NANOFIBERS

Abstract

The present invention relates to nanofibers. In particular, the present invention relates to potassium niobate nanofibers. In an aspect of the present invention, there is provided a method of preparing the nanofibers, the method comprising: (a) dissolving niobium chloride and potassium sorbate in a solvent to obtain a first solution; (b) removing chloride precipitates formed from the first solution; (c) adding a polymer, for example polymethylmethacrylate or polyvinylpyrrolidone to the solution to obtain a second spinnable solution; and (d) electrospinning the spinnable solution to produce the fibers. The application also discloses the application of such nanofibers in the manufacture of a humidity sensor device by sputtering a metal such as Tantalum on top of the nanofibers.

Claims

1. A method of preparing fibers, the method comprising: (a) dissolving niobium chloride and potassium sorbate in a solvent to obtain a first solution; (b) removing chloride precipitates formed from the first solution; (c) adding a polymer to the solution to obtain a second spinnable solution; and (d) electrospinning the spinnable solution to produce the fibers.

2. The method according to claim 1, wherein the polymer is any one selected from the group comprising: polyvinylpyrrolidone, poly(methyl methacrylate), cellulose acetate, polyacrylonitrile, polyvinyl alcohol and polyethylene oxide.

3. The method according to claim 1, wherein the solvent is an alcohol.

4. The method according to claim 3, wherein the alcohol is any one selected from the group comprising: methanol, ethanol and 2-methoxyethanol dimethylformamide.

5. The method according to claim 1, wherein the molar ratio between potassium and niobium after removing the chloride precipitates is about 1.

6. The method according to claim 1, wherein the electrospinning is carried out by ejecting the spinnable solution from a plastic syringe at a constant feed rate of 0.60 ml/hour.

7. The method according to claim 1, wherein the electrospun fibers are collected on a substrate.

8. The method according to claim 7, wherein the syringe and the substrate is separated by a distance of about 13 cm.

9. The method according to claim 8, wherein the applied electrical between the syringe and the substrate is 1.5 kV/cm.

10. The method according to claim 7, wherein the substrate is a SiO2/Si substrate or an aluminium foil.

11. The method according to claim 7, wherein the collection time for collecting the fibers on the substrate is between 2 to 5 minutes.

12. The method according to claim 1, further comprising drying the electrospun fibers at 60? C. for 1 hour.

13. The method according to claim 12, wherein the dried electrospun fibers undergo a calcination process at 550? C. for 5 hours at a heating rate of 5? C. per minute in atmosphere.

14. The method according to claim 1, wherein the first solution obtained in step (a) is magnetically stirred for 1 hour.

15. The method according to claim 1, wherein the spinnable solution is magnetically stirred for 3 hours prior to electrospinning.

16. A method of preparing a humidity sensor device, the method comprising: (a) obtaining a fiber according to any one of claims 1 to 15; and (b) sputtering a metal on top of the fiber to form interdigitated electrodes.

17. The method according to claim 16, wherein the metal is any one selected from the group comprising: aluminium, chromium, gold, molybdenum, platinum, silver, titanium.

18. An electrospun fiber obtained from a method according to any one of claims 1 to 15.

19. An electrospun fiber comprising potassium niobate and a polymer.

20. The fiber according to any one of claim 18 or 19, wherein the length of each fiber is about or greater than 500 ?m, and the average diameter of the fiber is between 100 nm to 500 nm.

21. A humidity sensor device comprising fibers according to any one of claim 18 or 19.

22. The device according to claim 22, wherein the fibers are composed of densely stacked grains of about 40 nm in size.

23. The device according to claim 22, further comprising a substrate for supporting the fibers.

24. The device according to claim 24, wherein the substrate is SiO2/Si.

25. The device according to claim 25, wherein the thickness of the SiO2/Si substrate is about 2 ?m and 285 nm respectively.

26. The device according to claim 22, wherein a metal is spluttered on top of the fibers to form interdigitated electrodes.

27. The device according to claim 27, wherein the interdigitated electrodes are spaced about 250 ?m apart.

28. The device according to claim 27, wherein the metal layer is about 350 nm.

29. The device according to claim 28, wherein the metal is any one selected from the group comprising: aluminium, chromium, gold, molybdenum, platinum, silver, titanium.

30. The device according to claim 21, wherein the length of each fiber is about or greater than 500 ?m.

31. The device according to claim 21, wherein the average diameter of the nanofiber is between 100 nm to 500 nm.

32. The device according to claim 21, wherein the fibers are stacked along the direction of the fiber axis.

33. The device according to claim 21, wherein the sensor is adapted to measure relative humidity of between 15-95% in atmospheric air at a room temperature of about 25? C.

Description

[0028] In the Figures:

[0029] FIG. 1. (a) SEM image of KNO precursor fibers. (b) KNO fibers calcinated at 550? C. KNO fibers collected for (c) 2 mins (d) 5 mins. (e) XRD patterns and (f) Raman spectra of the KNO nanofibers calcinated at 550? C. for 5 h.

[0030] FIG. 2. (a) Topography of the single nanofiber using AFM. (b) 3D plot the topography showing grain size of the KNO-550 nanofiber.

[0031] FIG. 3. (a) Conductance versus relative humidity during humidification and dehumidification cycle. Insetas fabricated humidity nanosensor. (b) Sensitivity curves for five different samples based on KNO-550 nanofibers collected for 5 minutes (c) Comparison of sensitivity between 2-minute and 5-minute samples.

[0032] FIG. 4. (a) FTIR spectra of KNO-550 nanofibers at different RH environments (b) Response and recovery time of the nanosensor (c) Stability of the KNO-550 based nanosensor.

[0033] FIG. 5. (a) Absolute value of current versus voltage of humidity nanosensor in different RH atmosphere at RT. Hysteresis curves of the humidity nanosensor for varying bias voltage at (b) 90% RH (c) 70% RH. (d) Hysteresis curves of the humidity nanosensor when biased at 3 V for 25 continuous cycles at 70% RH.

[0034] FIG. 6. Schematic illustration of electrospinning process.

[0035] FIGS. 7(a) and (b) shows SEM images of the porous and decorated fibers respectively.

EXAMPLE 1

[0036] Method and Material

[0037] Synthesis of KNbO.sub.3 Nanofibers

[0038] FIG. 6 shows a schematic illustration of the electrospinning process. Electrospinning is a facile and cost-effective technique capable of generating nanofibers that are ultra-long, have controllable diameter and has precise chemical composition.

[0039] For the synthesis of KNbO.sub.3 nanofibers, the precursor sol-gel was prepared by the following two step process. Firstly, 1 mmol of niobium chloride (0.27 g, >98%) and 6 mmol of potassium sorbate (0.907 g, >99%) were dissolved in methanol (4 ml) and the solution was magnetically stirred for 1 h at RT. During the mixing, color of the solution changes from fully transparent to white gradually, indicating the formation of potassium chloride precipitates which can be explained by the equation given below:

NbCl.sub.5+6C.sub.6H.sub.7KO.sub.25KCl?L+(Nb.sup.5++K.sup.++6C.sub.6H.sub.7O.sub.2.sup.?).sub.in.sub._.sub.solution

[0040] This mixture was centrifuged for 5 min at 4000 rpm to remove the solid precipitates from the solution. XRD analysis of the obtained precipitates confirms that they are predominantly potassium chloride crystals. After the removal of chloride particles, the remaining solution turns slightly yellowish and the molar ratio between potassium and niobium approximately equals to 1. Secondly, PVP (0.533 g, >99%, MW=1,300,000) and 2-methoxyethanol (2.66 ml, 98%) were added to the existing solution to maintain the viscosity and ionic concentration favorable for electrospinning process. The mixture was further magnetically stirred for 3 hours at room temperature (about 25? C.) to obtain a homogenous KNO precursor solution.

[0041] During electrospinning process, the precursor solution was ejected from a plastic syringe at a constant feeding rate of 0.60 ml h.sup.?1. The syringe and the collector was separated at a distance of 13 cm apart and the applied electrical between them was 1.5 kV cm.sup.?1. The nanofibers were collected on to SiO.sub.2 covered silicon substrate and dried at 60? C. for 1 h, followed by calcination process. The as-spun fibers were calcined at 550? C. at a heating rate of 5? C./min in atmosphere which are abbreviated to be KNO-550, respectively. All chemicals were purchased from Sigma-Aldrich and the measurements were carried out on calcined nanofibers.

[0042] The table below shows the amount and molecular weight of polymers used to obtain solid and porous KNbO.sub.3 nanofibers abbreviated as KNO-S and KNO-P respectively.

TABLE-US-00001 Fiber Molecular morphology Polymer Amount weight Solvent Solid KNO-S Polyvinylpyrrolidone, 0.4 g 1,300,000 3 ml of PVP 2-methoxyethanol Porous KNO-P Poly(methyl 0.8 g 120,000 3 ml of methacrylate), Dimethylformamide PMMA

[0043] FIGS. 7(a) and (b) shows SEM images of the porous and decorated fibers respectively.

[0044] Characterization of KNbO.sub.3 Nanofibers

[0045] Surface morphology and geometry of the KNO nanofibers were inspected using JEOL JSM7600F field-induced scanning electron microscope (FE-SEM) and Oxford Instruments MFP-3D was used for Atomic Force Microscopy. Elemental analysis (EDX) was performed using Oxford Instruments X-Max-50 silicon drift detector embedded in the FE-SEM system. Crystal structure of the as-synthesized nanofibers was analyzed using Bruker D8 advance XRD system (Cu K?). Raman spectra of the KNO nanofiber samples were obtained using Witec Alpha300M Raman System.

[0046] Fabrication of the Humidity Nanosensor

[0047] Humidity nanosensor was fabricated based on KNO-550 nanofibers collected on SiO.sub.2/Si substrate. Tantalum was sputtered on top of the fibers to form interdigitated electrodes (IDES) using DC sputtering (AJA Orion 5) to build the humidity nanosensor. Final device structure of the humidity sensor is Si (2 ?m)/SiO.sub.2 (285 nm)/KNO nanofibers/Ta (350 nm). The device dimensions are 2 cm?2 cm and IDE spacing of the sensor is 250 ?m. The collection time was controlled to obtain samples with different density of nanofibers on the substrate. Two different humidity sensors based on 2-minute and 5-minute collection time was fabricated.

[0048] Characterization of the Humidity Nanosensor.

[0049] The humidity nanosensor is placed inside the testing chamber with two inlets to introduce dry and humid air respectively. During humidity sensor testing, IDE electrodes are connected to the Keithley 6430 Sub-Femtoamp Remote Source meter for measuring sensor response with respect to the change in relative humidity of the testing chamber which is monitored using commercial humidity sensor (Sensirion SHT21). The reference sensor from Sensirion uses a capacitor to sense humidity. Its dielectric is realized through a polymer, which absorbs or desorbs water depending on the ambient humidity. The electrodes are realized with an interdigitated electrode structure. The reference sensor was biased at 3.3 V and the response time was 8 s from 10% to 63% RH [23]. The humidity control was achieved by passing both humid and dry air at various flow rates, while the total flow remains fixed at 0.5 l/min.

[0050] Results and Discussion

[0051] KNbO.sub.3 Nanofiber Phase and Morphology

[0052] The SEM micrographs (see FIGS. 1a-d) show that the fibers are non-woven and continuous. After calcination, KNO nanofibers are ultra-long in length ranging from 500 microns to several centimeters, while the average diameter of the fiber is approximately 100 nm. In alternative embodiments, the average diameter of the nanofiber may be between 100 nm to 500 nm. For example, when PMMA solution is used, the average diameter is around 300-500 nm while fibers from PVP solution ends up 100 nm. Presence of pure KNO with proper stoichiometry is confirmed by the EDX. The KNO nanofiber crystal structures were investigated using XRD analysis which showed that samples have perovskite structure (JCPDS card no. 32-822). As shown in FIG. 1e, there is a distinct diffraction peak appearing for the KNO-550 samples and the typical peak split (002 and 200 planes at 44? to) 46? indicates the formation of orthorhombic KNO nanofibers.

[0053] FIG. 1f shows the Raman spectra of the KNO nanofibers and the characteristic peaks at 280 cm.sup.?1 (?5), 597 cm.sup.?1 (?1) and 830 cm.sup.?1 corresponding to the vibrational modes of NbO.sub.6 octahedron. This implies the formation of perovskiteorthorhombic structure which is in agreement with the XRD profiles. Also, we observe that characteristic peak (u5) shifts to higher wave numbers from 256 cm.sup.?1 to 280 cm.sup.?1 as the temperature increases implying lattice distortion of KNO crystals at higher annealing temperature[24].

[0054] Furthermore, surface morphology of the nanofiber was investigated using contact mode of an atomic force microscopy. By scanning the tip across the sample area of 500 nm?500 nm, the topography of the fibers was obtained. FIG. 2a indicate(s) that the nano-grains are stacked to each other in one dimension to form the nanofiber. The grain size of KNO-550 is approximately 40 nm as can be seen from the 3D plot of the AFM topography image of 100 nm?100 nm sample area (see figure. 2b). Being an ABO.sub.3 metal oxide with higher surface to volume ratio (SEM micrographs) and grainy structures (AFM scans) may make these ultra-long KNO nanofibers a good candidate for nanosensor applications[24,25].

[0055] KNbO.sub.3 Nanofiber Based Humidity Nanosensor Characteristics

[0056] (a) Sensing Performance

[0057] Absorption of gases is expected to improve with smaller grain size, thus improving the sensing capability and sensitivity of the sensor[20]. KNO nanofibers calcinated at 550? C. have average grains of 40 nm in size when measured using AFM scans (see figure. 2) thereby giving rise to an increased area of grain boundary compared to the nanofibers calcinated at higher temperatures. Thus, humidity nanosensor was fabricated based on KNO-550 nanofibers collected on SiO.sub.2/Si substrate. To evaluate the humidity sensing properties of the fabricated device, we measured the variation in nanosensor's electrical characteristics at room temperature with varying relative humidity (RH). The dependence of conductance on the RH for KNO-550 nanofibers collected for 5 minutes is shown in FIG. 3a. When the KNO-550 sample is biased at 3 V, the conductance increases dramatically from 1.5?10.sup.?10 J to 4?10.sup.?6 (4 orders of magnitude) while RH values vary from 15% to 95% at room temperature respectively. When sensors based on 2-minute collection time were subjected to test for its humidity sensing properties, the conductance values changed from 9?10.sup.?12 to 7.6?10.sup.?8 for the same RH range. FIG. 3c shows the change in conductance for 2-minute and 5-minute samples with respect to RH, which suggests that the change in density of the nanofibers collected on the substrate does not affect the sensitivity of the sensor significantly. For each type of sensors, several samples were fabricated and sensing performance was tested. FIG. 3b shows the sensitivity for the humidity sensors based on KNO-550 collected for 5 minutes, showing same sensitivity values. The results directly confirm the excellent consistency of the humidity sensors. Table 1 shows the sensitivity of KNO nanofibers for change in relative humidity is higher when compared to existing reports on humidity sensors based on ZnO, TiO.sub.2 and BaTiO.sub.3 nano-materials. The sensing results were largely stable and the error percentage for conductance values between humidification and desiccation cycles were very close as shown in FIG. 3a, suggesting good reproducibility and stability.

[0058] (b) Sensing Mechanism

[0059] The humidity sensitivity observed is attributed to large surface area, grain size and distribution and number of grain boundaries of the KNO nanofibers as these properties facilitates the easy adsorption of water molecules on the surface of the nanosensor[21,26]. When these nanofibers are exposed to humid air, few water molecules are chemisorbed at the neck of the crystalline grains and on the grain surface. This interaction is accompanied with a dissociative mechanism of water molecules to form hydroxyl groups. KNO-550 due to its large surface to volume ratio immensely helps the dissociated hydroxyl group (OH.sup.?) to interact with metal cations (K.sup.+) to form KOH, thus providing mobile protons (H.sup.+). These protons migrate from site to site on the surface leading to increased conductivity in the material which is in agreement with similar nanofiber based humidity sensors reported earlier[27]. At environment with higher humidity levels, after the surface area is completely covered by the chemisorption, subsequent water molecules are physisorbed on the existing hydroxyl layer. When RH is getting higher, the physisorption continues to increase and large amount of water molecules are adsorbed on the grain boundaries and flat surfaces, mobile protons becomes dominant carrier responsible for the electrical conductivity[26,27].

[0060] Fourier Transform infrared spectroscopy (FTIR) characterization of KNO-550 nanofibers was carried out to understand the surface chemistry of the nanofibers when subjected to different RH environments and possibly explain the sorption mechanism. The nanofibers were equilibrated at each RH environment for 1 hour before loading the sample for FTIR characterization and spectra was obtained as shown in FIG. 4a. The strong band centered at 607 cm.sup.?1 represents the ONbO stretching vibration, which is attributed to the corner shared NbO.sub.6 octahedron[28,29]. The absorption bands at 1631 and 3451 cm.sup.?1 can be assigned to H.sub.2O adsorbed on the surface of the nanofibers[30]. In particular, the characteristic absorption between 3200-3600 cm.sup.?1 can be seen increasing with the increase in humidity. The strong and broad peaks confirm the stretching and H-bonding with the surface of the nanofibers associated with the adsorption of water molecules. Once the sample is heated to dry, this peaks disappears.

[0061] (c) Sensor Response & Recovery Time

[0062] From FIG. 4b, the nanosensor displays an impressive response time of ?2 s, as well as rapid recovery time of ?10 s when the relative humidity in the chamber is switched from 25% to 60% then back to 25%. This result indicates that the humidity sensing behavior of the KNO nanofibers should be attributed to physisorption of water molecules and conductivity is dominated by the mobile protons driven by the electric field. Excellent response and recovery time can be attributed to the greatly reduced interfacial area between the sensing active region of the nanofibers and the underlying substrate when compared to thin films and nano particles[21]. To test the stability of the KNO-550 nanofiber sensors, they were exposed in five different RH environments for 1 hour. As observed in FIG. 4c, the conductance had no obvious deviation, suggesting prominent stability.

TABLE-US-00002 TABLE 1 Sensing performance of reported humidity sensors based on semiconductor nanostructures. Response Recovery time Material Sensitivity time (s) (s) Reference SnO.sub.2 nanowires .sup.33 120-170 20-60 [17] LiCl doped TiO.sub.2 ~10.sup.3 3 7 [18] nanofiber ZnO nanowires 5400.sup. 3 30 [20] BaTiO.sub.3 nanofibers ~10.sup.2 4 5 [22] KNbO.sub.3 nanofibers 4 ? 10.sup.4 2 10 Present invention

[0063] Table 1 lists the room temperature performance of reported resistance-type humidity sensors based on other semiconductor nanostructures. The sensitivity of the humidity nanosensor based on KNO-550 is higher than other kinds of sensing materials. Moreover, the response time is comparable to ZnO nanowires and TiO.sub.2 nanofibers and shorter than SnO.sub.2 nanowires and BaTiO.sub.3 nanofibers.

[0064] (d) Hysteresis Versus Relative Humidity

[0065] From FIG. 5a, it is evident that the ferroelectric coercive field increases as relative humidity increases. At higher humidity, non-linear dielectric property of KNO nanofiber becomes dominant (wider hysteresis loop), as the dielectric response induced by water adsorption is found to be very sensitive to the amount of water molecules on adsorptive layer of the nanofiber surface. At 90% RH, the hysteresis loop is obtained (see FIG. 5b) which indicates that coercive field is substantially higher than the field observed at 70% RH (see FIG. 5c). This variation in coercive field at 90% RH could be ascribed to the large number of water molecules covering the surface of the KNO nanofibers negatively influencing the reorientation of the ferroelectric dipoles. This behavior affects the logarithmic dependence of resistivity with respect to RH if the sensor. However, when sensor's bias voltage increases (?3V), strong enough electric field helps to reduce the influence of the coercive field and it may improve the charge carrier velocity leading to the best linear response on conductance at higher RH (see FIG. 3a). As the sensor hysteresis has a wide loop, the repeatability of the loops was tested as shown in FIG. 5d. From several runs, the loop is repeatable, which would help in designing a stable calibration algorithm during practical usage of these nanosensors.

CONCLUSION

[0066] Here, high-quality perovskiteorthorhombic KNbO.sub.3 nanofibers were synthesized via electrospinning method using sol-gel precursor. After calcination at 550? C., the nanofibers were continuous with an average diameter of 100 nm and composed of densely stacked grains of about 40 nm in size. For the first time, resistive type humidity nanosensor based on as-synthesized KNO nanofibers was fabricated. The logarithmic dependence of conductance at different biasing conditions was investigated and compared with an off-the-shelf commercial humidity sensor. When biased at 3 V, the nanosensor exhibited excellent sensing characteristics: sensitivity of 4 orders in magnitude with respect to the varying relative humidity (15%-95%), faster response (2 s) & recovery (10 s), good linearity and reproducibility. Our findings on variations in coercive field with respect to relative humidity suggests that devices based on 1D KNO material should be encapsulated to avoid change in non-linear dielectric property at higher humidity levels for desired device performance. Moreover, this successful synthesis and demonstration of very high aspect ratio nanofibers would enable widespread applications of KNbO.sub.3 materials in photo catalysis, non-linear optical and ferroelectric devices such as flexible optoelectronics and nanogenerators.

[0067] By virtue of the non-toxicity, high Tc, non-linear optical and ferroelectric properties, one dimensional (1D) potassium niobate (KNbO3) may enable the development of numerous nanoscale devices. Despite the progresses in 1D perovskite materials, preparing high aspect ratio KNbO3 nanostructures is still a concern. This invention presents the successful synthesis of ultra-long KNbO3 nanofibers using a simple sol-gel assisted far-field electrospinning process. At optimized conditions, centimeters long, orthorhombic KNbO3 nanofibers with an average diameter of 100 nm have been obtained. The nanofibers are composed of uniform grains densely stacked along the direction of nanofiber axis. Due to large surface-to-volume ratio, a high sensitive humidity nanosensor based on KNbO3 nanofibers displaying a logarithmic-linear dependence behavior of the conductance with the relative humidity (RH) was demonstrated. The conductance increases dramatically from 10-10 to 10-6 while RH varies from 15% to 95% at room temperature. In addition, the nanosensor exhibits excellent sensing performance, including ultrafast response (?2 s) and recovery time 10 s), good linearity and reproducibility. Furthermore, the change in ferroelectric coercivity with respect to the RH and its effect in the sensing behaviour were unveiled. The work here could enable broad applications in the fields of environmental sensing and nano-electrical-mechanical systems.

[0068] Other potential applications include: [0069] Piezoelectric energy harvesters [0070] Ultrasound transducers [0071] Non-linear optical devicessecond harmonic generation [0072] Flexible and wearable electronics [0073] Photo-catalysisdye degradation, water splitting (H.sub.2 generation)

[0074] Whilst there has been described in the foregoing description preferred embodiments of the present invention, it will be understood by those skilled in the technology concerned that many variations or modifications in details of design or construction may be made without departing from the present invention.

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