Excellent Humidity Sensitivity Research Articles

Performance analysis of Fe2O3/CNT humidity sensor based on adsorption kinetics and DFT computations

Low sensitivity, long recovery times and unstable ability at high humidity has become a problem to develop humidity sensors. To overcome these problems, iron sesquioxide/carbon nanotube (Fe2O3/CNT) composite was prepared by hydrothermal technique as the humidity sensor material. The results of XRD and FI-IR reveal that Fe2O3/CNT with abundant hydrophilic functional groups are successfully synthesized. Meanwhile, First-principles (DFT) results demonstrate that Fe2O3 has higher binding energy and density of states (DOS) peak shift when worked with water molecules than ferric oxide (Fe3O4), indicating it has better adsorption kinetics thus to high humidity sensitivity on the surface. Furthermore, the capability of the Fe2O3 material to sense humidity was investigated via hysteresis and response/recovery test. Based on the hydrophilic functional groups and high adsorption capacity for water molecules, when compared with Fe3O4-based humidity sensor, this Fe2O3/CNT device has an excellent humidity sensitivity, which display low hysteresis, good repeatability, and short response time. This work suggests that Fe2O3/CNT could be a promising candidate material for developing high-performance humidity sensor.

A Simplified Method for the Preparation of Highly Conductive and Flexible Silk Nanofibrils/MXene Membrane.

Silk nanofibers (SNF) have great applications in high-performance functional nanocomposites due to their excellent mechanical properties, biocompatibility, and degradability. However, the preparation of SNF by traditional methods often requires the use of some environmentally harmful or toxic reagents, limiting its application in green chemistry. In this paper, we successfully prepared SNF using natural silk as raw material and solvent stripping technology by adjusting the solvent concentration and solution ratio (the diameter of about 120 nm). Using the above SNFs as raw materials, SNF membranes were prepared by vacuum filtration technology. In addition, we prepared an SNF/MXene nanocomposite material with excellent humidity sensitivity by simply coating MXene nanosheets with silk fibers. The conductivity of the material can approach 1400.6 S m-1 with excellent mechanical strength (51.34 MPa). The SNF/MXene nanocomposite material with high mechanical properties, high conductivity, and green degradability can be potentially applied in the field of electromagnetic interference (EMI) shielding, providing a feasible approach for the development of functional nanocomposite materials.

Flexible highly-sensitive humidity sensor based on CGO/SMPLAF for wearable human skin humidity detection

Conventional humidity sensors have poor flexibility and low sensitivity, which makes them difficult for wearable electronic device applications. This study aims to fabricate a flexible, high-sensitivity and breathable humidity sensor for skin humidity monitoring. The sensor includes two parts, i.e., shape memory polylactic acid fiber (SMPLAF) as the substrate and crumpled graphene oxide (CGO) membrane as the top humidity-sensitive layer. SMPLAF was firstly stretched and then heated to restore its original shape to obtain the CGO membrane. The unique crumpled morphology of CGO membrane provides a large specific surface area and high capillary force, which facilitates the rapid exchange of water molecules between the sensing CGO membrane and the external environment. Accordingly, the sensor exhibits excellent humidity sensitivity, fast response time (<5 s) and long term stability (15 days). Moreover, the CGO-based sensor demonstrates excellent flexibility. Bent at 120°, the response almost unchanged, whereas the response of the sensor without CGO decreased from 0.89 to 0.66. Though the sensor has the disadvantage of unstable performance at high RH, it could be overcome by controlling the wavelength of the crumpled structure of CGO. Due to the above superior properties, the sensor shows promising potentials for human breath monitoring and skin humidity test.

Adopted ion-pair effect to construct bicontinuous starch-based gel and its application in humidity sensitivity and strain-responsiveness

Along with the growing consume of disposable electronic devices, the development of biodegradable and sustainable resources is highly desired to address the contaminated footprints from petroleum-based materials. However, it is still a challenge to overcome obstacles of the dispersion of additives and flexibility for fabricating starch-based strain-responsiveness device. Herein, we developed a facile method to fabricate mechanically stretchable starch-based gel (SGZn gel) with humidity-conductivity through thermal mixing starch and ions-pairing solution, which was a kind of transparent gel without region of stress concentration and hysteresis effect. Attributed to disorganization effect and dipole interaction of ion pairs, SGZn gels achieved homogeneous matrix, increased elongation at break (243%) and high transparency (87%). Meanwhile, the continuous ionic conductivity paths in gel matrix constructed by ions-pairing solution could endow SGZn gels with excellent humidity sensitivity, obtaining an ionic conductivity of 10−3 S/cm after absorbing 10 wt% water from humidity environment. Benefiting from these features, SGZn gels were capable of sensing human motions, both tiny motion in fingers and intense force in feet, satisfying the basic requirements for sustainable wearable devices. This study provided a facile approach for preparing multifunctional starch-based gels to meet the requirements in various applications such as flexible material, humidity detection and smart devices.

Investigation on anneal-tuned properties of ZnFe2O4 nanoparticles for use in humidity sensors.

The effect of different annealing temperatures on structural, optical and magnetic properties of ZnFe2O4 nanoparticles prepared using the coprecipitation technique has been investigated. With the increase in annealing temperature, crystallinity and average crystallite size of nanoparticles increased. The average crystallite size was found to be 5.55 nm, 6.62 nm and 32.9 nm for the samples annealed at 300 °C, 500 °C and 700 °C, respectively. The X-ray diffraction and Fourier-transform infrared spectroscopy revealed the formation of a cubic spinel structure. The optical direct and indirect bandgap energy decreased with an increase in annealing temperature. The saturation magnetization increased from 16.38 emu/g to 25.91 emu/g. The M–H curves depicted the magnetic phase transition from superparamagnetic to ferrimagnetic. The electrical properties were investigated using an impedance analyzer in the frequency range of 300 Hz to 1 MHz. The conduction properties showed enhancement with increased annealing. The humidity sensing properties were investigated in the range of 15–90% RH and revealed a strong dependence of adsorption capacity on the annealing temperature. Electrical conductivity improved with increased humidity. Excellent humidity sensitivity was observed for ferrites annealed at 700 °C attributed to increased crystallinity and reduced lattice strain making them a potential candidate for use in humidity sensors.

Performance of the highly sensitive humidity sensor constructed with nanofibrillated cellulose/graphene oxide/polydimethylsiloxane aerogel via freeze drying.

A kind of capacitive humidity sensor with high sensitivity constructed with nanofibrillated cellulose (NFC), graphene oxide (GO) and polydimethylsiloxane (PDMS) is presented in this work, via a simple ultrasonic dispersion and freeze drying technology. The NFC and GO with a strong adsorption for water molecules were used as a substrate for the promotion of capacitive response of the humidity sensor. Moreover, anhydrous ethanol was added to inhibit the generation of big cracks in the humidity sensor in the freeze drying process, so as to obtain a regular network porous structure, then providing a great deal of conduction channels and active sites for molecular water. Also, the addition of PDMS can effectively enhance the flexibility and stability of its porous structure. The results confirmed that the humidity sensor with 30 wt% GO showed an excellent humidity sensitivity (6576.41 pF/% RH), remarkable reproducibility, low humidity hysteresis characteristic in 11–97% relative humidity (RH) at 25 °C, and short response/recovery times (57 s/2 s). In addition, the presented sensor exhibited small relative deviation of the measured relative humidity value compared with the commercial hygrometer. The realization of the high sensitivity can be attributed to the theories about interaction of the hydrophilic group, proton transfer of water molecules and the three-dimensional network transport structure model. Therefore, the NFC/GO/PDMS humidity sensor finally realizes stable, reproducible and fast humidity sensing via an eco-friendly process, exhibiting promising potential for wide practical application.

Mxene Nanosheets Preparation Sensitive Film for QCM Humidity Sensor

Fast, sensitive and stable detection of humidity is essential. In this paper, a 2D ultra-thin nanosheet Mxene was used as the material to form a sensitive film to construct the quartz crystal microbalance (QCM) humidity sensor by drop coating method. The element A in the MAX phase was dislodged by a simple liquid phase etching method. The morphology and composition of Mxene were studied by means of a series characterization methods, it was found the sample take on a ultrathin grapheme-like nanosheet. The test results show that the humidity sensor has 12.8Hz/% RH sensitivity, 6s and 2s (@ 90%)response/recovery time, maximum humidity hysteresis of 1.16%RH, good stability and selectivity. Finally, the adsorption enthalpy of water molecules on the surface of Mxene was explored by DFT calculation. It was found that residual F- played an important role in humitity sensing Introduction: The new two-dimensional (2D) material Mxene has been widely used in energy storage , battery , catalysis , electromagnetic shielding and other directions since it was discovered by the research team of Professor Yury Gogotsi of Drexel University in 2011.Ti3C2Tx has good electronic conductivity and hydrophilicity, it is the most widely studied and mature of all Mxenes. This work combines Mxene-Ti3C2Tx with the QCM test platform to further enhance the application of Mxene materials on humidity sensors.By optimizing the method in the previous literature, the Ti3AlC2 powder was etched in LiF / HCl solution to remove the Al element, some post-treatment such us ultrasonic was carry out to obtain a single-layer Ti3C2Tx solution that can be used to directly prepare the device. The obtained samples were characterized by XRD, SEM, TEM, EDS, XPS and BET, etc, it was found that Mxene with a large specific surface area has a good single-layer sheet structure, and the surface contains a large number of -OH, -O, -F functional groups. This single-layer sheet structure with large specific surface area and abundant active sites is more conducive to the adsorption of water molecules. By detecting the relative humidity of 11.3% - 97.3% shows that the sensor with the best performance has excellent sensitivity of 12.8Hz/% RH, 6s and 2s (@ 90%) rapid response/recovery time, maximum humidity hysteresis of 1.16%RH, good short-term repeatability, long-term stability, and excellent cross-sensitivity. Through characterization and simulation, it is found that -F functional group plays a certain role in the excellent humidity sensitivity. This not only provides a new possibility for the application of 2D materials in humidity sensor, but also widens the application of Mxene materials in sensor field. Figure 1

Stabilizing Ti3C2Tx-MXenes with TiOF2 nanospheres intercalation to improve hydrogen evolution reaction and humidity-sensing performance

MXenes, a new class of two-dimensional materials, possessing high electrical conductivity and unique surface properties，have exhibited great potential as non-noble metal catalyst for hydrogen evolution reaction (HER). However, due to poor oxidation resistance, they are easily oxidized in aqueous solution and high humidity environment, resulting in a great loss of electronic properties and surface reactivity. Herein, we report a simple and effective composite synthesis strategy with TiOF2 intercalation to improve the stability of MXenes materials. TiOF2@Ti3C2Tx prepared by this strategy shows ultra-long catalytic activity life of 1000 cycles in acid electrolyte. Besides，the catalyst also shows high HER activity with a low onset potential of 103 mV and a Tafel slope as low as 56.2 mV dec−1. Benefiting from the stabilization of TiOF2 and the physicochemical properties of Ti3C2Tx, TiOF2@Ti3C2Tx also exhibits excellent humidity sensitivity and stability.

Structural, optical and excellent humidity sensing behaviour of ZnSnO3 nanoparticles: effect of annealing

The chemi-resistive humidity sensing behaviour of as prepared and annealed fcc-ZnSnO3 nanoparticles synthesized using wet chemical synthesis method is reported here. The effect of annealing on the evolution of varied nano-morphology of ZnSnO3 is in accordance to Ostwald’s ripening law. The optical energy bandgap energy change from 4.64 to 3.84 eV for annealed samples confirms the role of annealing over improved sensing performance. At room temperature, an excellent humidity sensitivity of 4155% and response/recovery time of 19/22 s. is observed for 500 °C annealed ZnSnO3 sample within 08–97% relative humidity range. The experimental data observed over the entire range of RH values well fitted with the Freundlich adsorption isotherm model, and revealing two distinct water adsorption regimes. This indicates that with an increase in annealing temperature the samples show improved adsorption capacity and strength. The excellent humidity sensitivity observed in the annealed nanostructures is attributed to Grotthuss mechanism considering the availability and distribution of available adsorption sites. This present result proposes utilization of low cost synthesis technique of ZnSnO3 holds the promising capabilities as a potential candidate for the fabrication of next generation humidity sensors.

Controlled Zn1−xNixO nanostructures for an excellent humidity sensor and a plausible sensing mechanism

A Freundlich adsorption isotherm model confirms a plausible humidity sensing mechanism when using wet chemically prepared Zn1−xNixO nanostructures.

A Highly Sensitive Humidity Sensor Based on Ultrahigh-Frequency Microelectromechanical Resonator Coated with Nano-Assembled Polyelectrolyte Thin Films

We developed a highly sensitive humidity sensor based on the combination of ultrahigh-frequency film bulk acoustic resonator (FBAR) and nano-assembled polyelectrolyte (PET) thin films. The water molecule absorption efficiency was optimized by forming loosely-packed PET nanostructures. Then, the humidity sensing characteristics were analyzed in terms of sensitivity, linearity, reversibility, stability and detection limit. As a result, PET-coated FBAR exhibits excellent humidity sensitivity of 2202.20 Hz/ppm, which is five orders of magnitude higher than quartz crystal microbalance (QCM). Additionally, temperature dependence was investigated with the result that PET-coated FBAR possessed a higher sensitivity at low temperature. Furthermore, we realized the selective detection of water vapor from volatile organic compounds (VOCs) with respect to the polarity property. Owing to the high sensitivity, miniaturized size and ultrahigh operating frequency, PET-coated FBAR is uniquely favorable as a wireless humidity sensor node to integrate into wireless sensor networks (WSNs).

Facile fabrication of NaCl-added mesoporous silica HMS composite and its humidity responsing performance

A humidity-responsive material of NaCl-added mesoporous silica HMS composite was obtained successfully by a facile grind method where mesoporous silica HMS had been synthesized via a sol–gel route. Under room temperature, the sample of NaCl-added HMS with NaCl content of 0.2 g/g showed excellent humidity sensitivity. The impedance decreased linearly by more than four orders of magnitude from 11 to 95% relative humidity and the response time was about 18 s. In addition, the effect of pore order in mesoporous silica on its humidity-sensitive properties was discussed and complex impedance spectra were investigated to validate the humidity-sensing mechanism of Na-HMS(0.2).

Fabrication, structural and humidity sensing properties of BaTiO3 nanofibers via electrospinning

Barium titanate (BTO) nanofibers were synthesized by electrospinning and followed calcination. The morphologies and microstructures of the nanofibers were characterized by X-ray diffractometer (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Calcinating temperature and process greatly influenced the surface morphology of the nanofibers. In addition, parallel BTO nanofibers were also successfully produced by a centrifugal electrospinning technique. And the humidity sensing properties of the nanofibers were measured. The results showed an ultrafast response time (~0.5 s) and a recovery time (~0.4 s) to humidity at room temperature, which revealed excellent humidity sensitivity performance.

Humidity Sensing Properties of Screen-printed Carbon-black an Fe(II) Spin Crossover Compound Hybrid Films

We report on the humidity sensing properties of a screen-printed polymeric film loaded with carbon-black and Fe[(Htrz)2(trz)]BF4 nanoparticles. The new film shows excellent humidity sensitivity in a w de relative humidity range and the processing conditions make it suitable for a new eneration of inexpensive, resistive humidity sensors on flexible substrates tags

The humidity-sensitive property of MgO-SBA-15 composites in one-pot synthesis

Abstract MgO-SBA-15 has been successfully developed as a kind of humidity-sensitive material. The structure of MgO-SBA-15 materials was characterized by XRD, IR spectroscopy, N 2 adsorption–desorption, TEM and XPS. The study proved that the MgO did not destroy the mesoporous structure of SBA-15 via the characteristic results of XRD and N 2 adsorption–desorption. Researchers built the thick film humidity sensors on the Al 2 O 3 ceramic substrate with two Ag–Pd interdigital electrodes with five fingers. The sensor shows excellent humidity sensitivity from 11% to 95%RH, and it has very good response and recovery property. The response time is about 10 s and the recovery time is about 20 s. We found that the mesoporous structure of the sensing materials contributed to the humidity sensitivity. We discussed a possible mechanism based on the humidity sensor produced by MgO-SBA-15.

Direct‐current and alternating‐current analysis of the humidity‐sensing properties of nickel oxide doped polypyrrole encapsulated in mesoporous silica SBA‐15

AbstractNickel oxide (NiO) doped polypyrrole (PPy) was encapsulated in mesoporous SBA‐15. All of the synthesized samples were characterized by infrared spectroscopy, X‐ray diffraction, and scanning electron microscopy. They were investigated as humidity‐sensor materials at room temperature. The sensor showed excellent humidity sensitivity in the relative humidity range 11–95%. The humidity‐sensing properties were very much improved by encapsulation of the NiO‐doped PPy into mesoporous silica SBA‐15. Finally, the sensitivity mechanism was investigated by direct‐current (dc) and alternating‐current (ac) analysis. The dc circuit with the instantaneous polarity reversion method was designed by us to study the dc response in different humidity environments. The conductive mechanism was established through the dc and ac investigation, and the conductive particles were identified as ions and electrons. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Improvement of response characteristics of TiO 2 humidity sensors by simultaneous addition of Li 2O and V 2O 5

Porous TiO 2 ceramics were prepared by adding various amounts of Li 2O and V 2O 5 and the humidity sensitivity of the resulting ceramics was investigated by means of electrical measurements, scanning electron microscopy and mercury intrusion porosimetry. Simultaneous addition of Li 2O and V 2O 5 to TiO 2 enabled sintering at temperatures as low as 700 °C and also decreased the impedance of the ceramics. Furthermore, in the ceramics including these additives simultaneously, excellent humidity sensitivity as well as good response characteristics were observed. The microstructures of these ceramics depended on the firing temperature and the amount and ratio of Li 2O/V 2O 5, and optimum humidity sensitivity was observed for the sample including both 0.25 mol% Li 2O and 0.75 mol% V 2O 5 fired at 700 °C. These results indicated that the humidity sensitivity and its response characteristics were closely related to the microstructure, and that improving the uniformity of microstructure is important for improving humidity sensitivity and its response characteristics.