Electrospun fibers as humidity sensor

Commercial humidity meter.

Advancement in technology has brought humidity sensor from environment monitoring and industrial process control to our daily life. Increasing use of nanofibers with better sensitivity will lead to miniaturization of the sensor which can be incorporated into lifestyle technology. Many humidity sensors are based on changes in the material impedance. However, there are other methods of detecting humidity change such as measuring frequency change in quartz crystal microbalance due to absorption of water molecule by the electrospun fibrous material.

Inorganic materials are often used as humidity sensor as they are semi-conductive with their impedance altered by the adsorption of water molecules. Electrospinning is a common method used in laboratory to produce fibers down to the nanofiber dimension. Inorganic nanofibers are produced by sintering electrospun precursor fibers. The randomly oriented nanofibrous are deposited on a pair of electrodes the current is measured. Wang et al (2011) used BaTiO₃ electrospun fibers to measure the humidity level based on direct current (DC). Using Ag electrodes, they were able to record a response and recovery time of 20s and 3s respectively. Detection of the relative humidity was at a range from 11% to 95%. There is a large jump in the current measured at 95% relative humidity compared to other lower humidity level. A common hypothesis for the change in the material impedance relative to the humidity level is that at low humidity, only electrons on the fibers act as charge carriers. However, as more water molecules are absorbed onto the nanofiber surface, high local charge density on the fiber may lead to the dissociation of water and other molecules. These introduce more charge carriers in the form of ions which contributed to a jump in current. Guo et al (2022) investigated the use of electrospun CuO nanofiber as humidity sensor in the braking of autonomous vehicles. CuO senses humidity by adsorption of water molecules on the surface which leads to a change in conductivity. CuO nanofibers were produced by sintering of its electrospun precursor (copper chloride and PVA). The CuO nanofibers were then grounded into a paste and applied on a central interdigitated electrode of the MEMS chip, forming a MEMS humidity sensor. This humidity sensor shows good response in the humidity range of 20 to 100% with very small changes in the impedance response whether it is from low to high humidity or in reverse. Response and recovery is very fast at 1s and it is able to detect changes in the humidity when placed behind the car wheel.

To improve the sensitivity of nanofibrous sensor, one concept is to increase the number of charge carriers. Doping inorganic nanofibers with salt will increase the number of ions available when water is absorbed onto the nanofiber surface. Using a mixture of materials to form a composite nanofiber may also improve sensor sensitivity by introducing more defects and corresponding water molecule dissociation. Xu et al (2011) fabricated ZnO/TiO₂ nanofibers humidity sensor which showed high sensitivity and quick response (4s)/recovery<12s). Qi et al (2008) showed that KCL-doped TiO₂ nanofibers demonstrated impedance variance of more than four orders of magnitude in the range of 11% to 95% relative humidity over pure TiO₂ nanofibers. Li et al (2008) also reported improved impedance variance when TiO₂ nanofibers were doped with up to 30% LiCl. Su et al (2012) was able to increase the humidity range of ZrO₂ by doping with Mg salt. The resultant Zr_0.9Mg_0.1O_2?δ nanofibers showed an increased in the impedance variance of more than four orders of magnitude in the range of 11% to 97% relative humidity.

Measuring changes in frequency of a quartz crystal microbalance due to absorption of water by electrospun fibers on it is another effective method of measuring relative humidity. Wang et al (2010) used electrospinning to coat a polyelectrolyte, polyacrylic acid (PAA) onto a quartz crystal microbalance. As polyelectrolyte is very sensitive to environment moisture content, the polyacrylic acid nanofiber coating on the quartz crystal microbalance was able to respond to the ambient relative humidity by altering the frequency of the quartz crystal microbalance. Comparing a composite blend of electrospun PAA and polyvinyl alcohol (PVA) nanofibers with PAA/PVA film, the nanofibers form showed sensitivity that is twice that of film [Wang et al 2010]. Sheng et al (2011) has also developed a highly sensitive surface acoustic wave humidity sensor using electrospun multi-walled carbon nanotube/Nafion nanofiber films on the surface acoustic wave resonator. The sensor showed good linearity and a short response time of about 3s at 63% relative humidity.

Electrospun nanofibers have also been used in the development of humidity optical fiber sensor. A humidity sensitive material such as poly(acylic acid) (PAA) may be electrospun onto a silica fiber core. To render the nanofibers insoluble, Urrutia et al (2012) added β-cyclodextrin into the PAA solution before electrospinning followed by thermal treatment. The silica fiber with the nanofiber coating was subsequently loaded with Ag nanoparticles. Detection of humidity change is through monitoring the absorbance spectra. A fast response time of 400 ms was achieved using this relative humidity optical sensor.

There are other considerations when setting up the humidity sensor. Issues such as component arrangement and electrode materials selection will determine the performance of the sensor. Batool et al (2013) examined the effect of Ti, Ni and Au electrodes on the characteristics of TiO₂ nanofibers for use as humidity sensor. Their setup comprises of a base silicon substrate where the electrospun fibers were deposited. The 80 nm thick metal electrodes (Ti, Ni and Au) were deposited on top of the fibers. In their study, Ti was found to be the best electrode compared to the electrode made from Ni and Au. They attribute this to the more porous structure of Ti electrode which allows more water vapour to diffuse through the pores to reach the sensing material layer.

In a step towards the development of smart textiles, self-powered flexible humidity sensors based on electrospun fibers have been fabricated. Wang et al (2021) used electrospun poly(vinyl alcohol)/Ti₃C₂T_x (PVA/MXene) as the humidity sensing material. This electrospun membrane was deposited on an interdigital electrode. To power the humidity sensor, a monolayer molybdenum diselenide (MoSe₂) piezoelectric nanogenerator (PENG) was fabricated on a flexible terephthalate (PET) substrate. The MoSe₂ PENG converts mechanical energy to electric energy and the electrical current runs through the PVA/MXene sensor material. The PVA/MXene sensor material showed large response, fast response/recovery time, low hysteresis and excellent repeatability through the detection of output voltage with changes in humidity. At higher humidity, the resistance of PVA/MXene membrane increases. This is probably due to greater layer spacing between the highly conductive MXene layers as water molecules enter the hydrophilic membrane.