Abstract:
A self-cleaning humidity sensor based on Mg 2+ /Na + -doped TiO 2  nanofiber mats is provided. Examples show the response and recovery characteristic curves for ten circles with the RH changing from 11% to 95%. The nanofibers are manufactured by mixing together a metal salt comprising titanium, a magnesium compound, a sodium compound, and a high molecular weight material to form a mixture, electrospinning the mixture to form composite nanofibers, and calcining the composite nanofibers to yield a TiO 2  nanofiber material doped with magnesium and sodium.

Description:
RELATED APPLICATIONS 
     This application claims the benefit of earlier filed U.S. Provisional Application No. 61/090,516, filed Aug. 20, 2008, and also U.S. Provisional Application No. 61/090,511, filed Aug. 20, 2008, the disclosures of which are incorporated herein in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention is in the field of humidity sensors, more particularly humidity sensors that include nanofibers on an electrode for use in measuring the relative humidity of a gaseous mixture. 
     2. Technology Review 
     One-dimensional (1D) nanostructures such as wires, rods, tubes, and fibers have attracted intensive research driven by their unique applications in mesoscopic physics and fabrication of nanodevices. Until now, many types of functional devices based on 1D functional material have been successfully fabricated, including optoelectronic devices, gas/humidity sensors, photochromic devices, and supercapacitors. Among these functional devices, humidity sensors have gained special focus owing to their practical applications in air-quality control, environmental monitoring, healthcare, defense and security, etc. Since the humidity-sensitive electrical conduction has been established by Jain and his co-workers, many efforts have been focused on the synthesis of humidity sensors with perfect characteristics. With the need for high sensitive and stable humidity sensors, many efficient humidity sensors have been fabricated based on 1D metallic oxide nanostructures taking the advantages of thermal, physical and chemical stability and their porous nature. 
     BRIEF SUMMARY OF THE INVENTION 
     This invention relates to a method of fabricating a humidity nanosensor using an electrical spinning method. The humidity nanosensors manufactured according to the invention include nanofibers supported on an electrode. The nanofibers can be in the form of a mat and can include TiO 2  nanofibers doped with magnesium and sodium. The doped nanofibers have been found to be substantially more sensitive to humidity than TiO 2  sensors manufactured using other techniques and compositions. 
     In one embodiment, a method includes: 1) mixing together a titanium compound, a magnesium compound, a sodium compound, and a high molecular weight material; 2) electrospinning the mixture to form composite nanofibers; and 3) calcining the composite nanofibers to yield a TiO 2  nanofiber material doped with magnesium and sodium. The high molecular weight material used to manufacture the composite nanofibers can be a polymer such as, but not limited to, polyvinyl alcohol and/or polyvinyl pyrrolidone (Mw greater than about 250,000, preferably greater than about 500,000, more preferably greater than about 1,000,000. In one embodiment, the composite nanofiber can be calcined at a temperature in a range from 500° C. and 650° C., and preferably above about 600° C. 
     The nanofibers manufactured according to the invention can be deposited on an electrode to form a nanosensor. In one embodiment, the electrode is an interdigital electrode, and the nanofibers are deposited on the electrode as a mat of fibers. 
     The nanofibers can have a width in a range of about 10 nm to about 600 nm, preferably a width in a range of about 50 nm to about 300 nm. 
     These and other advantages and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       To further clarify the above and other advantages and features of the present invention, a more particular description of the invention will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. It is appreciated that these drawings depict only typical embodiments of the invention and are therefore not to be considered limiting of its scope. The invention will be described and explained with additional specificity and detail through the use of the accompanying drawings, in which: 
         FIG. 1  is a schematic diagram of the steps to fabricate Mg 2+ /Na + -doped TiO 2  nanofiber mats for humidity measurement via electrospinning and calcination; 
         FIGS. 2   a - d  are SEM images of the as-prepared products; the contents of MgCl 2  in the products are a) 14.3%, b) 22.2%, and c) 40.0%, respectively; d) shows XRD patterns of the pure TiO 2  nanofiber mats and hybrids systems; 
         FIGS. 3   a - b  are graphs that depict a) dependence of impedance on the relative humidity for the products and b) humidity hysteresis characteristic of the products containing 22.2% MgCl 2 ; 
         FIGS. 4   a - b  are graphs that depict a) response and recovery characteristic curves for one cycle and b) ten cycles with the relative humidity ranging from 11 to 95%; the content of MgCl 2  in the product is 22.2%; 
         FIGS. 5   a - f  are graphs that depict a) XRD patterns, b) dependence of impedance on the RH, c) response and recovery behaviors for the products calcinated at different temperatures, d) humidity hysteresis of the product annealed at 650° C., and e) and f) are response and recovery behaviors of the product annealed at 650° C. for one circle and ten circles, respectively; the content of MgCl 2  in the product is 22.2%; 
         FIG. 6  is a graph depicting stabilities of the products for 30 days; 
         FIG. 7  is a graph depicting affection of UV irradiation on the humidity sensing characteristics based on inventive products; 
         FIG. 8  is a graph depicting impedance dependence of relative humidity at various frequencies; and 
         FIGS. 9   a - b  are graphs depicting as-prepared humidity sensor working principles. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of this the present invention provide a method of fabricating ceramic nanometer fibers for humidity detection using an electrical spinning method. 
     I. Fabrication of Mg2+ and Na+-Doped TiO 2  Nanofiber Mats 
     Electrospinning is used to fabricate MgCl 2 /dioctyl sulfosuccinate sodium (AOT: C 20 H 37 OSNa)/poly (vinyl pyrrolidone) (PVP)/tetrabutyl titanate nanocomposite fibers. Thereafter, calcination at 500° C. in air for 3 hours is used to treat the as-prepared nanocomposite fibers to remove PVP and convert tetrabutyl titanate into crystal TiO 2  nanofibers. Finally, Mg 2+ - and Na + -doped TiO 2  nanofiber mats are ultrasonicated in deionized water for 5 minutes and lifted onto the working electrode for sensor measurement. To see the whole procedure clearly, all the steps have been schematically illustrated in  FIG. 1 . As illustrated in  FIG. 1 , the compositions in the final product can be easily controlled by varying the compositions in precursors. 
     II. Morphological and Structural Characterization 
       FIGS. 2   a - c  are SEM images of the products containing different contents of MgCl 2  in the final products by keeping the amount of AOT at 0.02 g, indicating the large scale of the products with the diameters ranging from 50 to 300 nm can be obtained. Increasing the contents of MgCl 2  in the products from 14.3%, to 22.2%, and to 40.0%, the diameters of the products become thinner for more charges have been added during the process of electrospinning.  FIG. 1   d  shows the XRD of the products. Comparing with the pure TiO 2  nanofiber mats (polycrystals containing rutile and anatase). Those TiO 2  nanofibers in the hybrids systems are anatase, implying that the addition of Mg 2+ /Na +  can change the microstructures of the TiO 2  nanofibers during the calcination. 
     III. Humidity Sensitive Properties 
     A. 2.3.1. Impedance—Relative Humidity (RH) Curves and Humidity Hysteresis Characteristic 
     During the process of humidity measurement, the operation AC voltage and frequency are kept at 1 V and 100 Hz (see Supporting Information below), respectively.  FIG. 3   a  is a graph that shows the dependence of impedance on the relative humidity (RH) for TiO 2  nanofibers containing different contents of Mg 2+ /Na + . In contrast to the pure TiO 2  nanofiber mat, Mg 2+ /Na +  doped TiO 2  nanofiber mats show better sensitivity. The TiO 2  nanofiber mat with the content of MgCl 2  at 22.2% shows the best linearity. From  FIG. 3   a , it can also be observed that when the relative humidity ranged from 11% to 54%, the impedance of the product did not vary much. This phenomenon might be interpreted in the way that, when MgCl 2  in the final products is incorporated in the TiO 2  matrices, it is difficult to dissolve into Mg 2+  and Cl − , as the RH is low. From the above results, the content of MgCl 2  is kept in the following experiment. 
       FIG. 3   b  is a graph that shows the humidity hysteresis characteristic of the as-prepared product. Two lines, representing adsorption and desorption process respectively, are perfectly inosculated, which surpassed all the results reported before. 
     B. Response and Recovery Behavior 
       FIGS. 4   a - b  indicate the response and recovery behaviors, an important humidity characteristic, have been measured on the inventive products.  FIG. 4   a  is a graph that shows the response and recovery characteristic curves for one cycle with the relative humidity changing from 11% to 95%. When the relative humidity increases from 11% to 95%, the response time is about 3 sec. As the relative humidity lowers from 95% to 11%, the recovery time is about 2 seconds. 
       FIG. 4   b  is a graph that shows the response and recovery curves for ten cycles, revealing that the highest and lowest impedance of the as-prepared product is relatively stable, confining the good stability. The response and recovery behaviors of the product might be caused by the structures of 1D TiO 2  nanofibers (substrates). The large surface of substrate facilitates absorption of the water molecules on the outer surface of the products so that MgCl 2  dissolves into Mg 2+  and Cl −  and changes the impedance. Moreover, the 1D substrate facilitates fast mass transfer of the water molecules to form an interaction region and cause the charge carriers to transverse the barriers induced by molecular recognition along the substrate. The humidity sensing working principles have been discussed in the Supporting Information below. 
     IV. Affection of TiO 2  Nanofibers&#39; Structurs on Humidity Characteristic 
     The crystal structures of TiO 2  substrates are changed by altering the calcinating temperatures in order to observe their effects on humidity characteristics.  FIG. 5   a  shows the XRD of the products by increasing the annealing temperatures from 500° C. to 600° C., and to 650° C. From the XRD patterns, it can be seen that those TiO 2  structures are rutile crystals when the temperatures are above 600° C. 
       FIG. 5   b  shows the dependence of impedance on the relative humidity (RH) for TiO 2  nanofibers with different crystal structures, indicating those Mg 2+ /Na +  doped TiO 2  nanofibers, calcinated at 600° C., has the best linearity owing to the rutile TiO 2  (110), which plays as active sites for water dissociation, making the impedance of the product change. When the temperature was 650° C., the linearity was destroyed in this particular example. This phenomenon could be explained in the way that when the temperature is too high, Mg 2+  will react with TiO 2  and MgTiO 3  will be formed, which is difficult to dissolve into Mg 2+  and TiO 3   2− , making impedance unchanged. 
       FIG. 5   c  reveals the response and recovery behaviors on the products as prepared at different temperatures. The humidity hysteresis characteristic of the as-prepared product at 600° C. is given in  FIG. 5   d , which confirms that there is no distinction between the absorption and desorption processes.  FIGS. 5   e  and  5   f  show the response and recovery behaviors on the products prepared at 600° C., indicating that the response time is about 2 second and recovery time is about 1 second. 
     V. Stability of the As-Prepared Humidity Sensor 
     As the stability is an important characteristic for humidity sensors in practical applications, we put in air the working electrode covered with the product (600° C.) for one month and then characterize its impedances as shown in the graph of  FIG. 6 . Those impedances of the product hardly changed, confirming a good stability. 
     Additionally, as illustrated by Fujishima,  Adv. Mater.  1998, 10, 135, TiO 2  is a photosensitive and self-cleaning material, and UV light can change the surface structures of TiO 2 . When the TiO 2  structures are irradiated with UV light, the photogenerated hole reacts with lattice oxygen to form surface oxygen vacancies and the water molecules kinetically coordinate, and thus the humidity sensing properties based on TiO 2  structures will change. 
       FIG. 7  is a graph that shows the effects of UV light on the humidity sensing characteristics of our product (600° C.). After UV irradiation, those characteristics were weakened. When the product was put in darkness for 20 days, however, the characteristics resume or recover. Those results demonstrate good stability of the inventive product, even with regard to UV irradiation. 
     VI. Conclusions 
     In summary, we present a simple, effective, and low-cost route for Mg 2+ /Na +  doped TiO 2  nanofiber mats. The as-prepared products show good humidity sensing characteristics (super-rapid response time and recovery time, good reproducibility, and stability), which show promise in being applied in fabricating humidity nanosensors. As TiO 2  has been used as substrates, the inventive products can be also applied in photocatalysts, photovoltaics, sensors, and electrochromic display devices. More importantly, the disclosed method might not only open a new avenue for the synthesis of highly efficient humidity sensors, but also offer a platform to better understand and construct high effectively humidity nanodectors. 
     VII. Experimental 
     Materials: MgCl 2  (&gt;95%), tetrabutyl titanate (&gt;95%), ethanol (&gt;95%) and acetic acid (&gt;95%) were purchased from Tianjin Chemical Company. Poly (vinyl pyrrolidone) (Mw: 1,300,000) and dioctyl sulfosuccinate sodium (AOT: C 20 H 37 OSNa) were purchased from Aldrich. 
     Preparation of Mg 2+ /Na +  doped TiO 2  nanofibers: In a typical procedure, 1.5 g of tetrabutyl titanate was mixed with 3 mL of acetic and 3 mL of ethanol in a glovebox under vigorous stirring for 10 minutes. Subsequently, this solution was added to 7.5 mL of ethanol containing 0.45 g of poly (vinyl pyrrolidone) (PVP), 0.02 g of dioctyl sulfosuccinate sodium (AOT: C 20 H 37 OSNa), and a suitable amount of MgCl 2  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 foil) and anode (syringe) at a distance of 20 cm. Then, calcination (500° C. in air for 3 hours) has been used to treat the as-prepared nanocomposite fibers to remove PVP and convert tetrabutyl titanate into crystal TiO 2  nanofiber. 
     Fabrication and measurement of humidity sensor based on the disclosed product: The as-prepared Mg 2+ /Na +  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 screen-printed onto a ceramic substrate (6 mm×3 mm, 0.5 mm in thickness) with five pairs of Ag—Pd interdigital electrodes (electrodes width and distance: 0.15 mm) to form a film with the thickness about 10 μm, and then the film was dried at 60° C. in air for 5 hours. Finally, the humidity sensor was fabricated after aging at 95% relative humidity with a voltage of 1V, and frequency of 100 Hz for 24 hours. 
     The characteristic curves of humidity sensitivity were measured on a ZL5 intelligent LCR test meter (Made in Shanghai, China) at room temperature. The voltage applied in the studies was AC 1 V. The controlled humidity environments were achieved using supersaturation aqueous solutions of different salts 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. 
     Characterization: The X-ray powder diffraction (XRD) data were collected on an X&#39;Pert MPD Philips diffractometer (Cu Kα X-radiation at 40 kV and 50 mA). Scanning electron microscopy (SEM) images was recorded on a SHIMADZU SSX-550 (Japan) instrument. The humidity measured machine was ZL5 intelligent LCR test meter made in Shanghai China. 
     A new type of highly efficient and self-cleaning humidity sensor based on Mg 2+ /Na + -doped TiO 2  nanofiber mats is provided. The examples show the response and recovery characteristic curves for ten circles with the RH changing from 11 to 95%. 
     Supporting Information 
     Explanation of  FIG. 8 : In order to find out the optimization relationship of sample between impedance and relative humidity, we measured the impedance at different frequencies. From the curve shown in  FIG. 8 , we found that the high humidity sensitivity and good linearity in whole relative humidity range are obtained in the low frequency region such as 40 and 100 Hz. The higher the frequency, the lower the sensitivity, which can be attributed to, that at higher frequencies, the adsorbed water cannot be polarized, therefore the dielectric phenomenon does not appear. At the same time, we found that when the frequency is 40 Hz, the measured data were unstable. In order to gain high relative humidity sensitivity over the entire relative humidity range and good linearity, low working frequency should be applied. So we confirm the operation condition at AC 1V, 100 Hz. 
     Explanation of  FIGS. 9   a - b : To understand our sensor working principles, the complex impedances have been measured. In our work, the frequency varies from 40 Hz to 100 Hz and the relative humidity ranges from 11% to 95% at room temperature. We measured the complex impedance and complex angle at different frequencies and different relative humidity, and then calculated the real part and the imaginary part of complex impedance. We can clearly see from the graphs in  FIGS. 9   a - b  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 a kind of polarization and can be modeled by an equivalent circuit of parallel resistor and capacitor. At 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.,  Phys. Rev. Lett.  2001, 87, 226104, 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 ( FIG. 9   a ). 
     When the relative humidity reaches a high value, a 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 ( FIG. 9   b ). In this condition, one or several serial water layers are formed among TiO 2  nanofibers, and ionic conduction between nanorods takes place along with protonic transport, and becomes dominating in the transport-process. The equivalent circuits of such complex impedance plots have been inserted in  FIGS. 9   a - b . Here R f  represents the resistance of the KCl doped TiO 2  nanofibers film, which decreases as relative humidity increases. C f  represents the capacitance of the film and Z i  the impedance at the electrode/sensing film interface. 
     According to  FIG. 9   a , R f &lt;&lt;Z i  at low relative humidity, and the impedance change of the sensor is mostly determined by R f . At high relative humidity ( FIG. 9   b ), 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. 
     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope