porous (Pb, La) (Zr, Ti) O 3, J. Mater.
Sci. 22 (1987) 3685–3692.
[35] Y.C. Yeh, T.Y. Tseng, Analysis of the d.c. and a.c. properties of K 2O-doped porous
Ba 0.5Sr 0.5TiO 3 ceramic humidity sensor, J. Mater. Sci. 24 (1989) 2739–2745.
[36] E. Traversa, A. Bearzotti, M. Miyayama, H. Yanagida, Study of the conduction
mechanism of La 0.2CuO 4–ZnO hetero-contracts at different relative humidity,
Sens. Actuators B: Chem. 24/25 (1995) 714–718.
[37] E. Traversa, G. Gnappi, A. Montenero, G. Gusmano, Ceramic thin films by sol–gel
processing as novel materials for integrated humidity sensors, Sens. Actuators
B: Chem. 31 (1996) 59–70.
[38] C.D. Feng, S.L. Sun, H. Wang, C.U. Segre, J.R. Stetter, Humidity sensing properties
of Nafion and sol–gel derived SiO 2/Nafion composite thin films, Sens. Actuators
B: Chem. 40 (1997) 217–222.
[39] G. Casalbore-Miceli, M.J. Yang, N. Camaioni, C.M. Mari, Y. Li, H. Sun, M. Ling,
Investigations on the ion transport mechanism in conducting polymer films,
Solid State Ionics 131 (2000) 311–321.
[40] E. Quartarone, P. Mustarelli, A. Magistris, M.V. Russo, I. Fratoddi, A. Furlani,
Investigations by impedance spectroscopy on the behaviour of poly(N,Ndimethylpropargylamine)
as humidity sensor, Solid State Ionics 136/137 (2000)
667–670.
[41] B.M. Kulwicki, Humidity sensors, J. Am. Ceram. Soc. 74 (1991) 697–
708.
[42] R. Schaub, P. Thostrup, N. Lopez, E. Lægsgaard, I. Stensgaard, J.K. Nørskov, F.
Besenbacher, Oxygen vacancies as active sites for water dissociation on rutile
TiO 2(1 1 0), Phys. Rev. Lett. 87 (2001), 266104/1–266104/4.
Biographies
Qi Qi received his BS degree from the College of Electronics Science and Engineering,
Jilin University, China in 2003. He entered the PhD course in 2006, majored in
microelectronics and solid state electronics.
Tong Zhang received her MS degree in major of semiconductor materials in 1992,
and PhD degree in the field of microelectronics and solid state electronics in
2001 from Jilin University. She was appointed as a full professor in College of
Electronics Science and Engineering, Jilin University in 2001. Now, she is interested
in the field of sensing functional materials and gas sensors and humidity
sensors.
Qingjiang Yu received his MS degree from National Laboratory of Superhard Materials,
Jilin University, China in 2005. He entered the PhD course in 2005, majored in
condensed matter physics.
Rui Wang received her MS degree from the College of Electronics Science and Engineering,
Jilin University, China in 2007. She entered the PhD course in 2007, majored
in microelectronics and solid state electronics.
Yi Zeng received his MS degree from National Laboratory of Superhard Materials,
Jilin University, China in 2007. He entered the PhD course in 2007, majored in
microelectronics and solid state electronics.
Li Liu received her MS degree from the College of Electronics Science and Engineering,
Jilin University, China in 2000. She entered the PhD course in 2002, majored in
microelectronics and solid state electronics.
Haibin Yang received his MS in major of materials in 1986, and PhD degree from
National Laboratory of Superhard Materials, Jilin University, China in 1990. Now, he
is interested in the field of functional nanomaterials.