Supporting Information for: Highly Sensitive and Stable Humidity Nanosensors based on LiCl Doped TiO 2 Electrospun Nanofibers Zhenyu Li 1, Hongnan Zhang 1, Wei Zheng 1, Wei Wang 1, Huimin Huang 1, Ce Wang 1*, Alan G. MacDiarmid 1, and Yen Wei 1,2* 1 Alan G. MacDiarmid Institute, Jilin University, Changchun 130012, P. R. China 2 Department of Chemistry, Drexel University, Philadelphia, PA 19104 USA Experimental Materials: Tetrabutyl titanate (> 95%), ethanol (>95%), and acetic acid (> 95%) were purchased from Tianjin Chemical Company. Poly (vinyl pyrrolidone) (Mw: 1,300,000) and LiCl were purchased from Aldrich. Preparation of TiO 2 /LiCl nanofibers: In a typical procedure, 1.5 g of tetrabutyl titanate was mixed with 3 ml of acetic acid and 3 ml of ethanol in glovebox under vigorous stirring for 10 min. Subsequently, this solution was added to 7.5 ml of ethanol containing 0.45 g of poly(vinyl pyrrolidone) (PVP) and a suitable amount (0.05, 0.10, 0.15 or 0.20 g) of LiCl under vigorous stirring for 30 min. Then, the mixture was loaded into a glass syringe and connected to high-voltage power supply. 12 kv was provided between the cathode (a flat aluminum foil) and anode (syringe) at a distance of 20 cm. The conversion of tetrabutyl titanate to TiO 2 and the complete removal of PVP in the as-spun nanofibers were achieved by calcining at 500 for 3 h in air. All the measurements were carried out on the calcined fibers. Fabrication and measurement of humidity sensor based on our products: The as-prepared LiCl doped TiO 2 nanofibers were mixed in a weight ratio of 100: 5 and were ground with deionized water to form a dilute paste. The paste was S1
spin-coated onto a ceramic substrate (10 mm 5 mm 1 mm) with four pairs of Ag-Pd interdigital electrodes to form a film with the thickness about 10 μm, and then the film was dried at 60 in air for 5 h. Finally, the humidity sensor was fabricated after aging at 95 % RH with a voltage of 1 V, 100 Hz for 24 h. The characteristic curves of humidity sensitivity were measured on a ZL-5 model LCR analyzer (Made in Shanghai, China) at room temperature. The voltage applied in our studies was ac 1 V. The controlled humidity environments were achieved using supersaturation aqueous solutions of different salts of LiCl, MgCl 2, Mg(NO 3 ) 2, NaCl, KCl, and KNO 3 in a closed glass vessel at room temperature, which yielded 11, 33, 54, 75, 85 and 95% RH, respectively. Measurement process of observation: The schematic diagram of the experimental set-up is shown blew. Two chambers were used in our measurements, and the sensor was switched between them. Hitherto, the quick response sensors can be measured by switch between chambers. This method S1, S2 has been developed by T. H. Wang. Scheme S1. Schematic diagram of the experimental set-sup. The calibration procedure before measurements and the data on relative humidities over salt solutions at specific temperature used in our experiments: We usually spend 12 h (one night) to ensue the air in chamber reach equilibrate state, and there is also a standard humidity sensor in our system to monitor the RH in the chamber. This procedure for preparing a hydrostatic solution is following the National Bureau of Standards established by L. Greenspan. S3 Characterization: S2
The X-ray powder diffraction (XRD) data were collected on an X Pert MPD Philips diffractometer (Cu Kα X-radiation at 40 kv and 50 ma). Scanning electron microscopy (SEM) images were recorded on a SHIMADZU SSX-550 (Japan) instrument. The humidity measured machine was ZL5 intelligent LCR test meter made in Shanghai China. Results Scheme S2. Schematic diagram of the processing steps used to fabricate LiCl doped TiO 2 nanofiber mats on Al 2 O 3 substrate with interdigitated Pt electrode arrays. 100000 10000 20HZ 100HZ 1kHZ 10kHZ 100kHZ Impedance (ko) 1000 100 10 1 0 20 40 60 80 100 Relative humidity (RH%) Figure S1. The RH dependence of impedance based on the product containing 30.0% LiCl at various frequencies. S3
Figure S2. Complex impedance plots of the product containing 30.0% LiCl at different relative humidity. Explanation of Figure S2: In our work, the frequency varies from 20 Hz to 100 Hz and the RH ranges from 11 to 95 % at room temperature. We measured the complex impedance and complex angle at different frequencies and different RH, and then calculated the real part and the imaginary part of complex impedance. We can see clearly from the Figure S2 that at low relative humidity a half semicircle is observed in complex impedance plots. With increasing of relative humidity, the half semicircle increases and becomes a semicircle. Many authors have explained that the semicircle is due to the intrinsic impedance of the materials. S4-S6 This time, only a few water molecules are adsorbed. Since coverage of water on the surface is not continuous, the ionic conduction is difficult. Based on the mechanism of R. Schaub et. al, S7 the tips and defects of the TiO 2 nanofibers present a high local charge density and a strong electrostatic field, which promotes water dissociation. The dissociation provides protons as charge carriers of the hopping transport (Figure S2a). When the relative humidity reaches a high value, a little straight line appears after the semicircle in the low frequency, which was caused by the diffusion process of redox ions at the electrode/sensing film interface (Figure S2b). S8-S10 In this condition, one or several serial water layers are formed among TiO 2 nanofibers, and ionic conduction between nanofibers takes place along with protonic transport, and becomes dominating in the transport-process. An equivalent circuit of S4
such complex impedance plots has been inserted in Figure S2. Here R f represents the resistance of the LiCl doped TiO 2 nanofibers film, which decreases as RH increases; C f the capacitance of the film and Z i the impedance at the electrode/sensing film interface. According to Figure S2a, Rf << Zi at relatively low RH, and the impedance change of the sensor is mostly determined by R f. At relatively high RH (Figure S2b), the magnitude 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. So from the view of the complex impedance plots, the sensing principle of this material is proton and ionic conductivity in low and high relative humidity, respectively. Reference: [S1] Feng, P.; Yue, X. Y.; Liu, Y. G.; Wang, T. H. Appl. Phys. Lett. 2006, 89, 243514(1)-243514(3). [S2] Liang, Y. X.; Chen, Y. J.; Wang, T. H. Appl. Phys. Lett. 2004, 85, 666-668. [S3] Greenspan, L. J Res NBS, 1977, 81A(1), 89-96. [S4] Yeh, Y. C.; Tseng, T. Y. J. Mater. Sci. 1989, 24, 2739-2745. [S5] Traversa, E.; Bearzotti, A.; Miyayama, M.; Yanagida, H. Sens. Actuators B 1995, 25, 714-718. [S6] Traversa, E.; Gnappi, G.; Montenero, A.; Gusmano, G. Sens. Actuators B 1996, 31, 59-70. [S7] Schaub, R.; Thostrup, P.; Lopez, N.; Lægsgaard, E.; Stensgaard, I.; Nørskov, J. K.; Besenbacher, F. Phys. Rev. Lett. 2001, 87, 266104-266107. [S8] Feng, C. D.; Sun, S. L.; Wang, H.; Segre, C. U.; Stetter, J. R. Sens. Actuators B 1997, 40, 217-222. [S9] Casalbore-Miceli, G.; Yang, M. J.; Camaioni, N.; Mari, C. M.; Li, Y.; Sun, H.; Ling, M. Solid State Ionics 2000, 131, 311-321. [S10] Quartarone, E.; Mustarelli, P.; Magistris, A.; Russo, M. V.; Fratoddi, I.; Furlani, A. Solid State Ionics 2000, 136-137, 667-670. S5