Highly Sensitive and Stable Humidity Nanosensors Based on
LiCl Doped TiO2Electrospun Nanofibers
Zhenyu Li,†Hongnan Zhang,†Wei Zheng,†Wei Wang,†Huimin Huang,†
Ce Wang,*,†Alan G. MacDiarmid,†and Yen Wei†,‡
Alan G. MacDiarmid Institute, Jilin UniVersity, Chang Chun 130012, PR China, and
Department of Chemistry, Drexel UniVersity, Philadelphia, PennsylVania 19104
Received January 19, 2008; E-mail: firstname.lastname@example.org
Design and fabrication of chemical sensors has become one
of the most active research fields due to their diverse practical
and potential applications.1To improve the sensing character-
istics, a general route is to make chemical sensors at the
nanoscale, taking advantage of the large surface areas of
nanoscale structures.2Chemical nanosensors based on one-
dimensional (1D) carbon,3silicon,4and ceramic5nanostructures
are of particular interest because of their high surface to volume
ratio and special physical and chemical properties.6Among the
chemical nanosensors, the humidity nanosensor is very important
for their practical applications in environment monitoring,
industrial process control, and our daily life.1a,c,7Many humidity
nanosensors based on 1D nanostructure have been successfully
obtained. However, the sensing characteristics (e.g., response,
recovery, reproducibility, stability, and linearity) still need to
TiO2 is an important ceramic material with a variety of
applications in environmental cleaning and protection, photo-
catalysis, sensors, and solar cells.8These excellent properties
make TiO2a good candidate in fabricating humidity nanosen-
sors. On the other hand, LiCl has been often used in fabricating
humidity sensors.9In this communication, we report a new type
of humidity nanosensors based on LiCl-doped TiO2electrospun
nanofibers using the electrospinning technique10and calcination.
The as-prepared humidity sensor exhibits excellent sensing
characteristics, including ultrafast response time (e3 s) and
recovery time (e7 s) for measuring relative humidity (RH) in
a wide range of 11-95% in air at room temperature (25 °C)
with the impendence changing from 107to 104Ω. In addition,
the nanosensor has good reproducibility, linearity, and stability.
Thus, a solution of tetrabutyltitanate, LiCl, and poly(vinyl
pyrrolidone) (PVP) in acetic acid and ethanol was electrospun
into nanofibers followed by calcination to remove PVP and to
afford LiCl-doped TiO2nanofibers as the humidity sensors. The
experimental details (Scheme S1) and schematic steps (Scheme
S2) are given in Supporting Information. Figure 1 shows the
SEM images of the electrospun TiO2 nanofibers containing
different amounts of LiCl, indicating a large scale of product
uniformity with the fiber diameters ranging from 150 to 260
nm. The corresponding XRD patterns (inset in Figure 1d)
demonstrate that the structure of TiO2changes from pure anatase
to a mixture of anatase and rutile and to pure rutile as the amount
of LiCl is increased. This result reveals that the addition of LiCl
can change the structures of TiO2nanofibers upon calcination.
The impedance of the nanosensor has been measured at
different frequencies at 25 °C. The results, as shown in Figure
S1 in Supporting Information, indicate that the high humidity
sensitivity and good linearity in the entire RH range were
obtained in the low frequency region of 20-100 Hz. At higher
frequencies, the dielectric phenomenon did not appear because
the adsorbed water molecules could not be polarized. Therefore,
we kept the operation AC voltage and frequency at 1 V and
100 Hz, respectively, in the following experiments.
The dependence of impedance on the RH for TiO2nanofibers
containing different amounts of LiCl is shown in Figure 2a.
Compared to the pure TiO2 nanofibers, LiCl-doped TiO2
nanofibers exhibited greatly improved sensitivity. In the same
time, by keeping the RH range of 11–95%, the humidity
nanosensor containing 30.0% LiCl shows the best linearity with
the impedance varying more than 3 orders of magnitude
Figure 1. SEM images of the TiO2nanofibers containing different contents
of LiCl. The contents of (a), (b), (c), and (d) are 12.5, 22.2, 30.0, and 36.4%,
respectively. The inset in (d) is XRD patterns of the products.
Figure 2. (a) The dependence of impedance on the RH for TiO2nanofibers
containing different contents of LiCl. (b) The humidity hysteresis charac-
teristics of the as-prepared humidity nanosensors containing 30.0% LiCl.
The AC voltage and the frequency are 1 V and 100 Hz, respectively.
Published on Web 03/15/2008
10.1021/ja800176s CCC: $40.75 2008 American Chemical Society
5036 9 J. AM. CHEM. SOC. 2008, 130, 5036–5037
(107-104Ω). Figure 2b shows the humidity hysteresis char-
acteristic of the as-prepared humidity nanosensors (30.0% LiCl).
The lines for adsorption and desorption processes are very close
to the maximum humidity hysteresis being less than 2.5% RH
under 65% RH for our nanosensors.
The response and recovery behavior is an important charac-
teristic for evaluating the performance of humidity sensors.
Figure 3a,b shows the response and recovery characteristic
curves based on the product containing 30.0% LiCl-doped TiO2
nanofibers for 1 cycle and 10 cycles with the RH changing from
11 to 95%. When the humidity was increased from 11 to 95%,
the response time for our sensor was less than 3 s. When the
RH was decreased from 95 to 11%, the recovery time was less
than 7 s. Such an ultrafast response and recovery behavior could
be explained by the structures of 1D TiO2nanofibers. The large
surface of the nanofiber makes the absorption of water molecules
on the surface of our sensors easy. The 1D structure of the fibers
can facilitate fast mass transfer of the water molecules to and
from the interaction region as well as improve the rate for charge
carriers to transverse the barriers induced by molecular recogni-
tion along the fibers.11Additionally, comparing with 2D
nanoscale films, the interfacial areas between the active sensing
region of the nanofibers and the underlying substrate is greatly
reduced. Those advantages lead to significant gain in the sensing
signal and good stability.12From the curves for 10 cycles, the
highest and lowest impedance values varied little, suggesting
that the as-prepared humidity nanosensors have good reproduc-
ibility. (More descriptions on basic humidity sensing principles
are given in Figure S2 in Supporting Information.) To test the
stability, the sensor containing 30.0% LiCl was exposed in air
for 30 days followed by measuring impedances at various RHs.
As shown in Figure 3c, there were almost no changes in the
impedances, which directly confirms the good stability of our
sensors. From the criteria as discussed above, our humidity
nanosensors based on LiCl-doped TiO2nanofibers surpass all
the previous humidity sensors reported in the literature.3–5
In summary, we reported a highly sensitive and stable
humidity nanosensor based on LiCl-doped TiO2 nanofibers
through electrospinning and calcination techniques. The sensor
exhibited excellent characteristics (ultrafast response and re-
covery behavior, good reproducibility, linearity, and stability),
which are of great importance in humidity detection and control.
Moreover, our method provides a useful platform to design and
construct highly effective humidity nanodetectors.
Acknowledgment. The authors thank the deceased Noble
Prize winner Prof. A. G. MacDiarmid for his guidance in the
field of humidity nanosensor. This work has been supported by
the National 973 project (No. 2007CD936203), National 863
project (No. 2007AA03z324), Headwaters Nanokinetic. Inc.,
and NIH (No. DE09848). Dedicated to the memory of Professor
Alan G. MacDiarmid.
Supporting Information Available: Experimental details, sche-
matic steps for product, impedance dependence of RH at various
frequencies, improving properties of LiCl, and the humidity sensing
principles based on our products. This material is available free of
charge via the Internet at http://pubs.acs.org.
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Figure 3. Response and recovery characteristic curves based on the product
containing 30.0% LiCl-doped TiO2nanofibers for 1 cycle (a) and 10 cycles
(b). (c) Stability of the sensor after exposing in air for 30 days.
J. AM. CHEM. SOC. 9 VOL. 130, NO. 15, 2008